Merge branch 'canvas-formats'

The canvas was two spin buttons, so "an Instagram story" or "A4 at 300 dpi"
was a number the user had to work out. src/core/canvas.cpp now holds one
table of standard sizes both front-ends read: a web format is a fixed pixel
size, a print format a sheet in millimetres that becomes pixels at a dpi.
Adding a format is a line in canvasFormats() — the dropdown, --format and
--help all walk the table.

The pixels stay the truth everywhere; renderChain never hears about formats.
A format is the presentation of a canvas, carried alongside the pixels in
Options and Project and empty for a custom size, and where one is named it
decides the pixels — so a project saved as A4 reopens as A4 rather than as
whatever it happened to measure.

In the interface a format fills the width and height in and greys them out
rather than hiding them: the size a sheet works out to is worth seeing. A
print format also shows a DPI row, and Custom hands the spins back keeping
the last size, which makes starting from a format and then tweaking it the
ordinary path.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CLAUDE.md

@@ -14,11 +14,11 @@ Everything is warning-clean under `-Wall -Wextra -Wpedantic`; keep it that way.
 
 The test suite is a single hand-rolled binary (no framework), so there is no
 per-test filter: run `./build/tests/effector_tests` directly to see the section
-headings (`rng`, `registry`, `scanlines`, `batch loading`) and the `ok`/`FAIL`
+headings (`rng`, `registry`, `scanlines`, `tiles`, `blur`, `chain`, `project`,
-lines. To iterate on one area, comment out the other blocks in
+`image io`, `batch loading`) and the `ok`/`FAIL` lines. To iterate on one area, comment
-`tests/core_tests.cpp` or add a new `std::cout << "section\n"` block — checks are
+out the other blocks in `tests/core_tests.cpp` or add a new
-plain `check(condition, "description")` calls and `main()` returns the failure
+`std::cout << "section\n"` block — checks are plain
-count.
+`check(condition, "description")` calls and `main()` returns the failure count.
 
 Headless render (no display needed, the fastest end-to-end smoke test):
 
@@ -41,21 +41,77 @@ env -u WAYLAND_DISPLAY GDK_BACKEND=x11 DISPLAY=:79 build/effector ...
 
 `Effect` (`src/core/effect.hpp`) exposes `params()` returning `std::vector<ParamSpec>`
 (`src/core/params.hpp`). Those specs are the *only* thing the GUI and CLI know
-about an effect: `src/gui/window.cpp` builds SpinButtons/Switches/DropDowns from
+about an effect: `src/gui/window.cpp` builds SpinButtons/Switches/DropDowns/file
-them and `src/cli/options.cpp` generates flags from them (`strip_height` →
+choosers from them and `src/cli/options.cpp` generates flags from them
-`--strip-height`). Never add effect-specific branches to the GUI or the parser —
+(`strip_height` → `--strip-height`). Never add effect-specific branches to the
-add a `ParamSpec` instead. Values travel as `ParamValues` (a
+GUI or the parser — add a `ParamSpec` instead. A `Path` param names a file on
+disk; like a source path it is written relative to the project folder. A `Text`
+param is free-form text the user types, and a `Color` param is a colour written
+`#rrggbb` (`parseColor`/`formatColor`/`paramColor` in `src/core/params.hpp`),
+shown as a colour button in the interface. Values
+travel as `ParamValues` (a
 `map<string, variant<long,double,bool,string>>`), read back with
 `paramInt`/`paramDouble`/`paramBool`/`paramChoice`.
 
+Whether an effect works from the loaded photographs is part of the same
+contract: `Effect::needsSources()` (default true, false for blur). Only the
+first stage ever sees the sources, so `chainNeedsSources` (`src/core/chain.cpp`)
+just asks the first stage — a chain led by blur renders with no images loaded at
+all, and both front-ends gate on that rather than on "are there any images".
+
 Adding an effect: new `src/effects/<name>.{hpp,cpp}` subclassing `Effect`, one
 line in the registry list in `src/core/registry.cpp`, one line in the
-`effector_core` sources in `CMakeLists.txt`. Nothing else.
+`effector_core` sources in `CMakeLists.txt`. Nothing else — it shows up in the
+CLI, in the chain menu and as a cascade stage on its own.
+
+### Effects cascade
+
+Effects are never applied on their own: `renderChain` (`src/core/chain.cpp`)
+runs a `std::vector<ChainStage>` in order, handing stage 0 the loaded sources
+and every later stage the previous stage's output as its *only* source. That is
+the whole of the cascade — an effect neither knows nor cares whether its input
+came from disk. All stages share one `Rng` and run strictly in sequence, so a
+seed still reproduces the cascade byte for byte.
+
+The interface builds the chain in the sidebar ("Effect chain": add from the
+menu, reorder, remove, one stage selected at a time with its params below), the
+CLI builds it from repeated `-e`: `-e scanlines --strip-height 6 -e blur
+--radius 5`. Flags share one namespace across all effects, so `buildChain`
+(`src/cli/options.cpp`) hands each flag to the stage whose effect declares it —
+two effects must not declare the same param key.
+
+### The canvas is a format or plain pixels
+
+`src/core/canvas.cpp` holds the one table of standard sizes both front-ends
+offer: web formats are a fixed pixel size, print formats a sheet in millimetres
+that becomes pixels at a dpi. Adding a format is a line in `canvasFormats()` —
+the dropdown, `--format` and `--help` all walk that table. The pixels stay the
+truth everywhere (`renderChain` never hears about formats); a format is the
+*presentation* of a canvas, carried alongside the pixels in `Options` and
+`Project` and empty for a custom size. Where a format is named it decides the
+pixels, so a project saved as A4 reopens as A4.
+
+### Projects are plain JSON
+
+`src/core/project.cpp` reads and writes the whole setup — source paths, the
+chain with its params, canvas and seed — as a small JSON file, with its own
+minimal reader rather than a JSON dependency. Writing is driven by the effects'
+`ParamSpec`s, not by whatever happens to sit in a stage's `ParamValues`, so a
+key an effect does not declare is never written and never read back: an old
+project keeps the defaults for params that did not exist when it was saved.
+Source paths under the project's own folder are written relative, so a folder
+can be moved or shared whole.
+
+The interface opens and saves projects from the header-bar menu (New, Open,
+Save, Save As, with the usual Ctrl+N/O/S accelerators); the window title shows
+the project name. Opening one replaces the sources, so a decode still in flight
+is discarded rather than mixed into the new project (`dropLoadedImages_`).
 
 ### Two link targets, deliberately
 
 `effector_core` (static) holds image IO, params, effects, rng and links only
-`gdk-pixbuf-2.0` + Threads — **no gtkmm, no display**. The `effector` executable
+`gdk-pixbuf-2.0`, `pangocairo` (the text effect's glyphs; Pango and Cairo need
+no display) + Threads — **no gtkmm, no display**. The `effector` executable
 adds `src/main.cpp`, `src/cli/`, `src/gui/` and links gtkmm. Keep GTK out of
 `src/core/` and `src/effects/`; the tests link the core alone and rely on this.
 
@@ -64,7 +120,10 @@ adds `src/main.cpp`, `src/cli/`, `src/gui/` and links gtkmm. Keep GTK out of
 `Rng` (`src/core/rng.hpp`) is a hand-written xoshiro256\*\* rather than
 `<random>`, because libstdc++ distributions are not portable across
 implementations. Same sources + settings + seed must produce byte-identical
-output on any machine — the tests assert this. Effects must draw *every* random
+output on any machine — the tests assert this. The one thing outside this is the
+text effect's glyphs, which are whatever shapes the machine's fonts and
+fontconfig give: everything it is handed, the rng included, is still used the
+same way. Effects must draw *every* random
 value from the `Rng&` they are handed, and must not depend on thread scheduling:
 the scanlines effect first draws the whole strip plan serially from the rng,
 then copies pixels in parallel.

CMakeLists.txt

@@ -15,22 +15,34 @@ set(CMAKE_CXX_FLAGS_RELEASE "-O3 -DNDEBUG")
 find_package(PkgConfig REQUIRED)
 pkg_check_modules(GTKMM REQUIRED IMPORTED_TARGET gtkmm-4.0)
 pkg_check_modules(GDKPIXBUF REQUIRED IMPORTED_TARGET gdk-pixbuf-2.0)
+# Pango and Cairo draw the text effect's glyphs. Neither needs a display, so
+# the core stays headless.
+pkg_check_modules(PANGOCAIRO REQUIRED IMPORTED_TARGET pangocairo)
 
 find_package(Threads REQUIRED)
 
 # --- core: effects and image handling, no GUI ---------------------------------
 add_library(effector_core STATIC
+    src/core/canvas.cpp
+    src/core/chain.cpp
     src/core/effect.cpp
     src/core/image.cpp
     src/core/params.cpp
+    src/core/project.cpp
     src/core/registry.cpp
     src/core/rng.cpp
+    src/effects/blur.cpp
+    src/effects/grain.cpp
+    src/effects/monochrome.cpp
+    src/effects/overlay.cpp
     src/effects/scanlines.cpp
+    src/effects/text.cpp
+    src/effects/tiles.cpp
 )
 target_include_directories(effector_core PUBLIC src)
 # The core is GUI-free: it only needs gdk-pixbuf for decoding and encoding,
 # so headless rendering works without a display.
-target_link_libraries(effector_core PUBLIC PkgConfig::GDKPIXBUF Threads::Threads)
+target_link_libraries(effector_core PUBLIC PkgConfig::GDKPIXBUF PkgConfig::PANGOCAIRO Threads::Threads)
 target_compile_options(effector_core PRIVATE -Wall -Wextra -Wpedantic)
 
 # --- application --------------------------------------------------------------

README.md

@@ -34,6 +34,22 @@ build/effector -o out.png -s 1920x1080 \
     --strip-height 8 --jitter 4 --seed 42 photo1.jpg photo2.png
 ```
 
+The canvas is either a size in pixels (`-s 1920x1080`, the Custom entry in the
+interface) or one of the standard formats, which `--help` lists in full:
+
+```bash
+build/effector -o post.png --format instagram-post -e tiles
+build/effector -o sheet.png --format a4 --dpi 300 -e scanlines
+```
+
+A web format — the Instagram sizes, Full HD, 4K UHD — is a fixed pixel size. A
+print format — A5, A4, A3, US Letter — is a sheet of paper, and becomes pixels
+at `--dpi` (36–1200, 300 by default), so A4 at 300 dpi is 2480×3508. Picking a
+format in the interface fills the width and height in and greys them out;
+Custom hands them back, keeping whatever the format last worked out to. A
+format is saved with the project, so an A4 project reopens as A4 rather than as
+the pixels it happened to be.
+
 Any option can also be passed when launching the interface; it becomes the
 starting state (images, canvas size, effect settings, seed).
 

src/cli/options.cpp

@@ -1,9 +1,12 @@
 #include "cli/options.hpp"
 
 #include <charconv>
+#include <cstdint>
 #include <cstring>
 #include <sstream>
 
+#include "core/canvas.hpp"
+#include "core/chain.hpp"
 #include "core/effect.hpp"
 #include "core/registry.hpp"
 
@@ -23,7 +26,10 @@ bool parseSize(const std::string& text, int& width, int& height) {
     long w = 0;
     long h = 0;
     if (!parseLong(text.substr(0, sep), w) || !parseLong(text.substr(sep + 1), h)) return false;
-    if (w <= 0 || h <= 0) return false;
+    // Checked as longs, before the narrowing: 4294968296 cast straight to int
+    // is 1000, which would render a plausible canvas nobody asked for.
+    if (w < kMinCanvasSide || w > kMaxCanvasSide) return false;
+    if (h < kMinCanvasSide || h > kMaxCanvasSide) return false;
     width = static_cast<int>(w);
     height = static_cast<int>(h);
     return true;
@@ -48,7 +54,6 @@ std::string requireValue(int argc, char** argv, int& i, const std::string& flag)
 
 Options parseOptions(int argc, char** argv) {
     Options options;
-    options.effect = defaultEffectName();
 
     for (int i = 1; i < argc; ++i) {
         const std::string arg = argv[i];
@@ -58,15 +63,32 @@ Options parseOptions(int argc, char** argv) {
         } else if (arg == "--list-effects") {
             options.listEffects = true;
         } else if (arg == "-e" || arg == "--effect") {
-            options.effect = requireValue(argc, argv, i, arg);
+            const std::string name = requireValue(argc, argv, i, arg);
-            effectByName(options.effect);  // validate now, fail early
+            effectByName(name);  // validate now, fail early
+            options.effects.push_back(name);
         } else if (arg == "-o" || arg == "--output") {
             options.output = requireValue(argc, argv, i, arg);
         } else if (arg == "-s" || arg == "--size") {
             const std::string value = requireValue(argc, argv, i, arg);
             if (!parseSize(value, options.width, options.height)) {
-                throw EffectError("invalid size '" + value + "', expected e.g. 1920x1080");
+                throw EffectError("invalid size '" + value + "', expected e.g. 1920x1080 with "
+                                  "each side between " + std::to_string(kMinCanvasSide) + " and " +
+                                  std::to_string(kMaxCanvasSide));
             }
+        } else if (arg == "--format") {
+            const std::string value = requireValue(argc, argv, i, arg);
+            if (canvasFormatByKey(value) == nullptr) {
+                throw EffectError("unknown canvas format '" + value + "', see --help");
+            }
+            options.format = value;
+        } else if (arg == "--dpi") {
+            const std::string value = requireValue(argc, argv, i, arg);
+            long dpi = 0;
+            if (!parseLong(value, dpi) || dpi < kMinDpi || dpi > kMaxDpi) {
+                throw EffectError("invalid dpi '" + value + "', expected " +
+                                  std::to_string(kMinDpi) + "-" + std::to_string(kMaxDpi));
+            }
+            options.dpi = static_cast<int>(dpi);
         } else if (arg == "--seed") {
             const std::string value = requireValue(argc, argv, i, arg);
             long seed = 0;
@@ -104,6 +126,20 @@ Options parseOptions(int argc, char** argv) {
                     }
                     break;
                 }
+                case ParamSpec::Type::Path:
+                case ParamSpec::Type::Text:
+                    options.params[spec->key] = requireValue(argc, argv, i, arg);
+                    break;
+                case ParamSpec::Type::Color: {
+                    const std::string text = requireValue(argc, argv, i, arg);
+                    std::uint8_t rgb[3];
+                    if (!parseColor(text, rgb)) {
+                        throw EffectError(arg + ": '" + text +
+                                          "' is not a colour, expected e.g. #ff8800");
+                    }
+                    options.params[spec->key] = formatColor(rgb);
+                    break;
+                }
                 case ParamSpec::Type::Choice: {
                     const std::string text = requireValue(argc, argv, i, arg);
                     bool valid = false;
@@ -119,6 +155,12 @@ Options parseOptions(int argc, char** argv) {
             options.images.push_back(arg);
         }
     }
+    // Applied once everything is parsed, so --dpi may come before --format and
+    // so a format always wins over an -s given alongside it.
+    if (const auto* format = canvasFormatByKey(options.format)) {
+        canvasFormatSize(*format, options.dpi, options.width, options.height);
+    }
+    if (options.effects.empty()) options.effects.push_back(defaultEffectName());
     return options;
 }
 
@@ -127,13 +169,25 @@ std::string usage(const char* programName) {
     out << "Usage: " << programName << " [options] [image ...]\n\n"
         << "Image effects app. Runs the GTK interface unless --output is given.\n\n"
         << "Options:\n"
-        << "  -e, --effect NAME     effect to apply (default: " << defaultEffectName() << ")\n"
+        << "  -e, --effect NAME     effect to apply, repeat to cascade (default: " << defaultEffectName() << ")\n"
         << "  -o, --output FILE     render headlessly to this file and exit\n"
-        << "  -s, --size WxH        canvas size (default: 1920x1080)\n"
+        << "  -s, --size WxH        canvas size, each side " << kMinCanvasSide << "-"
+        << kMaxCanvasSide << " (default: 1920x1080)\n"
+        << "      --format NAME     canvas format, overrides --size (see below)\n"
+        << "      --dpi N           resolution of a print format, " << kMinDpi << "-" << kMaxDpi
+        << " (default: " << kDefaultDpi << ")\n"
         << "      --seed N          random seed (default: random)\n"
         << "      --list-effects    list available effects and exit\n"
         << "  -h, --help            show this help and exit\n";
 
+    out << "\nCanvas formats (--format):\n";
+    for (const auto& format : canvasFormats()) {
+        out << "  " << format.key;
+        if (format.key.size() < 20) out << std::string(20 - format.key.size(), ' ');
+        out << "  " << format.label << ", " << canvasFormatDetail(format)
+            << (format.print ? " at --dpi" : "") << "\n";
+    }
+
     for (const auto& effect : effects()) {
         out << "\n" << effect->title() << " options:\n";
         for (const auto& spec : effect->params()) {
@@ -147,4 +201,19 @@ std::string usage(const char* programName) {
     return out.str();
 }
 
+std::vector<ChainStage> buildChain(const Options& options) {
+    std::vector<ChainStage> stages;
+    for (const auto& name : options.effects) {
+        ChainStage stage;
+        stage.effect = effectByName(name);
+        stage.values = stage.effect->defaults();
+        for (const auto& spec : stage.effect->params()) {
+            const auto it = options.params.find(spec.key);
+            if (it != options.params.end()) stage.values[spec.key] = it->second;
+        }
+        stages.push_back(std::move(stage));
+    }
+    return stages;
+}
+
 }  // namespace effector

src/cli/options.hpp

@@ -5,16 +5,20 @@
 #include <string>
 #include <vector>
 
+#include "core/canvas.hpp"
+#include "core/chain.hpp"
 #include "core/params.hpp"
 
 namespace effector {
 
 struct Options {
     std::vector<std::string> images;
-    std::string effect;
+    std::vector<std::string> effects;  // one per cascade stage, in order
     std::string output;              // empty -> launch the interface
     int width = 1920;
     int height = 1080;
+    std::string format;      // a named canvas format, empty for plain pixels
+    int dpi = kDefaultDpi;   // only a print format is rendered at a resolution
     std::optional<std::uint64_t> seed;
     ParamValues params;              // only the keys the user actually passed
     bool listEffects = false;
@@ -27,4 +31,9 @@ Options parseOptions(int argc, char** argv);
 
 std::string usage(const char* programName);
 
+// Turns the parsed names and flags into cascade stages: every flag goes to the
+// stage whose effect declares it, so --strip-height and --radius can be given
+// together for `-e scanlines -e blur`.
+std::vector<ChainStage> buildChain(const Options& options);
+
 }  // namespace effector

src/core/canvas.cpp

@@ -0,0 +1,76 @@
+#include "core/canvas.hpp"
+
+#include <algorithm>
+#include <cmath>
+#include <locale>
+#include <sstream>
+
+#include "core/chain.hpp"
+
+namespace effector {
+namespace {
+
+// 210 mm reads as "210", 215.9 mm as "215.9". In the C locale whatever the
+// user's is: gtkmm sets the global locale from the environment, so a stream
+// left to its default writes "210,0" on a German desktop.
+std::string millimetres(double mm) {
+    std::ostringstream out;
+    out.imbue(std::locale::classic());
+    out.setf(std::ios::fixed);
+    out.precision(1);
+    out << mm;
+    std::string text = out.str();
+    if (text.size() > 2 && text.compare(text.size() - 2, 2, ".0") == 0) text.resize(text.size() - 2);
+    return text;
+}
+
+}  // namespace
+
+const std::vector<CanvasFormat>& canvasFormats() {
+    // Written out once and handed back by reference: the table is fixed, and
+    // both front-ends walk it for their menu and their --help.
+    static const std::vector<CanvasFormat> formats = {
+        {"instagram-post", "Instagram post", false, 1080, 1080, 0.0, 0.0},
+        {"instagram-portrait", "Instagram portrait", false, 1080, 1350, 0.0, 0.0},
+        {"instagram-story", "Instagram story", false, 1080, 1920, 0.0, 0.0},
+        {"hd", "Full HD", false, 1920, 1080, 0.0, 0.0},
+        {"uhd", "4K UHD", false, 3840, 2160, 0.0, 0.0},
+        {"a5", "A5", true, 0, 0, 148.0, 210.0},
+        {"a4", "A4", true, 0, 0, 210.0, 297.0},
+        {"a3", "A3", true, 0, 0, 297.0, 420.0},
+        {"letter", "US Letter", true, 0, 0, 215.9, 279.4},
+    };
+    return formats;
+}
+
+const CanvasFormat* canvasFormatByKey(const std::string& key) {
+    if (key.empty()) return nullptr;
+    for (const auto& format : canvasFormats()) {
+        if (format.key == key) return &format;
+    }
+    return nullptr;
+}
+
+void canvasFormatSize(const CanvasFormat& format, int dpi, int& width, int& height) {
+    if (!format.print) {
+        width = format.width;
+        height = format.height;
+        return;
+    }
+    const int clamped = std::clamp(dpi, kMinDpi, kMaxDpi);
+    const auto pixels = [clamped](double mm) {
+        const long value = std::lround(mm / 25.4 * clamped);
+        return static_cast<int>(std::clamp<long>(value, kMinCanvasSide, kMaxCanvasSide));
+    };
+    width = pixels(format.widthMm);
+    height = pixels(format.heightMm);
+}
+
+std::string canvasFormatDetail(const CanvasFormat& format) {
+    if (format.print) {
+        return millimetres(format.widthMm) + " × " + millimetres(format.heightMm) + " mm";
+    }
+    return std::to_string(format.width) + " × " + std::to_string(format.height) + " px";
+}
+
+}  // namespace effector

src/core/canvas.hpp

@@ -0,0 +1,45 @@
+#pragma once
+
+#include <string>
+#include <vector>
+
+namespace effector {
+
+// A named canvas size both front-ends offer, so "A4 at 300 dpi" is one choice
+// rather than a pixel count the user has to work out. Web formats are a fixed
+// pixel size; print formats are a physical size in millimetres and only become
+// pixels once a resolution is given, which is why the dpi belongs to the
+// canvas rather than to any one format.
+struct CanvasFormat {
+    std::string key;    // written in a project and given to --format, e.g. "a4"
+    std::string label;  // shown in the interface, e.g. "A4"
+    bool print = false;
+    int width = 0;         // pixels, web formats only
+    int height = 0;        //
+    double widthMm = 0.0;  // millimetres, print formats only
+    double heightMm = 0.0;
+};
+
+// The dpi a print format is rendered at. The bounds keep A3 — the largest
+// sheet here — inside kMaxCanvasSide at the top end, and keep a sheet from
+// coming out too small to see at the bottom.
+inline constexpr int kMinDpi = 36;
+inline constexpr int kMaxDpi = 1200;
+inline constexpr int kDefaultDpi = 300;
+
+// In the order they are offered: the web formats first, then the paper sizes.
+const std::vector<CanvasFormat>& canvasFormats();
+
+// Null when the key names no format, which is how a custom canvas — the user's
+// own pixel sizes — is spelled everywhere: an empty or unknown key.
+const CanvasFormat* canvasFormatByKey(const std::string& key);
+
+// The pixel size of a format. `dpi` is ignored by a web format, which already
+// is a pixel size; a print format is rounded to the nearest pixel and clamped
+// to the canvas bounds.
+void canvasFormatSize(const CanvasFormat& format, int dpi, int& width, int& height);
+
+// "1080 × 1080 px" or "210 × 297 mm", for the dropdown and for --help.
+std::string canvasFormatDetail(const CanvasFormat& format);
+
+}  // namespace effector

src/core/chain.cpp

@@ -0,0 +1,54 @@
+#include "core/chain.hpp"
+
+namespace effector {
+
+Image renderChain(const std::vector<ChainStage>& stages, const std::vector<Image>& sources,
+                  int width, int height, Rng& rng) {
+    if (stages.empty()) throw EffectError("The effect chain is empty.");
+
+    Image image;
+    for (std::size_t i = 0; i < stages.size(); ++i) {
+        const auto& stage = stages[i];
+        if (!stage.effect) throw EffectError("The effect chain has an empty stage.");
+
+        auto values = stage.effect->defaults();
+        for (const auto& [key, value] : stage.values) values[key] = value;
+
+        if (i == 0) {
+            image = stage.effect->render(sources, width, height, rng, values);
+        } else {
+            // Feeding the previous result back in as the only source is what
+            // cascading means here; effects need to know nothing about it.
+            const std::vector<Image> previous = {std::move(image)};
+            image = stage.effect->render(previous, width, height, rng, values);
+        }
+    }
+    return image;
+}
+
+bool chainNeedsSources(const std::vector<ChainStage>& stages) {
+    if (stages.empty() || !stages.front().effect) return false;
+    return stages.front().effect->needsSources();
+}
+
+std::string chainTitle(const std::vector<ChainStage>& stages) {
+    std::string text;
+    for (const auto& stage : stages) {
+        if (!stage.effect) continue;
+        if (!text.empty()) text += " → ";
+        text += stage.effect->title();
+    }
+    return text;
+}
+
+std::string chainSlug(const std::vector<ChainStage>& stages) {
+    std::string text;
+    for (const auto& stage : stages) {
+        if (!stage.effect) continue;
+        if (!text.empty()) text += "-";
+        text += stage.effect->name();
+    }
+    return text;
+}
+
+}  // namespace effector

src/core/chain.hpp

@@ -0,0 +1,46 @@
+#pragma once
+
+#include <string>
+#include <vector>
+
+#include "core/effect.hpp"
+#include "core/image.hpp"
+#include "core/params.hpp"
+#include "core/rng.hpp"
+
+namespace effector {
+
+// The canvas sides both front-ends accept. The upper bound is not a hardware
+// limit but the point past which a side stops fitting comfortably in an int
+// once multiplied out, so both the CLI and the project reader can reject a
+// number rather than silently wrap it into a plausible-looking one.
+inline constexpr int kMinCanvasSide = 1;
+inline constexpr int kMaxCanvasSide = 20000;
+
+// One link of a cascade: an effect together with the values it runs with.
+struct ChainStage {
+    EffectPtr effect;
+    ParamValues values;  // missing keys fall back to the effect's defaults
+};
+
+// Renders the stages in order. The first one sees the loaded sources; every
+// later one sees the previous stage's output as its only source, so blur after
+// scanlines softens the scanlines rather than the photographs.
+//
+// All stages share one Rng and run strictly in sequence, so a seed still
+// reproduces the cascade byte for byte.
+Image renderChain(const std::vector<ChainStage>& stages, const std::vector<Image>& sources,
+                  int width, int height, Rng& rng);
+
+// Whether the chain has to be given source images. Only the first stage ever
+// sees them — every later one works on the stage before it — so this is simply
+// whether the first effect declares that it needs them.
+bool chainNeedsSources(const std::vector<ChainStage>& stages);
+
+// "Scanlines → Blur", for window titles and status lines.
+std::string chainTitle(const std::vector<ChainStage>& stages);
+
+// "scanlines-blur", for file names.
+std::string chainSlug(const std::vector<ChainStage>& stages);
+
+}  // namespace effector

src/core/effect.cpp

@@ -16,6 +16,9 @@ ParamValues Effect::defaults() const {
                 values[spec.key] = spec.defaultValue != 0.0;
                 break;
             case ParamSpec::Type::Choice:
+            case ParamSpec::Type::Path:
+            case ParamSpec::Type::Text:
+            case ParamSpec::Type::Color:
                 values[spec.key] = spec.defaultChoice;
                 break;
         }

src/core/effect.hpp

@@ -27,6 +27,11 @@ public:
     virtual std::string description() const = 0;
     virtual const std::vector<ParamSpec>& params() const = 0;
 
+    // Whether the effect builds its image out of the loaded source images.
+    // Effects that only transform what they are handed (blur) say no, and a
+    // chain that starts with one of them needs no sources at all.
+    virtual bool needsSources() const { return true; }
+
     virtual Image render(const std::vector<Image>& sources, int width, int height, Rng& rng,
                          const ParamValues& values) const = 0;
 

src/core/image.cpp

@@ -43,7 +43,7 @@ Image::Image(int width, int height)
       pixels_(static_cast<std::size_t>(std::max(width, 0)) * 3 *
               static_cast<std::size_t>(std::max(height, 0))) {}
 
-Image Image::load(const std::string& path) {
+Image Image::load(const std::string& path, std::vector<std::uint8_t>* alphaOut) {
     GError* error = nullptr;
     PixbufPtr pixbuf = wrap(gdk_pixbuf_new_from_file(path.c_str(), &error));
     if (!pixbuf) throw std::runtime_error(takeError(error));
@@ -59,11 +59,23 @@ Image Image::load(const std::string& path) {
     const guint8* src = gdk_pixbuf_get_pixels(pixbuf.get());
 
     Image image(width, height);
+    if (alphaOut != nullptr) {
+        alphaOut->assign(static_cast<std::size_t>(std::max(width, 0)) *
+                             static_cast<std::size_t>(std::max(height, 0)),
+                         255);
+    }
     for (int y = 0; y < height; ++y) {
         const guint8* in = src + static_cast<std::size_t>(y) * srcStride;
         std::uint8_t* out = image.row(y);
         if (channels == 3) {
             std::memcpy(out, in, static_cast<std::size_t>(width) * 3);
+        } else if (alphaOut != nullptr) {
+            // Kept apart rather than flattened: the caller does the compositing.
+            std::uint8_t* alpha = alphaOut->data() + static_cast<std::size_t>(y) * width;
+            for (int x = 0; x < width; ++x) {
+                for (int c = 0; c < 3; ++c) out[x * 3 + c] = in[x * channels + c];
+                alpha[x] = in[x * channels + 3];
+            }
         } else {
             // Flatten alpha onto white, matching what a viewer would show.
             for (int x = 0; x < width; ++x) {

src/core/image.hpp

@@ -34,7 +34,11 @@ public:
 
     // Decodes via GdkPixbuf (PNG, JPEG, WebP, TIFF, BMP, ...), applying the
     // embedded EXIF orientation. Throws std::runtime_error on failure.
-    static Image load(const std::string& path);
+    // Transparency is flattened onto white, as a viewer would show it, unless
+    // `alphaOut` is given: it then receives one byte per pixel (255 throughout
+    // for a file without an alpha channel) and the colours are left untouched,
+    // which is what an effect compositing the image needs.
+    static Image load(const std::string& path, std::vector<std::uint8_t>* alphaOut = nullptr);
 
     // Encodes by file extension; png is used when the extension is unknown.
     void save(const std::string& path) const;

src/core/params.cpp

@@ -63,6 +63,37 @@ ParamSpec ParamSpec::makeChoice(std::string key, std::string label, std::string
     return spec;
 }
 
+ParamSpec ParamSpec::makePath(std::string key, std::string label, std::string help) {
+    ParamSpec spec;
+    spec.type = Type::Path;
+    spec.key = std::move(key);
+    spec.label = std::move(label);
+    spec.help = std::move(help);
+    return spec;
+}
+
+ParamSpec ParamSpec::makeText(std::string key, std::string label, std::string defaultText,
+                              std::string help) {
+    ParamSpec spec;
+    spec.type = Type::Text;
+    spec.key = std::move(key);
+    spec.label = std::move(label);
+    spec.help = std::move(help);
+    spec.defaultChoice = std::move(defaultText);
+    return spec;
+}
+
+ParamSpec ParamSpec::makeColor(std::string key, std::string label, std::string defaultColor,
+                               std::string help) {
+    ParamSpec spec;
+    spec.type = Type::Color;
+    spec.key = std::move(key);
+    spec.label = std::move(label);
+    spec.help = std::move(help);
+    spec.defaultChoice = std::move(defaultColor);
+    return spec;
+}
+
 namespace {
 
 const ParamValue* find(const ParamValues& values, const std::string& key) {
@@ -99,10 +130,61 @@ bool paramBool(const ParamValues& values, const std::string& key, bool fallback)
 
 std::string paramChoice(const ParamValues& values, const std::string& key,
                         const std::string& fallback) {
+    return paramText(values, key, fallback);
+}
+
+std::string paramText(const ParamValues& values, const std::string& key,
+                      const std::string& fallback) {
     if (const auto* value = find(values, key)) {
         if (const auto* v = std::get_if<std::string>(value)) return *v;
     }
     return fallback;
 }
 
+namespace {
+
+int hexDigit(char c) {
+    if (c >= '0' && c <= '9') return c - '0';
+    if (c >= 'a' && c <= 'f') return c - 'a' + 10;
+    if (c >= 'A' && c <= 'F') return c - 'A' + 10;
+    return -1;
+}
+
+}  // namespace
+
+bool parseColor(const std::string& text, std::uint8_t rgb[3]) {
+    std::string digits = text;
+    if (!digits.empty() && digits.front() == '#') digits.erase(digits.begin());
+    if (digits.size() != 3 && digits.size() != 6) return false;
+
+    int parsed[6];
+    for (std::size_t i = 0; i < digits.size(); ++i) {
+        parsed[i] = hexDigit(digits[i]);
+        if (parsed[i] < 0) return false;
+    }
+    for (int c = 0; c < 3; ++c) {
+        // "#abc" is the usual shorthand for "#aabbcc".
+        const int value = digits.size() == 3 ? parsed[c] * 17 : parsed[c * 2] * 16 + parsed[c * 2 + 1];
+        rgb[c] = static_cast<std::uint8_t>(value);
+    }
+    return true;
+}
+
+std::string formatColor(const std::uint8_t rgb[3]) {
+    static const char* const kDigits = "0123456789abcdef";
+    std::string text = "#";
+    for (int c = 0; c < 3; ++c) {
+        text += kDigits[rgb[c] >> 4];
+        text += kDigits[rgb[c] & 0x0F];
+    }
+    return text;
+}
+
+void paramColor(const ParamValues& values, const std::string& key, const std::string& fallback,
+                std::uint8_t rgb[3]) {
+    if (parseColor(paramText(values, key, fallback), rgb)) return;
+    if (parseColor(fallback, rgb)) return;
+    rgb[0] = rgb[1] = rgb[2] = 0;
+}
+
 }  // namespace effector

src/core/params.hpp

@@ -1,5 +1,6 @@
 #pragma once
 
+#include <cstdint>
 #include <map>
 #include <string>
 #include <variant>
@@ -10,7 +11,7 @@ namespace effector {
 // Declarative description of one effect knob. The GUI turns these into widgets
 // and the CLI turns them into flags, so neither knows about any effect.
 struct ParamSpec {
-    enum class Type { Int, Double, Bool, Choice };
+    enum class Type { Int, Double, Bool, Choice, Path, Text, Color };
 
     Type type = Type::Int;
     std::string key;
@@ -35,6 +36,18 @@ struct ParamSpec {
                               std::string help = {});
     static ParamSpec makeChoice(std::string key, std::string label, std::string defaultChoice,
                                 std::vector<std::string> choices, std::string help = {});
+    // A path to a file on disk, empty by default: a chooser in the interface, a
+    // flag taking any string on the command line, written relative to the
+    // project folder so that a folder still opens after it is moved.
+    static ParamSpec makePath(std::string key, std::string label, std::string help = {});
+    // Free-form text the user types: an entry in the interface, a flag taking
+    // any string on the command line.
+    static ParamSpec makeText(std::string key, std::string label, std::string defaultText,
+                              std::string help = {});
+    // A colour written "#rrggbb": a colour button in the interface, the same
+    // spelling as a flag value.
+    static ParamSpec makeColor(std::string key, std::string label, std::string defaultColor,
+                               std::string help = {});
 };
 
 using ParamValue = std::variant<long, double, bool, std::string>;
@@ -45,5 +58,17 @@ double paramDouble(const ParamValues& values, const std::string& key, double fal
 bool paramBool(const ParamValues& values, const std::string& key, bool fallback);
 std::string paramChoice(const ParamValues& values, const std::string& key,
                         const std::string& fallback);
+std::string paramText(const ParamValues& values, const std::string& key,
+                      const std::string& fallback);
+
+// Reads "#rgb" or "#rrggbb" (the leading hash optional) into three bytes.
+// Returns false and leaves `rgb` untouched for anything else, so a malformed
+// value falls back to the default rather than painting an arbitrary colour.
+bool parseColor(const std::string& text, std::uint8_t rgb[3]);
+std::string formatColor(const std::uint8_t rgb[3]);
+
+// The colour of a param, or `fallback` when the value does not parse.
+void paramColor(const ParamValues& values, const std::string& key, const std::string& fallback,
+                std::uint8_t rgb[3]);
 
 }  // namespace effector

src/core/project.cpp

@@ -0,0 +1,543 @@
+#include "core/project.hpp"
+
+#include <charconv>
+#include <cmath>
+#include <cstdint>
+#include <cstdio>
+#include <cstdlib>
+#include <filesystem>
+#include <fstream>
+#include <locale>
+#include <sstream>
+
+#include "core/canvas.hpp"
+#include "core/effect.hpp"
+#include "core/registry.hpp"
+
+namespace effector {
+namespace {
+
+// A project file is small and its shape is fixed, so it carries its own
+// reader rather than a JSON dependency: enough of the grammar to read back
+// what projectToJson writes, and to give a line number when a hand-edited
+// file is wrong.
+struct Json {
+    enum class Kind { Null, Bool, Number, String, Array, Object };
+
+    Kind kind = Kind::Null;
+    bool boolean = false;
+    double number = 0.0;
+    bool integral = true;  // written without a fraction or exponent
+    std::string text;      // the string, or the digits a number was written as
+    std::vector<Json> items;
+    std::vector<std::pair<std::string, Json>> members;
+
+    const Json* member(const std::string& key) const {
+        for (const auto& entry : members) {
+            if (entry.first == key) return &entry.second;
+        }
+        return nullptr;
+    }
+};
+
+class JsonParser {
+public:
+    explicit JsonParser(const std::string& text) : text_(text) {}
+
+    Json parse() {
+        skipSpace();
+        Json value = parseValue();
+        skipSpace();
+        if (pos_ != text_.size()) fail("trailing text after the end of the document");
+        return value;
+    }
+
+private:
+    [[noreturn]] void fail(const std::string& what) const {
+        int line = 1;
+        for (std::size_t i = 0; i < pos_ && i < text_.size(); ++i) {
+            if (text_[i] == '\n') ++line;
+        }
+        throw EffectError("line " + std::to_string(line) + ": " + what);
+    }
+
+    char peek() const { return pos_ < text_.size() ? text_[pos_] : '\0'; }
+
+    void skipSpace() {
+        while (pos_ < text_.size()) {
+            const char c = text_[pos_];
+            if (c == ' ' || c == '\t' || c == '\n' || c == '\r') {
+                ++pos_;
+            } else {
+                break;
+            }
+        }
+    }
+
+    void expect(char c) {
+        if (peek() != c) fail(std::string("expected '") + c + "'");
+        ++pos_;
+    }
+
+    bool literal(const char* word) {
+        const std::size_t length = std::char_traits<char>::length(word);
+        if (text_.compare(pos_, length, word) != 0) return false;
+        pos_ += length;
+        return true;
+    }
+
+    Json parseValue() {
+        switch (peek()) {
+            case '{': return parseObject();
+            case '[': return parseArray();
+            case '"': {
+                Json value;
+                value.kind = Json::Kind::String;
+                value.text = parseString();
+                return value;
+            }
+            default: break;
+        }
+        if (literal("true")) return boolean(true);
+        if (literal("false")) return boolean(false);
+        if (literal("null")) return Json();
+        return parseNumber();
+    }
+
+    static Json boolean(bool state) {
+        Json value;
+        value.kind = Json::Kind::Bool;
+        value.boolean = state;
+        return value;
+    }
+
+    Json parseObject() {
+        Json value;
+        value.kind = Json::Kind::Object;
+        expect('{');
+        skipSpace();
+        if (peek() == '}') {
+            ++pos_;
+            return value;
+        }
+        while (true) {
+            skipSpace();
+            std::string key = parseString();
+            skipSpace();
+            expect(':');
+            skipSpace();
+            value.members.emplace_back(std::move(key), parseValue());
+            skipSpace();
+            if (peek() == ',') {
+                ++pos_;
+                continue;
+            }
+            expect('}');
+            return value;
+        }
+    }
+
+    Json parseArray() {
+        Json value;
+        value.kind = Json::Kind::Array;
+        expect('[');
+        skipSpace();
+        if (peek() == ']') {
+            ++pos_;
+            return value;
+        }
+        while (true) {
+            skipSpace();
+            value.items.push_back(parseValue());
+            skipSpace();
+            if (peek() == ',') {
+                ++pos_;
+                continue;
+            }
+            expect(']');
+            return value;
+        }
+    }
+
+    std::string parseString() {
+        expect('"');
+        std::string out;
+        while (true) {
+            if (pos_ >= text_.size()) fail("unterminated string");
+            const char c = text_[pos_++];
+            if (c == '"') return out;
+            if (c != '\\') {
+                out.push_back(c);
+                continue;
+            }
+            if (pos_ >= text_.size()) fail("unterminated escape");
+            switch (text_[pos_++]) {
+                case '"': out.push_back('"'); break;
+                case '\\': out.push_back('\\'); break;
+                case '/': out.push_back('/'); break;
+                case 'b': out.push_back('\b'); break;
+                case 'f': out.push_back('\f'); break;
+                case 'n': out.push_back('\n'); break;
+                case 'r': out.push_back('\r'); break;
+                case 't': out.push_back('\t'); break;
+                case 'u': {
+                    // Only the basic plane, which is all a path or a choice
+                    // name can need; surrogate pairs are passed through as is.
+                    if (pos_ + 4 > text_.size()) fail("short \\u escape");
+                    const unsigned code = static_cast<unsigned>(
+                        std::strtoul(text_.substr(pos_, 4).c_str(), nullptr, 16));
+                    pos_ += 4;
+                    appendUtf8(out, code);
+                    break;
+                }
+                default: fail("unknown escape");
+            }
+        }
+    }
+
+    static void appendUtf8(std::string& out, unsigned code) {
+        if (code < 0x80) {
+            out.push_back(static_cast<char>(code));
+        } else if (code < 0x800) {
+            out.push_back(static_cast<char>(0xC0 | (code >> 6)));
+            out.push_back(static_cast<char>(0x80 | (code & 0x3F)));
+        } else {
+            out.push_back(static_cast<char>(0xE0 | (code >> 12)));
+            out.push_back(static_cast<char>(0x80 | ((code >> 6) & 0x3F)));
+            out.push_back(static_cast<char>(0x80 | (code & 0x3F)));
+        }
+    }
+
+    Json parseNumber() {
+        const std::size_t start = pos_;
+        if (peek() == '-' || peek() == '+') ++pos_;
+        bool integral = true;
+        while (pos_ < text_.size()) {
+            const char c = text_[pos_];
+            if (c >= '0' && c <= '9') {
+                ++pos_;
+            } else if (c == '.' || c == 'e' || c == 'E') {
+                integral = false;
+                ++pos_;
+            } else if (c == '-' || c == '+') {  // an exponent's sign
+                ++pos_;
+            } else {
+                break;
+            }
+        }
+        if (pos_ == start) fail("expected a value");
+
+        Json value;
+        value.kind = Json::Kind::Number;
+        value.integral = integral;
+        // from_chars rather than stod: a number in JSON always has a point for
+        // a decimal mark, whatever LC_NUMERIC says the user's decimal mark is.
+        const char* const first = text_.data() + start + (text_[start] == '+' ? 1 : 0);
+        const char* const last = text_.data() + pos_;
+        const auto parsed = std::from_chars(first, last, value.number);
+        if (parsed.ec != std::errc() || parsed.ptr != last) {
+            fail("'" + text_.substr(start, pos_ - start) + "' is not a number");
+        }
+        value.text.assign(first, last);  // kept whole for the seed, see below
+        return value;
+    }
+
+    const std::string& text_;
+    std::size_t pos_ = 0;
+};
+
+std::string quote(const std::string& text) {
+    std::string out = "\"";
+    for (const char c : text) {
+        switch (c) {
+            case '"': out += "\\\""; break;
+            case '\\': out += "\\\\"; break;
+            case '\n': out += "\\n"; break;
+            case '\r': out += "\\r"; break;
+            case '\t': out += "\\t"; break;
+            default:
+                if (static_cast<unsigned char>(c) < 0x20) {
+                    char buffer[7];
+                    std::snprintf(buffer, sizeof(buffer), "\\u%04x",
+                                  static_cast<unsigned>(static_cast<unsigned char>(c)));
+                    out += buffer;
+                } else {
+                    out.push_back(c);
+                }
+        }
+    }
+    return out + "\"";
+}
+
+std::string number(double value, int digits) {
+    std::ostringstream out;
+    out.imbue(std::locale::classic());
+    out.setf(std::ios::fixed);
+    out.precision(digits);
+    out << value;
+    std::string text = out.str();
+    // 5.00 reads better as 5.0 and 0.250 as 0.25.
+    while (text.size() > 1 && text.back() == '0' && text[text.size() - 2] != '.') text.pop_back();
+    return text;
+}
+
+// Relative when the image lives under the project folder, absolute otherwise:
+// a folder holding the project and its images stays movable.
+std::string relativeTo(const std::string& path, const std::string& directory) {
+    if (directory.empty()) return path;
+    std::error_code error;
+    const auto relative = std::filesystem::relative(path, directory, error);
+    if (error || relative.empty()) return path;
+    const auto text = relative.generic_string();
+    return text.rfind("..", 0) == 0 ? path : text;
+}
+
+// A canvas side is checked as a double, before it is narrowed: casting a
+// number outside the range of an int is undefined, and even where it wraps
+// quietly it turns a size the file never named into one that looks ordinary.
+int canvasSide(const Json& value, const char* what) {
+    if (value.kind != Json::Kind::Number || value.number != std::trunc(value.number)) {
+        throw EffectError(std::string("the canvas ") + what + " must be a whole number");
+    }
+    if (!(value.number >= kMinCanvasSide) || !(value.number <= kMaxCanvasSide)) {
+        throw EffectError(std::string("the canvas ") + what + " must be between " +
+                          std::to_string(kMinCanvasSide) + " and " +
+                          std::to_string(kMaxCanvasSide) + " pixels");
+    }
+    return static_cast<int>(value.number);
+}
+
+// Checked as a double before narrowing, exactly as a canvas side is.
+int canvasDpi(const Json& value) {
+    if (value.kind != Json::Kind::Number || value.number != std::trunc(value.number)) {
+        throw EffectError("the canvas dpi must be a whole number");
+    }
+    if (!(value.number >= kMinDpi) || !(value.number <= kMaxDpi)) {
+        throw EffectError("the canvas dpi must be between " + std::to_string(kMinDpi) + " and " +
+                          std::to_string(kMaxDpi));
+    }
+    return static_cast<int>(value.number);
+}
+
+std::string resolve(const std::string& path, const std::string& directory) {
+    std::filesystem::path file(path);
+    if (file.is_absolute() || directory.empty()) return path;
+    return (std::filesystem::path(directory) / file).lexically_normal().string();
+}
+
+}  // namespace
+
+std::string projectToJson(const Project& project, const std::string& directory) {
+    // JSON is written in the C locale whatever the user's is: a decimal comma
+    // or a digit grouping would make the file something this reader — or any
+    // other — cannot read back. gtkmm sets the global locale from the
+    // environment, so a stream left to its default is the user's locale.
+    std::ostringstream out;
+    out.imbue(std::locale::classic());
+    out << "{\n";
+    out << "  \"effector_project\": 1,\n";
+    out << "  \"canvas\": {\"width\": " << project.width << ", \"height\": " << project.height;
+    // The pixels are always written, so a project opens the same in a reader
+    // that knows nothing of formats; the format and its dpi come after them
+    // and only when there is one to name.
+    if (const auto* format = canvasFormatByKey(project.format)) {
+        out << ", \"format\": " << quote(format->key);
+        if (format->print) out << ", \"dpi\": " << project.dpi;
+    }
+    out << "},\n";
+    if (project.seed) out << "  \"seed\": " << *project.seed << ",\n";
+
+    out << "  \"sources\": [";
+    for (std::size_t i = 0; i < project.sources.size(); ++i) {
+        out << (i == 0 ? "\n" : ",\n") << "    " << quote(relativeTo(project.sources[i], directory));
+    }
+    out << (project.sources.empty() ? "" : "\n  ") << "],\n";
+
+    out << "  \"chain\": [";
+    for (std::size_t i = 0; i < project.stages.size(); ++i) {
+        const auto& stage = project.stages[i];
+        if (!stage.effect) continue;
+        out << (i == 0 ? "\n" : ",\n");
+        out << "    {\n      \"effect\": " << quote(stage.effect->name()) << ",\n";
+        out << "      \"params\": {";
+
+        const auto& specs = stage.effect->params();
+        bool first = true;
+        for (const auto& spec : specs) {
+            out << (first ? "\n" : ",\n") << "        " << quote(spec.key) << ": ";
+            first = false;
+            switch (spec.type) {
+                case ParamSpec::Type::Bool:
+                    out << (paramBool(stage.values, spec.key, spec.defaultValue != 0.0) ? "true"
+                                                                                       : "false");
+                    break;
+                case ParamSpec::Type::Int:
+                    out << paramInt(stage.values, spec.key, static_cast<long>(spec.defaultValue));
+                    break;
+                case ParamSpec::Type::Double:
+                    out << number(paramDouble(stage.values, spec.key, spec.defaultValue),
+                                  spec.digits);
+                    break;
+                case ParamSpec::Type::Choice:
+                case ParamSpec::Type::Text:
+                    out << quote(paramText(stage.values, spec.key, spec.defaultChoice));
+                    break;
+                case ParamSpec::Type::Color: {
+                    std::uint8_t rgb[3];
+                    paramColor(stage.values, spec.key, spec.defaultChoice, rgb);
+                    out << quote(formatColor(rgb));
+                    break;
+                }
+                case ParamSpec::Type::Path: {
+                    // Written relative to the project folder, exactly as the
+                    // sources are, so a folder still opens once it is moved.
+                    const auto path = paramText(stage.values, spec.key, "");
+                    out << quote(path.empty() ? path : relativeTo(path, directory));
+                    break;
+                }
+            }
+        }
+        out << (specs.empty() ? "}" : "\n      }") << "\n    }";
+    }
+    out << (project.stages.empty() ? "" : "\n  ") << "]\n";
+    out << "}\n";
+    return out.str();
+}
+
+Project projectFromJson(const std::string& text, const std::string& directory) {
+    JsonParser parser(text);
+    const Json root = parser.parse();
+    if (root.kind != Json::Kind::Object) throw EffectError("a project must be a JSON object");
+
+    Project project;
+
+    if (const auto* canvas = root.member("canvas")) {
+        if (const auto* width = canvas->member("width")) project.width = canvasSide(*width, "width");
+        if (const auto* height = canvas->member("height")) {
+            project.height = canvasSide(*height, "height");
+        }
+        if (const auto* dpi = canvas->member("dpi")) project.dpi = canvasDpi(*dpi);
+        // A format the build does not know is simply a custom canvas: the
+        // pixels were written too, so the project still renders as it was
+        // saved. Where the format is known it decides the pixels, so a
+        // hand-edited dpi takes effect and a preset stays true to its name.
+        if (const auto* name = canvas->member("format"); name != nullptr &&
+                                                         name->kind == Json::Kind::String) {
+            if (const auto* format = canvasFormatByKey(name->text)) {
+                project.format = format->key;
+                canvasFormatSize(*format, project.dpi, project.width, project.height);
+            }
+        }
+    }
+
+    if (const auto* seed = root.member("seed")) {
+        // Taken from the digits rather than from the double: a 64-bit seed has
+        // more bits than a double has mantissa, and a seed that comes back off
+        // by a few is a project that no longer renders its own picture.
+        std::uint64_t value = 0;
+        const char* const first = seed->text.data();
+        const char* const last = first + seed->text.size();
+        if (seed->kind == Json::Kind::Number && seed->integral &&
+            std::from_chars(first, last, value).ptr == last) {
+            project.seed = value;
+        } else if (seed->kind == Json::Kind::Number) {
+            throw EffectError("the seed must be a whole number that fits in 64 bits");
+        }
+    }
+
+    if (const auto* sources = root.member("sources")) {
+        for (const auto& item : sources->items) {
+            if (item.kind == Json::Kind::String) {
+                project.sources.push_back(resolve(item.text, directory));
+            }
+        }
+    }
+
+    const auto* chain = root.member("chain");
+    if (chain == nullptr || chain->kind != Json::Kind::Array || chain->items.empty()) {
+        throw EffectError("the project has no effect chain");
+    }
+    for (const auto& item : chain->items) {
+        const auto* name = item.member("effect");
+        if (name == nullptr || name->kind != Json::Kind::String) {
+            throw EffectError("a chain stage has no effect name");
+        }
+
+        ChainStage stage;
+        stage.effect = effectByName(name->text);  // throws on an unknown effect
+        stage.values = stage.effect->defaults();
+
+        // Driven by the specs, not by the file: a key the effect does not
+        // declare is dropped, and an old project simply keeps the defaults for
+        // params that did not exist when it was written.
+        if (const auto* params = item.member("params")) {
+            for (const auto& spec : stage.effect->params()) {
+                const auto* value = params->member(spec.key);
+                if (value == nullptr) continue;
+                switch (spec.type) {
+                    case ParamSpec::Type::Bool:
+                        if (value->kind == Json::Kind::Bool) stage.values[spec.key] = value->boolean;
+                        break;
+                    case ParamSpec::Type::Int:
+                        if (value->kind == Json::Kind::Number) {
+                            stage.values[spec.key] = static_cast<long>(std::llround(value->number));
+                        }
+                        break;
+                    case ParamSpec::Type::Double:
+                        if (value->kind == Json::Kind::Number) stage.values[spec.key] = value->number;
+                        break;
+                    case ParamSpec::Type::Path:
+                        if (value->kind == Json::Kind::String) {
+                            stage.values[spec.key] =
+                                value->text.empty() ? value->text : resolve(value->text, directory);
+                        }
+                        break;
+                    case ParamSpec::Type::Text:
+                        if (value->kind == Json::Kind::String) stage.values[spec.key] = value->text;
+                        break;
+                    case ParamSpec::Type::Color: {
+                        // A colour that does not parse keeps the default,
+                        // exactly as an unknown choice does.
+                        std::uint8_t rgb[3];
+                        if (value->kind == Json::Kind::String && parseColor(value->text, rgb)) {
+                            stage.values[spec.key] = formatColor(rgb);
+                        }
+                        break;
+                    }
+                    case ParamSpec::Type::Choice:
+                        for (const auto& choice : spec.choices) {
+                            if (value->kind == Json::Kind::String && value->text == choice) {
+                                stage.values[spec.key] = choice;
+                            }
+                        }
+                        break;
+                }
+            }
+        }
+        project.stages.push_back(std::move(stage));
+    }
+
+    return project;
+}
+
+void saveProject(const std::string& path, const Project& project) {
+    const auto directory = std::filesystem::path(path).parent_path().string();
+    std::ofstream file(path, std::ios::binary | std::ios::trunc);
+    if (!file) throw EffectError("cannot write " + path);
+    file << projectToJson(project, directory);
+    if (!file) throw EffectError("could not write " + path);
+}
+
+Project loadProject(const std::string& path) {
+    std::ifstream file(path, std::ios::binary);
+    if (!file) throw EffectError("cannot read " + path);
+    std::ostringstream buffer;
+    buffer << file.rdbuf();
+    try {
+        return projectFromJson(buffer.str(), std::filesystem::path(path).parent_path().string());
+    } catch (const EffectError& error) {
+        throw EffectError(std::filesystem::path(path).filename().string() + ": " + error.what());
+    }
+}
+
+}  // namespace effector

src/core/project.hpp

@@ -0,0 +1,47 @@
+#pragma once
+
+#include <cstdint>
+#include <optional>
+#include <string>
+#include <vector>
+
+#include "core/canvas.hpp"
+#include "core/chain.hpp"
+#include "core/params.hpp"
+
+namespace effector {
+
+// Everything needed to recreate a render: which images went in, the cascade
+// with its settings, the canvas and the seed. Deliberately no pixels — a
+// project is a small text file that points at the sources on disk.
+struct Project {
+    std::vector<std::string> sources;  // absolute paths
+    std::vector<ChainStage> stages;
+    int width = 1920;
+    int height = 1080;
+    // The named format the canvas came from, empty for a custom pixel size.
+    // The pixels above stay the truth for the render; this is what the
+    // interface puts back in its dropdown, and what a print format needs its
+    // resolution for.
+    std::string format;
+    int dpi = kDefaultDpi;
+    std::optional<std::uint64_t> seed;
+};
+
+// JSON, written for humans: two-space indent, one source path per line, and
+// the params of a stage spelled with the same keys the effect declares.
+// Source paths are written relative to `directory` when they live under it, so
+// a project folder can be moved or shared whole.
+std::string projectToJson(const Project& project, const std::string& directory = {});
+
+// Throws EffectError on malformed JSON, an unknown effect or a missing chain.
+// Relative source paths are resolved against `directory`; params the effect
+// does not declare are dropped, and missing ones keep the effect's default.
+Project projectFromJson(const std::string& text, const std::string& directory = {});
+
+// Both throw EffectError: the file is the user's, so failures are reported
+// rather than silently swallowed.
+void saveProject(const std::string& path, const Project& project);
+Project loadProject(const std::string& path);
+
+}  // namespace effector

src/core/registry.cpp

@@ -1,6 +1,12 @@
 #include "core/registry.hpp"
 
+#include "effects/blur.hpp"
+#include "effects/grain.hpp"
+#include "effects/monochrome.hpp"
+#include "effects/overlay.hpp"
 #include "effects/scanlines.hpp"
+#include "effects/text.hpp"
+#include "effects/tiles.hpp"
 
 namespace effector {
 
@@ -8,6 +14,12 @@ namespace effector {
 const std::vector<EffectPtr>& effects() {
     static const std::vector<EffectPtr> registry = {
         std::make_shared<ScanlinesEffect>(),
+        std::make_shared<BlurEffect>(),
+        std::make_shared<GrainEffect>(),
+        std::make_shared<MonochromeEffect>(),
+        std::make_shared<OverlayEffect>(),
+        std::make_shared<TextEffect>(),
+        std::make_shared<TilesEffect>(),
     };
     return registry;
 }

src/effects/blur.cpp

@@ -0,0 +1,112 @@
+#include "effects/blur.hpp"
+
+#include <algorithm>
+#include <cstring>
+#include <vector>
+
+#include "core/parallel.hpp"
+#include "effects/source_fill.hpp"
+
+namespace effector {
+
+const std::vector<ParamSpec>& BlurEffect::params() const {
+    static const std::vector<ParamSpec> specs = {
+        ParamSpec::makeInt("radius", "Radius", 8, 1, 256,
+                           "Half width in pixels of the blur window."),
+        ParamSpec::makeInt("passes", "Passes", 2, 1, 8,
+                           "How often the box blur is repeated; more passes look more gaussian."),
+        ParamSpec::makeChoice("direction", "Direction", "both", {"both", "horizontal", "vertical"},
+                              "Axis the blur is applied along."),
+        ParamSpec::makeBool("first_source", "Use the first image", false,
+                            "Always blur the first source instead of picking one at random."),
+    };
+    return specs;
+}
+
+namespace {
+
+// One separable box blur pass. `stride` is the distance in bytes between two
+// neighbouring samples along the blurred axis, `count` how many of them there
+// are; the window is clamped at both ends.
+void boxPass(const std::uint8_t* in, std::uint8_t* out, int count, std::size_t stride, int radius) {
+    const int window = 2 * radius + 1;
+    int sum[3] = {0, 0, 0};
+
+    for (int c = 0; c < 3; ++c) {
+        sum[c] = in[c] * (radius + 1);
+        for (int i = 1; i <= radius; ++i) {
+            sum[c] += in[static_cast<std::size_t>(std::min(i, count - 1)) * stride + c];
+        }
+    }
+
+    for (int i = 0; i < count; ++i) {
+        for (int c = 0; c < 3; ++c) {
+            out[static_cast<std::size_t>(i) * stride + c] =
+                static_cast<std::uint8_t>(sum[c] / window);
+        }
+        const int add = std::min(i + radius + 1, count - 1);
+        const int drop = std::max(i - radius, 0);
+        for (int c = 0; c < 3; ++c) {
+            sum[c] += in[static_cast<std::size_t>(add) * stride + c] -
+                      in[static_cast<std::size_t>(drop) * stride + c];
+        }
+    }
+}
+
+void blurHorizontal(const Image& src, Image& dst, int radius) {
+    parallelFor(static_cast<std::size_t>(src.height()), [&](std::size_t begin, std::size_t end) {
+        for (std::size_t y = begin; y < end; ++y) {
+            boxPass(src.row(static_cast<int>(y)), dst.row(static_cast<int>(y)), src.width(), 3,
+                    radius);
+        }
+    });
+}
+
+void blurVertical(const Image& src, Image& dst, int radius) {
+    const std::size_t stride = src.stride();
+    parallelFor(static_cast<std::size_t>(src.width()), [&](std::size_t begin, std::size_t end) {
+        for (std::size_t x = begin; x < end; ++x) {
+            const std::size_t offset = x * 3;
+            boxPass(src.data() + offset, dst.data() + offset, src.height(), stride, radius);
+        }
+    });
+}
+
+}  // namespace
+
+Image BlurEffect::render(const std::vector<Image>& sources, int width, int height, Rng& rng,
+                         const ParamValues& values) const {
+    if (width < 1 || height < 1) throw EffectError("Canvas size must be at least 1x1 pixels.");
+
+    const int radius = static_cast<int>(std::max(1L, paramInt(values, "radius", 8)));
+    const int passes = static_cast<int>(std::max(1L, paramInt(values, "passes", 2)));
+    const std::string direction = paramChoice(values, "direction", "both");
+    const bool firstSource = paramBool(values, "first_source", false);
+
+    Image canvas(width, height);
+    if (!sources.empty()) {
+        // Every random value is drawn here, before any threading, so that a
+        // seed always gives the same image.
+        const std::size_t index = firstSource ? 0 : rng.below(sources.size() - 1);
+        const Image& src = sources[index];
+        const int sx = static_cast<int>(rng.inRange(0, std::max(0, src.width() - width)));
+        const int sy = static_cast<int>(rng.inRange(0, std::max(0, src.height() - height)));
+        fillFromSource(canvas, src, sx, sy);
+    }
+
+    Image scratch(width, height);
+    for (int pass = 0; pass < passes; ++pass) {
+        if (direction != "vertical") {
+            blurHorizontal(canvas, scratch, std::min(radius, std::max(1, width - 1)));
+            std::swap(canvas, scratch);
+        }
+        if (direction != "horizontal") {
+            blurVertical(canvas, scratch, std::min(radius, std::max(1, height - 1)));
+            std::swap(canvas, scratch);
+        }
+    }
+
+    return canvas;
+}
+
+}  // namespace effector

src/effects/blur.hpp

@@ -0,0 +1,25 @@
+#pragma once
+
+#include "core/effect.hpp"
+
+namespace effector {
+
+// Softens its input with a separable box blur, repeated a few times to
+// approximate a gaussian. It needs no source images of its own: given none it
+// blurs an empty canvas, which is what makes it usable as a pure filter stage.
+class BlurEffect : public Effect {
+public:
+    std::string name() const override { return "blur"; }
+    std::string title() const override { return "Blur"; }
+    std::string description() const override {
+        return "Softens its input with a repeated box blur, horizontally, vertically or both; "
+               "with sources loaded it fills the canvas from a randomly chosen one first.";
+    }
+    const std::vector<ParamSpec>& params() const override;
+    bool needsSources() const override { return false; }
+
+    Image render(const std::vector<Image>& sources, int width, int height, Rng& rng,
+                 const ParamValues& values) const override;
+};
+
+}  // namespace effector

src/effects/grain.cpp

@@ -0,0 +1,90 @@
+#include "effects/grain.hpp"
+
+#include <algorithm>
+#include <cstdint>
+#include <vector>
+
+#include "core/parallel.hpp"
+#include "effects/source_fill.hpp"
+
+namespace effector {
+
+const std::vector<ParamSpec>& GrainEffect::params() const {
+    static const std::vector<ParamSpec> specs = {
+        ParamSpec::makeDouble("grain_amount", "Amount", 0.35, 0.0, 1.0,
+                              "How far a grain may push a pixel, as a fraction of full scale."),
+        ParamSpec::makeInt("grain_size", "Grain size", 1, 1, 32,
+                           "Width in pixels of one grain; larger sizes look coarser."),
+        ParamSpec::makeBool("grain_colour", "Coloured grain", false,
+                            "Draw the channels separately instead of one grey grain per pixel."),
+        ParamSpec::makeBool("grain_midtones", "Weight towards midtones", true,
+                            "Fade the grain out in the shadows and highlights, as film does."),
+    };
+    return specs;
+}
+
+namespace {
+
+// Response curve: 1 at mid grey, falling to 0 at black and at white, which is
+// roughly how grain behaves on film.
+inline int midtoneWeight(int value) { return 255 - std::abs(2 * value - 255); }
+
+}  // namespace
+
+Image GrainEffect::render(const std::vector<Image>& sources, int width, int height, Rng& rng,
+                          const ParamValues& values) const {
+    if (width < 1 || height < 1) throw EffectError("Canvas size must be at least 1x1 pixels.");
+
+    const double amount = std::clamp(paramDouble(values, "grain_amount", 0.35), 0.0, 1.0);
+    const int size = static_cast<int>(std::max(1L, paramInt(values, "grain_size", 1)));
+    const bool colour = paramBool(values, "grain_colour", false);
+    const bool midtones = paramBool(values, "grain_midtones", true);
+
+    Image canvas(width, height);
+    if (!sources.empty()) {
+        // Drawn before any threading, so that a seed always gives the same image.
+        const std::size_t index = rng.below(sources.size() - 1);
+        const Image& src = sources[index];
+        const int sx = static_cast<int>(rng.inRange(0, std::max(0, src.width() - width)));
+        const int sy = static_cast<int>(rng.inRange(0, std::max(0, src.height() - height)));
+        fillFromSource(canvas, src, sx, sy);
+    }
+
+    if (amount <= 0.0) return canvas;
+
+    // The whole grain field is drawn serially from the rng, exactly like the
+    // scanlines plan, and only the pixel arithmetic afterwards is parallel.
+    const int cellsX = (width + size - 1) / size;
+    const int cellsY = (height + size - 1) / size;
+    const int channels = colour ? 3 : 1;
+    std::vector<std::int8_t> field(static_cast<std::size_t>(cellsX) * cellsY * channels);
+    for (auto& cell : field) cell = static_cast<std::int8_t>(rng.inRange(-127, 127));
+
+    // 127 * 255 is the largest product of a grain and a midtone weight; scaling
+    // by `amount` there keeps the whole thing in integers.
+    const int scale = static_cast<int>(amount * 255.0 * 65536.0 / 127.0);
+
+    parallelFor(static_cast<std::size_t>(height), [&](std::size_t begin, std::size_t end) {
+        for (std::size_t y = begin; y < end; ++y) {
+            std::uint8_t* out = canvas.row(static_cast<int>(y));
+            const std::size_t rowCell =
+                static_cast<std::size_t>(static_cast<int>(y) / size) * cellsX;
+
+            for (int x = 0; x < width; ++x) {
+                const std::size_t cell = (rowCell + static_cast<std::size_t>(x / size)) * channels;
+                for (int c = 0; c < 3; ++c) {
+                    const int value = out[static_cast<std::size_t>(x) * 3 + c];
+                    const int grain = field[cell + (colour ? static_cast<std::size_t>(c) : 0)];
+                    const int weight = midtones ? midtoneWeight(value) : 255;
+                    const int delta = grain * weight / 255 * scale / 65536;
+                    out[static_cast<std::size_t>(x) * 3 + c] =
+                        static_cast<std::uint8_t>(std::clamp(value + delta, 0, 255));
+                }
+            }
+        }
+    });
+
+    return canvas;
+}
+
+}  // namespace effector

src/effects/grain.hpp

@@ -0,0 +1,27 @@
+#pragma once
+
+#include "core/effect.hpp"
+
+namespace effector {
+
+// Scatters film-like grain over its input. Like the blur it needs no source
+// images of its own, so it can lead a chain: given sources it fills the canvas
+// from one of them first, given none it grains an empty canvas — which with the
+// midtone weighting on stays black, exactly as an unexposed film would.
+class GrainEffect : public Effect {
+public:
+    std::string name() const override { return "grain"; }
+    std::string title() const override { return "Film grain"; }
+    std::string description() const override {
+        return "Scatters film-like grain over its input, optionally coloured and weighted towards "
+               "the midtones; with sources loaded it fills the canvas from a randomly chosen one "
+               "first.";
+    }
+    const std::vector<ParamSpec>& params() const override;
+    bool needsSources() const override { return false; }
+
+    Image render(const std::vector<Image>& sources, int width, int height, Rng& rng,
+                 const ParamValues& values) const override;
+};
+
+}  // namespace effector

src/effects/monochrome.cpp

@@ -0,0 +1,105 @@
+#include "effects/monochrome.hpp"
+
+#include <algorithm>
+#include <array>
+#include <cstdint>
+
+#include "core/parallel.hpp"
+#include "effects/source_fill.hpp"
+
+namespace effector {
+
+const std::vector<ParamSpec>& MonochromeEffect::params() const {
+    static const std::vector<ParamSpec> specs = {
+        ParamSpec::makeChoice("mono_weighting", "Weighting", "luminance",
+                              {"luminance", "average", "red", "green", "blue"},
+                              "How the three channels are mixed into one grey; the colour names "
+                              "weigh that channel most, like a filter over the lens."),
+        ParamSpec::makeDouble("mono_contrast", "Contrast", 1.0, 0.1, 4.0,
+                              "Pull of the greys away from mid grey; 1 leaves them as they are."),
+        ParamSpec::makeChoice("mono_tone", "Toner", "none",
+                              {"none", "sepia", "selenium", "cyanotype"},
+                              "Tints the finished greys, as a darkroom toner would."),
+        ParamSpec::makeDouble("mono_strength", "Strength", 1.0, 0.0, 1.0,
+                              "How far the result is mixed back over the colour input."),
+    };
+    return specs;
+}
+
+namespace {
+
+// Channel weights summing to 1024, so a grey is one integer multiply-add away.
+std::array<int, 3> weightsFor(const std::string& weighting) {
+    if (weighting == "average") return {341, 342, 341};
+    if (weighting == "red") return {819, 154, 51};
+    if (weighting == "green") return {154, 819, 51};
+    if (weighting == "blue") return {154, 51, 819};
+    return {218, 732, 74};  // Rec. 709 luminance.
+}
+
+// Tint multipliers per channel, as 256ths.
+std::array<int, 3> tintFor(const std::string& tone) {
+    if (tone == "sepia") return {282, 251, 205};
+    if (tone == "selenium") return {261, 251, 271};
+    if (tone == "cyanotype") return {205, 243, 307};
+    return {256, 256, 256};
+}
+
+}  // namespace
+
+Image MonochromeEffect::render(const std::vector<Image>& sources, int width, int height, Rng& rng,
+                               const ParamValues& values) const {
+    if (width < 1 || height < 1) throw EffectError("Canvas size must be at least 1x1 pixels.");
+
+    const auto weights = weightsFor(paramChoice(values, "mono_weighting", "luminance"));
+    const auto tint = tintFor(paramChoice(values, "mono_tone", "none"));
+    const double contrast = std::clamp(paramDouble(values, "mono_contrast", 1.0), 0.1, 4.0);
+    const double strength = std::clamp(paramDouble(values, "mono_strength", 1.0), 0.0, 1.0);
+
+    Image canvas(width, height);
+    if (!sources.empty()) {
+        // Drawn before any threading, so that a seed always gives the same image.
+        const std::size_t index = rng.below(sources.size() - 1);
+        const Image& src = sources[index];
+        const int sx = static_cast<int>(rng.inRange(0, std::max(0, src.width() - width)));
+        const int sy = static_cast<int>(rng.inRange(0, std::max(0, src.height() - height)));
+        fillFromSource(canvas, src, sx, sy);
+    }
+
+    if (strength <= 0.0) return canvas;
+
+    // Contrast and toner only ever see one grey, so the whole tail of the
+    // conversion is a table: grey in, the three toned channels out.
+    std::array<std::array<std::uint8_t, 256>, 3> table{};
+    for (int grey = 0; grey < 256; ++grey) {
+        const int pulled =
+            std::clamp(static_cast<int>(128.0 + (grey - 128) * contrast + 0.5), 0, 255);
+        for (int c = 0; c < 3; ++c) {
+            table[static_cast<std::size_t>(c)][static_cast<std::size_t>(grey)] =
+                static_cast<std::uint8_t>(
+                    std::clamp(pulled * tint[static_cast<std::size_t>(c)] / 256, 0, 255));
+        }
+    }
+
+    const int mix = static_cast<int>(strength * 256.0 + 0.5);
+
+    parallelFor(static_cast<std::size_t>(height), [&](std::size_t begin, std::size_t end) {
+        for (std::size_t y = begin; y < end; ++y) {
+            std::uint8_t* out = canvas.row(static_cast<int>(y));
+            for (int x = 0; x < width; ++x) {
+                std::uint8_t* px = out + static_cast<std::size_t>(x) * 3;
+                const int grey =
+                    (px[0] * weights[0] + px[1] * weights[1] + px[2] * weights[2]) / 1024;
+                for (int c = 0; c < 3; ++c) {
+                    const int toned = table[static_cast<std::size_t>(c)]
+                                           [static_cast<std::size_t>(std::clamp(grey, 0, 255))];
+                    px[c] = static_cast<std::uint8_t>(px[c] + (toned - px[c]) * mix / 256);
+                }
+            }
+        }
+    });
+
+    return canvas;
+}
+
+}  // namespace effector

src/effects/monochrome.hpp

@@ -0,0 +1,28 @@
+#pragma once
+
+#include "core/effect.hpp"
+
+namespace effector {
+
+// Converts its input to black and white, with the choice of channel weighting
+// a photographer would make behind the lens (a red filter darkening a sky, for
+// instance), an optional contrast pull and an optional toner. Like the blur it
+// needs no source images of its own, so it can lead a chain; given sources it
+// fills the canvas from one of them first.
+class MonochromeEffect : public Effect {
+   public:
+    std::string name() const override { return "monochrome"; }
+    std::string title() const override { return "Black and white"; }
+    std::string description() const override {
+        return "Converts its input to black and white with a choice of channel weighting, an "
+               "optional contrast pull and an optional toner; with sources loaded it fills the "
+               "canvas from a randomly chosen one first.";
+    }
+    const std::vector<ParamSpec>& params() const override;
+    bool needsSources() const override { return false; }
+
+    Image render(const std::vector<Image>& sources, int width, int height, Rng& rng,
+                 const ParamValues& values) const override;
+};
+
+}  // namespace effector

src/effects/overlay.cpp

@@ -0,0 +1,220 @@
+#include "effects/overlay.hpp"
+
+#include <algorithm>
+#include <cmath>
+#include <cstdint>
+
+#include "core/parallel.hpp"
+#include "effects/source_fill.hpp"
+
+namespace effector {
+
+const std::vector<ParamSpec>& OverlayEffect::params() const {
+    static const std::vector<ParamSpec> specs = {
+        ParamSpec::makePath("overlay_image", "Image",
+                            "The picture laid over the input: a logo, a texture, a frame."),
+        ParamSpec::makeChoice("overlay_fit", "Fitting", "fit",
+                              {"fit", "fill", "stretch", "original", "tile"},
+                              "fit keeps the proportions and shows the whole image; fill keeps "
+                              "them and covers the canvas, cropping what does not fit; stretch "
+                              "distorts it to the canvas exactly; original leaves it at its own "
+                              "size; tile repeats it at its own size over the whole canvas."),
+        ParamSpec::makeDouble("overlay_scale", "Scale", 1.0, 0.05, 8.0,
+                              "Resizes the fitted image (the tile, for tiling) by this factor.",
+                              0.05),
+        ParamSpec::makeBool("overlay_center", "Centre", true,
+                            "Places the image in the middle of the canvas rather than at its top "
+                            "left corner; the offset is counted from there either way."),
+        ParamSpec::makeInt("overlay_offset_x", "Offset X", 0, -20000, 20000,
+                           "Moves the image right by this many pixels (left when negative)."),
+        ParamSpec::makeInt("overlay_offset_y", "Offset Y", 0, -20000, 20000,
+                           "Moves the image down by this many pixels (up when negative)."),
+        ParamSpec::makeDouble("overlay_opacity", "Opacity", 1.0, 0.0, 1.0,
+                              "How strongly the image is mixed over the input; its own "
+                              "transparency is taken into account as well."),
+    };
+    return specs;
+}
+
+namespace {
+
+// Where the overlay lands on the canvas, in canvas pixels.
+struct Placement {
+    int x = 0;
+    int y = 0;
+    int width = 1;
+    int height = 1;
+};
+
+Placement placeOverlay(const std::string& fit, double scale, bool centre, int offsetX, int offsetY,
+                       int srcWidth, int srcHeight, int width, int height) {
+    double drawWidth = srcWidth;
+    double drawHeight = srcHeight;
+    if (fit == "stretch") {
+        drawWidth = width;
+        drawHeight = height;
+    } else if (fit == "fit" || fit == "fill") {
+        const double byWidth = static_cast<double>(width) / srcWidth;
+        const double byHeight = static_cast<double>(height) / srcHeight;
+        const double factor = fit == "fit" ? std::min(byWidth, byHeight) : std::max(byWidth, byHeight);
+        drawWidth = srcWidth * factor;
+        drawHeight = srcHeight * factor;
+    }
+    // "original" and "tile" keep the file's own size; the scale below applies
+    // to every mode, so a fitted image can still be nudged a little larger.
+
+    Placement placement;
+    placement.width = std::max(1, static_cast<int>(std::lround(drawWidth * scale)));
+    placement.height = std::max(1, static_cast<int>(std::lround(drawHeight * scale)));
+    placement.x = centre ? (width - placement.width) / 2 : 0;
+    placement.y = centre ? (height - placement.height) / 2 : 0;
+    placement.x += offsetX;
+    placement.y += offsetY;
+    return placement;
+}
+
+int wrapIndex(int value, int limit) {
+    const int wrapped = value % limit;
+    return wrapped < 0 ? wrapped + limit : wrapped;
+}
+
+// Bilinear sample of the overlay at a point in its own pixel coordinates,
+// clamped at the edges or wrapped around when the image is being tiled.
+void sampleOverlay(const Image& src, const std::uint8_t* alpha, double u, double v, bool wrap,
+                   int rgb[3], int& outAlpha) {
+    const int w = src.width();
+    const int h = src.height();
+    const double fu = std::floor(u);
+    const double fv = std::floor(v);
+    const int u0 = static_cast<int>(fu);
+    const int v0 = static_cast<int>(fv);
+    const double du = u - fu;
+    const double dv = v - fv;
+
+    int xs[2];
+    int ys[2];
+    for (int i = 0; i < 2; ++i) {
+        xs[i] = wrap ? wrapIndex(u0 + i, w) : std::clamp(u0 + i, 0, w - 1);
+        ys[i] = wrap ? wrapIndex(v0 + i, h) : std::clamp(v0 + i, 0, h - 1);
+    }
+
+    double accum[3] = {0.0, 0.0, 0.0};
+    double accumAlpha = 0.0;
+    for (int j = 0; j < 2; ++j) {
+        const double wy = j == 0 ? 1.0 - dv : dv;
+        const std::uint8_t* row = src.row(ys[j]);
+        const std::uint8_t* alphaRow = alpha + static_cast<std::size_t>(ys[j]) * w;
+        for (int i = 0; i < 2; ++i) {
+            const double weight = wy * (i == 0 ? 1.0 - du : du);
+            const std::uint8_t* px = row + static_cast<std::size_t>(xs[i]) * 3;
+            const double alphaWeight = weight * alphaRow[xs[i]];
+            // Colour weighted by alpha: a transparent pixel in a PNG is usually
+            // black, and averaging it in unweighted draws a dark fringe around
+            // everything the image cuts out.
+            for (int c = 0; c < 3; ++c) accum[c] += px[c] * alphaWeight;
+            accumAlpha += alphaWeight;
+        }
+    }
+    outAlpha = std::clamp(static_cast<int>(accumAlpha + 0.5), 0, 255);
+    for (int c = 0; c < 3; ++c) {
+        rgb[c] = accumAlpha <= 0.0 ? 0
+                                   : std::clamp(static_cast<int>(accum[c] / accumAlpha + 0.5), 0,
+                                                255);
+    }
+}
+
+}  // namespace
+
+void OverlayEffect::loadOverlay(const std::string& path, Image& image,
+                                std::vector<std::uint8_t>& alpha) const {
+    std::lock_guard<std::mutex> lock(cacheMutex_);
+    if (cachedPath_ != path || cached_.empty()) {
+        std::vector<std::uint8_t> loadedAlpha;
+        Image loaded;
+        try {
+            loaded = Image::load(path, &loadedAlpha);
+        } catch (const std::exception& error) {
+            throw EffectError("Cannot read the overlay image '" + path + "': " + error.what());
+        }
+        if (loaded.empty()) throw EffectError("The overlay image '" + path + "' is empty.");
+        cached_ = std::move(loaded);
+        cachedAlpha_ = std::move(loadedAlpha);
+        cachedPath_ = path;
+    }
+    image = cached_;
+    alpha = cachedAlpha_;
+}
+
+Image OverlayEffect::render(const std::vector<Image>& sources, int width, int height, Rng& rng,
+                            const ParamValues& values) const {
+    if (width < 1 || height < 1) throw EffectError("Canvas size must be at least 1x1 pixels.");
+
+    Image canvas(width, height);
+    if (!sources.empty()) {
+        // Drawn before any threading, so that a seed always gives the same image.
+        const std::size_t index = rng.below(sources.size() - 1);
+        const Image& src = sources[index];
+        const int sx = static_cast<int>(rng.inRange(0, std::max(0, src.width() - width)));
+        const int sy = static_cast<int>(rng.inRange(0, std::max(0, src.height() - height)));
+        fillFromSource(canvas, src, sx, sy);
+    }
+
+    const std::string path = paramText(values, "overlay_image", "");
+    const double opacity = std::clamp(paramDouble(values, "overlay_opacity", 1.0), 0.0, 1.0);
+    // No image chosen yet is the state the interface starts in, not an error:
+    // the stage simply passes its input through.
+    if (path.empty() || opacity <= 0.0) return canvas;
+
+    Image overlay;
+    std::vector<std::uint8_t> alpha;
+    loadOverlay(path, overlay, alpha);
+
+    const std::string fit = paramChoice(values, "overlay_fit", "fit");
+    const Placement place = placeOverlay(
+        fit, std::clamp(paramDouble(values, "overlay_scale", 1.0), 0.05, 8.0),
+        paramBool(values, "overlay_center", true),
+        static_cast<int>(paramInt(values, "overlay_offset_x", 0)),
+        static_cast<int>(paramInt(values, "overlay_offset_y", 0)), overlay.width(),
+        overlay.height(), width, height);
+
+    const bool tiling = fit == "tile";
+    // Source pixels per canvas pixel, the same both ways for every mode but
+    // stretch, where the two axes are scaled apart on purpose.
+    const double stepX = static_cast<double>(overlay.width()) / place.width;
+    const double stepY = static_cast<double>(overlay.height()) / place.height;
+
+    const int firstRow = tiling ? 0 : std::max(0, place.y);
+    const int lastRow = tiling ? height : std::min(height, place.y + place.height);
+    const int firstCol = tiling ? 0 : std::max(0, place.x);
+    const int lastCol = tiling ? width : std::min(width, place.x + place.width);
+    if (firstRow >= lastRow || firstCol >= lastCol) return canvas;  // entirely off the canvas
+
+    const int mix = static_cast<int>(opacity * 255.0 + 0.5);
+    const std::uint8_t* alphaData = alpha.data();
+
+    parallelFor(static_cast<std::size_t>(lastRow - firstRow), [&](std::size_t begin,
+                                                                 std::size_t end) {
+        int rgb[3];
+        int pixelAlpha = 255;
+        for (std::size_t i = begin; i < end; ++i) {
+            const int y = firstRow + static_cast<int>(i);
+            const double v = (y - place.y + 0.5) * stepY - 0.5;
+            std::uint8_t* out = canvas.row(y);
+            for (int x = firstCol; x < lastCol; ++x) {
+                const double u = (x - place.x + 0.5) * stepX - 0.5;
+                sampleOverlay(overlay, alphaData, u, v, tiling, rgb, pixelAlpha);
+
+                const int weight = pixelAlpha * mix / 255;
+                if (weight == 0) continue;
+                std::uint8_t* px = out + static_cast<std::size_t>(x) * 3;
+                for (int c = 0; c < 3; ++c) {
+                    px[c] = static_cast<std::uint8_t>(px[c] + (rgb[c] - px[c]) * weight / 255);
+                }
+            }
+        }
+    });
+
+    return canvas;
+}
+
+}  // namespace effector

src/effects/overlay.hpp

@@ -0,0 +1,45 @@
+#pragma once
+
+#include <cstdint>
+#include <mutex>
+#include <string>
+#include <vector>
+
+#include "core/effect.hpp"
+
+namespace effector {
+
+// Lays one picture from disk over its input: a logo, a texture, a frame. The
+// overlay is fitted to the canvas in the chosen way, placed from the top left
+// or from the centre, nudged by an offset and mixed in by its opacity. Like the
+// blur it needs no source images of its own, so it can lead a chain; given
+// sources it fills the canvas from a randomly chosen one first.
+class OverlayEffect : public Effect {
+public:
+    std::string name() const override { return "overlay"; }
+    std::string title() const override { return "Image overlay"; }
+    std::string description() const override {
+        return "Lays a single image from disk over its input, fitted to the canvas, centred or "
+               "not, nudged by an offset and mixed in by its opacity.";
+    }
+    const std::vector<ParamSpec>& params() const override;
+    bool needsSources() const override { return false; }
+
+    Image render(const std::vector<Image>& sources, int width, int height, Rng& rng,
+                 const ParamValues& values) const override;
+
+private:
+    // The same file is re-read on every param change otherwise, and a render is
+    // a keystroke away in the interface. Renders can overlap, so it is guarded.
+    // Returns the decoded file and its alpha, reading it only when the path
+    // differs from the one already held.
+    void loadOverlay(const std::string& path, Image& image,
+                     std::vector<std::uint8_t>& alpha) const;
+
+    mutable std::mutex cacheMutex_;
+    mutable std::string cachedPath_;
+    mutable Image cached_;
+    mutable std::vector<std::uint8_t> cachedAlpha_;
+};
+
+}  // namespace effector

src/effects/source_fill.hpp

@@ -0,0 +1,38 @@
+#pragma once
+
+#include <algorithm>
+#include <cstring>
+
+#include "core/image.hpp"
+#include "core/parallel.hpp"
+
+namespace effector {
+
+// Copies `src` into the canvas starting at (sx, sy), repeating it as needed so
+// that every pixel is covered even when the source is smaller than the canvas.
+// Shared by the effects that do not build an image of their own but still want
+// something to work on when they lead a chain.
+inline void fillFromSource(Image& canvas, const Image& src, int sx, int sy) {
+    const int width = canvas.width();
+    const std::size_t total = static_cast<std::size_t>(width) * 3;
+    const int copyWidth = std::min(width, src.width() - sx);
+    const std::size_t copyBytes = static_cast<std::size_t>(copyWidth) * 3;
+
+    parallelFor(static_cast<std::size_t>(canvas.height()), [&](std::size_t begin, std::size_t end) {
+        for (std::size_t y = begin; y < end; ++y) {
+            const int srcY = sy + (static_cast<int>(y) % std::max(1, src.height() - sy));
+            const std::uint8_t* in = src.row(srcY) + static_cast<std::size_t>(sx) * 3;
+            std::uint8_t* out = canvas.row(static_cast<int>(y));
+
+            std::memcpy(out, in, copyBytes);
+            std::size_t filled = copyBytes;
+            while (filled < total) {
+                const std::size_t chunk = std::min(filled, total - filled);
+                std::memcpy(out + filled, out, chunk);
+                filled += chunk;
+            }
+        }
+    });
+}
+
+}  // namespace effector

src/effects/text.cpp

@@ -0,0 +1,306 @@
+#include "effects/text.hpp"
+
+#include <pango/pangocairo.h>
+
+#include <algorithm>
+#include <cmath>
+#include <cstdint>
+
+#include "core/parallel.hpp"
+#include "effects/source_fill.hpp"
+
+namespace effector {
+
+const std::vector<ParamSpec>& TextEffect::params() const {
+    static const std::vector<ParamSpec> specs = {
+        ParamSpec::makeText("text", "Text", "Effector",
+                            "What to write; a line break starts a new line."),
+        ParamSpec::makeText("font_family", "Font", "Sans",
+                            "Font family, as it is named on this machine: Sans, Serif, Monospace, "
+                            "\"DejaVu Serif\", ..."),
+        ParamSpec::makeChoice("font_weight", "Weight", "normal",
+                              {"thin", "light", "normal", "medium", "semibold", "bold", "heavy"},
+                              "How bold the glyphs are; a family without the chosen weight is "
+                              "rendered in the nearest one it has."),
+        ParamSpec::makeBool("font_italic", "Italic", false, "Slants the glyphs."),
+        ParamSpec::makeDouble("font_size", "Size", 96.0, 4.0, 2000.0,
+                              "Height of the glyphs in pixels.", 1.0, 1),
+        ParamSpec::makeColor("text_color", "Colour", "#ffffff", "The colour of the glyphs."),
+        ParamSpec::makeChoice("text_align", "Alignment", "center", {"left", "center", "right"},
+                              "How several lines line up against each other."),
+        ParamSpec::makeInt("text_wrap", "Wrap width", 0, 0, 20000,
+                           "Breaks the text into lines no wider than this many pixels; 0 writes "
+                           "it as given."),
+        ParamSpec::makeChoice("text_box", "Box", "none", {"none", "colour", "image"},
+                              "What the text sits on: nothing, a filled rectangle, or a picture "
+                              "from disk cropped to the rectangle."),
+        ParamSpec::makeColor("box_color", "Box colour", "#000000",
+                            "The colour the box is filled with."),
+        ParamSpec::makePath("box_image", "Box picture",
+                            "The picture the box is filled with, scaled to cover it."),
+        ParamSpec::makeInt("box_padding", "Box padding", 24, 0, 2000,
+                           "How far the box reaches past the text on every side."),
+        ParamSpec::makeDouble("box_opacity", "Box opacity", 1.0, 0.0, 1.0,
+                              "How strongly the box is mixed over the input."),
+        ParamSpec::makeBool("text_center", "Centre", true,
+                            "Places the text in the middle of the canvas rather than at its top "
+                            "left corner; the offset is counted from there either way."),
+        ParamSpec::makeInt("text_offset_x", "Offset X", 0, -20000, 20000,
+                           "Moves the text right by this many pixels (left when negative)."),
+        ParamSpec::makeInt("text_offset_y", "Offset Y", 0, -20000, 20000,
+                           "Moves the text down by this many pixels (up when negative)."),
+        ParamSpec::makeDouble("text_opacity", "Text opacity", 1.0, 0.0, 1.0,
+                              "How strongly the glyphs are mixed over what is behind them."),
+    };
+    return specs;
+}
+
+namespace {
+
+// Small holders so that an EffectError thrown half way through still frees the
+// Cairo and Pango objects.
+struct SurfaceHandle {
+    cairo_surface_t* get = nullptr;
+    ~SurfaceHandle() {
+        if (get != nullptr) cairo_surface_destroy(get);
+    }
+};
+
+struct ContextHandle {
+    cairo_t* get = nullptr;
+    ~ContextHandle() {
+        if (get != nullptr) cairo_destroy(get);
+    }
+};
+
+struct LayoutHandle {
+    PangoLayout* get = nullptr;
+    ~LayoutHandle() {
+        if (get != nullptr) g_object_unref(get);
+    }
+};
+
+struct FontHandle {
+    PangoFontDescription* get = nullptr;
+    ~FontHandle() {
+        if (get != nullptr) pango_font_description_free(get);
+    }
+};
+
+PangoWeight weightFor(const std::string& name) {
+    if (name == "thin") return PANGO_WEIGHT_THIN;
+    if (name == "light") return PANGO_WEIGHT_LIGHT;
+    if (name == "medium") return PANGO_WEIGHT_MEDIUM;
+    if (name == "semibold") return PANGO_WEIGHT_SEMIBOLD;
+    if (name == "bold") return PANGO_WEIGHT_BOLD;
+    if (name == "heavy") return PANGO_WEIGHT_HEAVY;
+    return PANGO_WEIGHT_NORMAL;
+}
+
+PangoAlignment alignmentFor(const std::string& name) {
+    if (name == "left") return PANGO_ALIGN_LEFT;
+    if (name == "right") return PANGO_ALIGN_RIGHT;
+    return PANGO_ALIGN_CENTER;
+}
+
+// An RGB image plus its alpha as a Cairo ARGB32 surface, premultiplied, which
+// is the only layout Cairo paints from.
+cairo_surface_t* toSurface(const Image& image, const std::vector<std::uint8_t>& alpha) {
+    cairo_surface_t* surface =
+        cairo_image_surface_create(CAIRO_FORMAT_ARGB32, image.width(), image.height());
+    if (cairo_surface_status(surface) != CAIRO_STATUS_SUCCESS) {
+        cairo_surface_destroy(surface);
+        throw EffectError("Cannot hold the box picture in memory.");
+    }
+    const int stride = cairo_image_surface_get_stride(surface);
+    unsigned char* data = cairo_image_surface_get_data(surface);
+    const bool hasAlpha = alpha.size() == static_cast<std::size_t>(image.width()) *
+                                              static_cast<std::size_t>(image.height());
+
+    for (int y = 0; y < image.height(); ++y) {
+        const std::uint8_t* in = image.row(y);
+        auto* out = reinterpret_cast<std::uint32_t*>(data + static_cast<std::size_t>(y) * stride);
+        for (int x = 0; x < image.width(); ++x) {
+            const std::uint32_t a =
+                hasAlpha ? alpha[static_cast<std::size_t>(y) * image.width() + x] : 255;
+            const std::uint32_t r = in[x * 3 + 0] * a / 255;
+            const std::uint32_t g = in[x * 3 + 1] * a / 255;
+            const std::uint32_t b = in[x * 3 + 2] * a / 255;
+            out[x] = (a << 24) | (r << 16) | (g << 8) | b;
+        }
+    }
+    cairo_surface_mark_dirty(surface);
+    return surface;
+}
+
+void setColor(cairo_t* cr, const std::uint8_t rgb[3], double opacity) {
+    cairo_set_source_rgba(cr, rgb[0] / 255.0, rgb[1] / 255.0, rgb[2] / 255.0, opacity);
+}
+
+// Mixes the drawn layer, ARGB32 and premultiplied, over the canvas.
+void composite(Image& canvas, const cairo_surface_t* surface) {
+    const int stride = cairo_image_surface_get_stride(const_cast<cairo_surface_t*>(surface));
+    const unsigned char* data =
+        cairo_image_surface_get_data(const_cast<cairo_surface_t*>(surface));
+    const int width = canvas.width();
+
+    parallelFor(static_cast<std::size_t>(canvas.height()), [&](std::size_t begin, std::size_t end) {
+        for (std::size_t y = begin; y < end; ++y) {
+            const auto* in =
+                reinterpret_cast<const std::uint32_t*>(data + static_cast<std::size_t>(y) * stride);
+            std::uint8_t* out = canvas.row(static_cast<int>(y));
+            for (int x = 0; x < width; ++x) {
+                const std::uint32_t pixel = in[x];
+                const std::uint32_t a = pixel >> 24;
+                if (a == 0) continue;
+                // Premultiplied: the colour is already scaled by alpha, so the
+                // mix is an addition once the background is scaled down.
+                const std::uint32_t channel[3] = {(pixel >> 16) & 0xFF, (pixel >> 8) & 0xFF,
+                                                  pixel & 0xFF};
+                std::uint8_t* px = out + static_cast<std::size_t>(x) * 3;
+                for (int c = 0; c < 3; ++c) {
+                    const std::uint32_t value = channel[c] + px[c] * (255 - a) / 255;
+                    px[c] = static_cast<std::uint8_t>(std::min<std::uint32_t>(value, 255));
+                }
+            }
+        }
+    });
+}
+
+}  // namespace
+
+void TextEffect::loadBoxImage(const std::string& path, Image& image,
+                              std::vector<std::uint8_t>& alpha) const {
+    std::lock_guard<std::mutex> lock(cacheMutex_);
+    if (cachedPath_ != path || cached_.empty()) {
+        std::vector<std::uint8_t> loadedAlpha;
+        Image loaded;
+        try {
+            loaded = Image::load(path, &loadedAlpha);
+        } catch (const std::exception& error) {
+            throw EffectError("Cannot read the box picture '" + path + "': " + error.what());
+        }
+        if (loaded.empty()) throw EffectError("The box picture '" + path + "' is empty.");
+        cached_ = std::move(loaded);
+        cachedAlpha_ = std::move(loadedAlpha);
+        cachedPath_ = path;
+    }
+    image = cached_;
+    alpha = cachedAlpha_;
+}
+
+Image TextEffect::render(const std::vector<Image>& sources, int width, int height, Rng& rng,
+                         const ParamValues& values) const {
+    if (width < 1 || height < 1) throw EffectError("Canvas size must be at least 1x1 pixels.");
+
+    Image canvas(width, height);
+    if (!sources.empty()) {
+        // Drawn before any threading, so that a seed always gives the same image.
+        const std::size_t index = rng.below(sources.size() - 1);
+        const Image& src = sources[index];
+        const int sx = static_cast<int>(rng.inRange(0, std::max(0, src.width() - width)));
+        const int sy = static_cast<int>(rng.inRange(0, std::max(0, src.height() - height)));
+        fillFromSource(canvas, src, sx, sy);
+    }
+
+    const std::string text = paramText(values, "text", "");
+    const double textOpacity = std::clamp(paramDouble(values, "text_opacity", 1.0), 0.0, 1.0);
+    const std::string boxKind = paramChoice(values, "text_box", "none");
+    const double boxOpacity = std::clamp(paramDouble(values, "box_opacity", 1.0), 0.0, 1.0);
+    const bool drawsText = !text.empty() && textOpacity > 0.0;
+    const bool drawsBox = boxKind != "none" && boxOpacity > 0.0;
+    // No text typed yet is the state the interface starts in, not an error: the
+    // stage simply passes its input through.
+    if (!drawsText && !drawsBox) return canvas;
+
+    SurfaceHandle surface;
+    surface.get = cairo_image_surface_create(CAIRO_FORMAT_ARGB32, width, height);
+    if (cairo_surface_status(surface.get) != CAIRO_STATUS_SUCCESS) {
+        throw EffectError("Cannot hold the text layer in memory.");
+    }
+    ContextHandle cr;
+    cr.get = cairo_create(surface.get);
+
+    LayoutHandle layout;
+    layout.get = pango_cairo_create_layout(cr.get);
+    pango_layout_set_text(layout.get, text.c_str(), -1);
+    pango_layout_set_alignment(layout.get, alignmentFor(paramChoice(values, "text_align", "center")));
+    const int wrap = static_cast<int>(paramInt(values, "text_wrap", 0));
+    if (wrap > 0) {
+        pango_layout_set_width(layout.get, wrap * PANGO_SCALE);
+        pango_layout_set_wrap(layout.get, PANGO_WRAP_WORD_CHAR);
+    }
+
+    FontHandle font;
+    font.get = pango_font_description_new();
+    const std::string family = paramText(values, "font_family", "Sans");
+    pango_font_description_set_family(font.get, family.empty() ? "Sans" : family.c_str());
+    pango_font_description_set_weight(font.get, weightFor(paramChoice(values, "font_weight", "normal")));
+    pango_font_description_set_style(
+        font.get, paramBool(values, "font_italic", false) ? PANGO_STYLE_ITALIC : PANGO_STYLE_NORMAL);
+    // Absolute size: the param is in pixels, not in points at some unstated dpi.
+    pango_font_description_set_absolute_size(
+        font.get, std::clamp(paramDouble(values, "font_size", 96.0), 4.0, 2000.0) * PANGO_SCALE);
+    pango_layout_set_font_description(layout.get, font.get);
+
+    PangoRectangle logical;
+    pango_layout_get_pixel_extents(layout.get, nullptr, &logical);
+
+    const int padding = static_cast<int>(paramInt(values, "box_padding", 24));
+    const int boxWidth = std::max(1, logical.width + 2 * padding);
+    const int boxHeight = std::max(1, logical.height + 2 * padding);
+    const bool centre = paramBool(values, "text_center", true);
+    const double boxX = (centre ? (width - boxWidth) / 2.0 : 0.0) +
+                        static_cast<double>(paramInt(values, "text_offset_x", 0));
+    const double boxY = (centre ? (height - boxHeight) / 2.0 : 0.0) +
+                        static_cast<double>(paramInt(values, "text_offset_y", 0));
+
+    if (drawsBox) {
+        cairo_save(cr.get);
+        cairo_rectangle(cr.get, boxX, boxY, boxWidth, boxHeight);
+        cairo_clip(cr.get);
+        if (boxKind == "image") {
+            const std::string path = paramText(values, "box_image", "");
+            if (!path.empty()) {
+                Image picture;
+                std::vector<std::uint8_t> alpha;
+                loadBoxImage(path, picture, alpha);
+                SurfaceHandle fill;
+                fill.get = toSurface(picture, alpha);
+                // Scaled to cover the box, keeping its proportions, exactly as
+                // the overlay's "fill" does; what does not fit is clipped away.
+                const double factor = std::max(static_cast<double>(boxWidth) / picture.width(),
+                                               static_cast<double>(boxHeight) / picture.height());
+                cairo_translate(cr.get, boxX + (boxWidth - picture.width() * factor) / 2.0,
+                                boxY + (boxHeight - picture.height() * factor) / 2.0);
+                cairo_scale(cr.get, factor, factor);
+                cairo_set_source_surface(cr.get, fill.get, 0.0, 0.0);
+                cairo_pattern_set_filter(cairo_get_source(cr.get), CAIRO_FILTER_BILINEAR);
+                cairo_paint_with_alpha(cr.get, boxOpacity);
+            }
+        } else {
+            std::uint8_t rgb[3];
+            paramColor(values, "box_color", "#000000", rgb);
+            setColor(cr.get, rgb, boxOpacity);
+            cairo_paint(cr.get);
+        }
+        cairo_restore(cr.get);
+    }
+
+    if (drawsText) {
+        std::uint8_t rgb[3];
+        paramColor(values, "text_color", "#ffffff", rgb);
+        setColor(cr.get, rgb, textOpacity);
+        // logical.x/y are where the layout actually starts, which a wrapped or
+        // right-aligned layout shifts; subtracting them puts the glyphs inside
+        // the padding on every side.
+        cairo_move_to(cr.get, boxX + padding - logical.x, boxY + padding - logical.y);
+        pango_cairo_show_layout(cr.get, layout.get);
+    }
+
+    cairo_surface_flush(surface.get);
+    composite(canvas, surface.get);
+    return canvas;
+}
+
+}  // namespace effector

src/effects/text.hpp

@@ -0,0 +1,49 @@
+#pragma once
+
+#include <cstdint>
+#include <mutex>
+#include <string>
+#include <vector>
+
+#include "core/effect.hpp"
+
+namespace effector {
+
+// Writes a line (or several) over its input: a title, a caption, a watermark.
+// The family and the weight are chosen freely, and the text can sit on a box
+// filled with a colour or with a picture from disk. Like the blur and the
+// overlay it needs no source images of its own, so it can lead a chain; given
+// sources it fills the canvas from a randomly chosen one first.
+//
+// The glyphs come from Pango, so the exact pixels depend on the fonts the
+// machine has installed: a seed reproduces everything this effect draws, but a
+// machine without the chosen family renders the substitute its fontconfig
+// picks.
+class TextEffect : public Effect {
+public:
+    std::string name() const override { return "text"; }
+    std::string title() const override { return "Text"; }
+    std::string description() const override {
+        return "Writes text over its input in a chosen family, weight and size, optionally on a "
+               "box filled with a colour or with a picture.";
+    }
+    const std::vector<ParamSpec>& params() const override;
+    bool needsSources() const override { return false; }
+
+    Image render(const std::vector<Image>& sources, int width, int height, Rng& rng,
+                 const ParamValues& values) const override;
+
+private:
+    // The box picture is re-read on every param change otherwise, and a render
+    // is a keystroke away in the interface. Renders can overlap, so it is
+    // guarded.
+    void loadBoxImage(const std::string& path, Image& image,
+                      std::vector<std::uint8_t>& alpha) const;
+
+    mutable std::mutex cacheMutex_;
+    mutable std::string cachedPath_;
+    mutable Image cached_;
+    mutable std::vector<std::uint8_t> cachedAlpha_;
+};
+
+}  // namespace effector

src/effects/tiles.cpp

@@ -0,0 +1,180 @@
+#include "effects/tiles.hpp"
+
+#include <algorithm>
+#include <cstring>
+#include <vector>
+
+#include "core/parallel.hpp"
+#include "effects/source_fill.hpp"
+
+namespace effector {
+
+const std::vector<ParamSpec>& TilesEffect::params() const {
+    static const std::vector<ParamSpec> specs = {
+        ParamSpec::makeChoice("tile_layout", "Layout", "grid", {"grid", "brick", "scatter"},
+                              "grid lines the tiles up in rows and columns; brick packs every row "
+                              "on its own so the seams do not meet; scatter drops loose tiles over "
+                              "a filled background, overlapping."),
+        ParamSpec::makeInt("tile_width", "Tile width", 96, 1, 2048,
+                           "Width in pixels of each cut-out tile."),
+        ParamSpec::makeInt("tile_height", "Tile height", 96, 1, 2048,
+                           "Height in pixels of each cut-out tile."),
+        ParamSpec::makeInt("tile_jitter", "Size jitter", 0, 0, 512,
+                           "Random +/- variation added to both tile sizes (0 = uniform)."),
+        ParamSpec::makeInt("tile_count", "Tile count", 64, 1, 4096,
+                           "How many tiles are dropped on the canvas; scattered layout only."),
+        ParamSpec::makeBool("tile_keep_order", "Cycle images in order", false,
+                            "Take tiles from the images in order instead of picking randomly."),
+    };
+    return specs;
+}
+
+namespace {
+
+// One cut-out: `w` by `h` pixels taken from source `sourceIndex` at (sx, sy)
+// and laid down at (dstX, dstY).
+struct Tile {
+    int dstX;
+    int dstY;
+    int w;
+    int h;
+    int sourceIndex;
+    int sx;
+    int sy;
+};
+
+// Splits `span` into consecutive pieces of `base` pixels give or take `jitter`,
+// the last one clipped so that the pieces cover the span exactly.
+std::vector<int> planSpans(Rng& rng, int base, int jitter, int span) {
+    std::vector<int> spans;
+    spans.reserve(static_cast<std::size_t>(span / base) + 8);
+    int used = 0;
+    while (used < span) {
+        int size = base;
+        if (jitter > 0) size += static_cast<int>(rng.inRange(-jitter, jitter));
+        size = std::max(1, std::min(size, span - used));
+        spans.push_back(size);
+        used += size;
+    }
+    return spans;
+}
+
+void paintTile(Image& canvas, const Image& src, const Tile& tile) {
+    const int copyWidth = std::min(tile.w, src.width() - tile.sx);
+    const int copyHeight = std::min(tile.h, src.height() - tile.sy);
+    const std::size_t copyBytes = static_cast<std::size_t>(copyWidth) * 3;
+    const std::size_t total = static_cast<std::size_t>(tile.w) * 3;
+    const std::size_t offset = static_cast<std::size_t>(tile.dstX) * 3;
+
+    for (int line = 0; line < copyHeight; ++line) {
+        const std::uint8_t* in = src.row(tile.sy + line) + static_cast<std::size_t>(tile.sx) * 3;
+        std::uint8_t* out = canvas.row(tile.dstY + line) + offset;
+
+        std::memcpy(out, in, copyBytes);
+        // Repeat the cut sideways when the source is narrower than the tile,
+        // doubling the filled span on each pass.
+        std::size_t filled = copyBytes;
+        while (filled < total) {
+            const std::size_t chunk = std::min(filled, total - filled);
+            std::memcpy(out + filled, out, chunk);
+            filled += chunk;
+        }
+    }
+    // Same again downwards, out of the lines already written.
+    for (int line = copyHeight; line < tile.h; ++line) {
+        std::memcpy(canvas.row(tile.dstY + line) + offset,
+                    canvas.row(tile.dstY + (line % copyHeight)) + offset, total);
+    }
+}
+
+}  // namespace
+
+Image TilesEffect::render(const std::vector<Image>& sources, int width, int height, Rng& rng,
+                          const ParamValues& values) const {
+    if (sources.empty()) throw EffectError("No source images.");
+    if (width < 1 || height < 1) throw EffectError("Canvas size must be at least 1x1 pixels.");
+
+    const std::string layout = paramChoice(values, "tile_layout", "grid");
+    const int tileWidth = static_cast<int>(std::max(1L, paramInt(values, "tile_width", 96)));
+    const int tileHeight = static_cast<int>(std::max(1L, paramInt(values, "tile_height", 96)));
+    const int jitter = static_cast<int>(std::max(0L, paramInt(values, "tile_jitter", 0)));
+    const long count = std::max(1L, paramInt(values, "tile_count", 64));
+    const bool keepOrder = paramBool(values, "tile_keep_order", false);
+
+    Image canvas(width, height);
+
+    // The whole plan is drawn sequentially so that a seed always gives the same
+    // layout; only the pixel copying afterwards is parallelised.
+    std::size_t index = 0;
+    auto cut = [&](int dstX, int dstY, int w, int h) {
+        const int sourceIndex = keepOrder ? static_cast<int>(index++ % sources.size())
+                                          : static_cast<int>(rng.below(sources.size() - 1));
+        const Image& src = sources[static_cast<std::size_t>(sourceIndex)];
+        const int sx = static_cast<int>(rng.inRange(0, std::max(0, src.width() - w)));
+        const int sy = static_cast<int>(rng.inRange(0, std::max(0, src.height() - h)));
+        return Tile{dstX, dstY, w, h, sourceIndex, sx, sy};
+    };
+
+    std::vector<Tile> plan;
+    if (layout == "scatter") {
+        // Loose tiles leave gaps, so the canvas starts as one of the images
+        // rather than as black.
+        const std::size_t backdrop = rng.below(sources.size() - 1);
+        const Image& src = sources[backdrop];
+        const int sx = static_cast<int>(rng.inRange(0, std::max(0, src.width() - width)));
+        const int sy = static_cast<int>(rng.inRange(0, std::max(0, src.height() - height)));
+        fillFromSource(canvas, src, sx, sy);
+
+        plan.reserve(static_cast<std::size_t>(count));
+        for (long i = 0; i < count; ++i) {
+            int w = tileWidth;
+            int h = tileHeight;
+            if (jitter > 0) {
+                w += static_cast<int>(rng.inRange(-jitter, jitter));
+                h += static_cast<int>(rng.inRange(-jitter, jitter));
+            }
+            const int dstX = static_cast<int>(rng.inRange(0, width - 1));
+            const int dstY = static_cast<int>(rng.inRange(0, height - 1));
+            w = std::max(1, std::min(w, width - dstX));
+            h = std::max(1, std::min(h, height - dstY));
+            plan.push_back(cut(dstX, dstY, w, h));
+        }
+    } else {
+        const std::vector<int> rows = planSpans(rng, tileHeight, jitter, height);
+        // In a grid every row is cut at the same columns; in the brick layout
+        // each row is packed on its own, so the vertical seams do not line up.
+        std::vector<int> columns;
+        if (layout != "brick") columns = planSpans(rng, tileWidth, jitter, width);
+
+        plan.reserve(rows.size() * (columns.empty() ? 8 : columns.size()));
+        int y = 0;
+        for (const int rowHeight : rows) {
+            if (layout == "brick") columns = planSpans(rng, tileWidth, jitter, width);
+            int x = 0;
+            for (const int cellWidth : columns) {
+                plan.push_back(cut(x, y, cellWidth, rowHeight));
+                x += cellWidth;
+            }
+            y += rowHeight;
+        }
+    }
+
+    if (layout == "scatter") {
+        // Scattered tiles overlap, so they are painted in plan order on one
+        // thread: letting threads race for the same pixels would make the
+        // output depend on their scheduling.
+        for (const Tile& tile : plan) {
+            paintTile(canvas, sources[static_cast<std::size_t>(tile.sourceIndex)], tile);
+        }
+    } else {
+        parallelFor(plan.size(), [&](std::size_t begin, std::size_t end) {
+            for (std::size_t i = begin; i < end; ++i) {
+                paintTile(canvas, sources[static_cast<std::size_t>(plan[i].sourceIndex)], plan[i]);
+            }
+        });
+    }
+
+    return canvas;
+}
+
+}  // namespace effector

src/effects/tiles.hpp

@@ -0,0 +1,23 @@
+#pragma once
+
+#include "core/effect.hpp"
+
+namespace effector {
+
+// Covers the canvas with rectangular tiles cut at random positions from the
+// source images: the two-dimensional counterpart of the scanlines effect.
+class TilesEffect : public Effect {
+public:
+    std::string name() const override { return "tiles"; }
+    std::string title() const override { return "Tiles"; }
+    std::string description() const override {
+        return "Cuts rectangular tiles at random positions from the source images and lays "
+               "them over the canvas, in a grid, in offset rows or scattered about.";
+    }
+    const std::vector<ParamSpec>& params() const override;
+
+    Image render(const std::vector<Image>& sources, int width, int height, Rng& rng,
+                 const ParamValues& values) const override;
+};
+
+}  // namespace effector

src/gui/window.hpp

@@ -12,8 +12,10 @@
 #include <vector>
 
 #include "cli/options.hpp"
+#include "core/chain.hpp"
 #include "core/effect.hpp"
 #include "core/image.hpp"
+#include "core/project.hpp"
 
 namespace effector {
 
@@ -28,8 +30,12 @@ private:
     Gtk::Widget& buildPreview();
     void buildParamWidgets();
     ParamValues paramValues() const;
+    void storeParams();     // widgets -> the selected stage's values
     void applyInitial(const Options& initial);
 
+    bool canRender() const;     // a chain, and sources if the chain needs them
+    std::string missingText() const;  // what is still missing before a render
+    void showPlaceholder();     // drops the preview and says what is missing
     void requestRender();   // coalesced: restarts the short debounce timer
     void startRender();     // starts a worker, or marks the result stale
     void onAddImages();
@@ -42,10 +48,45 @@ private:
     void closeLoadWindow();
     void cancelLoad();
     void appendSourceRow(const std::string& path);
+    void updateSourcesLabel();  // the collapsed expander shows the count
+
+    Gtk::Widget& buildCanvasSection();
+    // "" while "Custom" is picked, otherwise the format's key.
+    std::string canvasFormatKey() const;
+    void setCanvasFormat(const std::string& key, int dpi);  // dropdown and dpi
+    // A format owns the pixel sizes, so the spins follow it and are greyed
+    // out; "Custom" hands them back to the user untouched.
+    void syncCanvasWidgets();
     void removeSource(Gtk::ListBoxRow* row);
 
-    void onEffectChanged();
+    Gtk::Widget& buildChainSection();
+    Gtk::Widget& buildAddMenu();      // the menu of effects that can be appended
+    void appendStage(const EffectPtr& effect);
+    void removeStage(int index);
+    void moveStage(int index, int delta);
+    void rebuildChainRows();
+    void selectStage(int index);
+    void onStageSelected();
+    Gtk::Widget& buildMainMenu();     // the header menu: new, open, save project
+    Project currentProject() const;
+    void applyProject(const Project& project, const std::string& path);
+    void clearSources();
+    void onNewProject();
+    void onOpenProject();
+    void onProjectChosen(const Glib::RefPtr<Gio::AsyncResult>& result,
+                         const Glib::RefPtr<Gtk::FileDialog>& dialog);
+    void onSaveProject();       // falls back to Save As while the project is untitled
+    void onSaveProjectAs();
+    void onSaveProjectChosen(const Glib::RefPtr<Gio::AsyncResult>& result,
+                             const Glib::RefPtr<Gtk::FileDialog>& dialog);
+    void writeProject(const std::string& path);
+    void updateTitle();
+
     void onParamChanged();
+    // The chooser behind a Path param. The chosen file goes straight into the
+    // stage's values rather than into the widget, which may well have been
+    // rebuilt by the time the dialog comes back.
+    void onChoosePath(int stage, const std::string& key, const std::string& label);
     void onSeedEntered();
     void onRecreate();
     void onSave();
@@ -66,9 +107,14 @@ private:
     // Model
     std::shared_ptr<const SourceList> sources_ = std::make_shared<SourceList>();
     std::vector<EffectPtr> effects_;
-    EffectPtr effect_;
+    // The cascade: stage 0 works on the sources, every later stage on the
+    // output of the one before it.
+    std::vector<ChainStage> stages_;
+    int selectedStage_ = 0;
+    bool suspendStageSelect_ = false;  // guards the rebuild of the chain rows
     Image result_;
     std::uint64_t seed_ = 0;
+    std::string projectPath_;  // empty until the project has been saved once
     bool suspendRender_ = true;
 
     // Source loading. Decoding a batch of large photographs takes seconds, so
@@ -85,6 +131,7 @@ private:
     std::vector<LoadFailure> loadFailures_;
     std::vector<std::string> loadQueue_;  // chosen while another batch was loading
     bool loading_ = false;
+    bool dropLoadedImages_ = false;  // the batch in flight belongs to a replaced project
     sigc::connection loadWindowDelay_;
 
     // Render job
@@ -105,12 +152,22 @@ private:
     std::unique_ptr<Gtk::Window> loadWindow_;
     Gtk::ProgressBar* loadBar_ = nullptr;
     Gtk::Label* loadFileLabel_ = nullptr;
+    Gtk::Expander sourcesExpander_;
     Gtk::ListBox sourceList_;
+    Gtk::DropDown formatDrop_;
     Gtk::SpinButton widthSpin_;
     Gtk::SpinButton heightSpin_;
+    Gtk::SpinButton dpiSpin_;
+    Gtk::Label* dpiLabel_ = nullptr;  // hidden along with the dpi spin
+    // Set while a format fills the spins, so their own handler does not queue
+    // a second render on top of the one the format change asks for.
+    bool suspendCanvasSync_ = false;
     Gtk::Grid paramGrid_;
+    Gtk::Label paramHeading_;
     Gtk::Entry seedEntry_;
-    Gtk::DropDown effectDropdown_;
+    Gtk::ListBox chainList_;
+    Gtk::MenuButton addEffectButton_;
+    Gtk::MenuButton menuButton_;
     Gtk::Button recreateButton_{"Recreate"};
     Gtk::Button saveButton_{"Save As…"};
     Gtk::Picture picture_;

src/main.cpp

@@ -8,6 +8,7 @@
 #include <mutex>
 
 #include "cli/options.hpp"
+#include "core/chain.hpp"
 #include "core/effect.hpp"
 #include "core/image.hpp"
 #include "core/registry.hpp"
@@ -17,7 +18,7 @@
 namespace {
 
 int renderHeadless(const effector::Options& options) {
-    const auto effect = effector::effectByName(options.effect);
+    const auto stages = effector::buildChain(options);
 
     std::vector<effector::LoadFailure> failures;
 
@@ -39,7 +40,7 @@ int renderHeadless(const effector::Options& options) {
     for (const auto& failure : failures) {
         std::cerr << "effector: skipping " << failure.path << ": " << failure.message << "\n";
     }
-    if (sources.empty()) {
+    if (sources.empty() && effector::chainNeedsSources(stages)) {
         std::cerr << "effector: no usable source images\n";
         return 2;
     }
@@ -47,17 +48,15 @@ int renderHeadless(const effector::Options& options) {
     const std::uint64_t seed = options.seed.value_or(effector::Rng::randomSeed());
     effector::Rng rng(seed);
 
-    auto values = effect->defaults();
-    for (const auto& [key, value] : options.params) values[key] = value;
-
     const auto start = std::chrono::steady_clock::now();
-    const auto image = effect->render(sources, options.width, options.height, rng, values);
+    const auto image =
+        effector::renderChain(stages, sources, options.width, options.height, rng);
     const auto renderMillis =
         std::chrono::duration<double, std::milli>(std::chrono::steady_clock::now() - start).count();
     image.save(options.output);
 
     std::cout << options.output << "  " << options.width << "x" << options.height
-              << "  effect=" << effect->name() << "  seed=" << seed << "  render="
+              << "  effect=" << effector::chainSlug(stages) << "  seed=" << seed << "  render="
               << std::fixed << std::setprecision(1) << renderMillis << " ms\n";
     return 0;
 }

tests/CMakeLists.txt

@@ -1,4 +1,6 @@
-add_executable(effector_tests core_tests.cpp)
+# The CLI parser joins the core here: it is GUI-free too, and the canvas size
+# it produces has to obey the same bounds a project file does.
+add_executable(effector_tests core_tests.cpp ../src/cli/options.cpp)
 target_link_libraries(effector_tests PRIVATE effector_core)
 target_compile_options(effector_tests PRIVATE -Wall -Wextra -Wpedantic)