Merge branch 'empty-chain-start'

A picture with a title, a caption and a credit meant typing the same
font, size and colour into every text stage, and changing the look meant
typing them all again. A project now holds named styles: a stage follows
one, and editing it moves every stage that follows it at once.

The front-ends learn nothing effect-specific. A ParamSpec can be marked
.styled(), saying a style carries it rather than the stage, and
Effect::styleKind() names the pool it comes from -- so styleSpecs(kind)
is to a style what params() is to a stage, and a second text-ish effect
would slot into the styles that already exist. The look is what a style
carries; the words and the placement stay on the stage, which is what
lets two stages share a typography in two different corners.

stageValues() resolves one in a single place, and a stage keeps a full
set of values of its own beside the style, so switching it to Custom,
dropping the style, or opening the project in a build that does not know
it renders exactly the same picture. A style moves stages together; it
never holds the only copy of anything.

The command line builds no styles and passes none, so it renders what it
always did.

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

CLAUDE.md

@@ -53,6 +53,17 @@ travel as `ParamValues` (a
 `map<string, variant<long,double,bool,string>>`), read back with
 `paramInt`/`paramDouble`/`paramBool`/`paramChoice`.
 
+A param can also be marked `.styled()`, which says a *named style* carries it
+rather than the stage it sits on: the font and the colour of a text stage, but
+never its words or its placement. The pool a style comes from is named by
+`Effect::styleKind()` (empty by default — the effect takes no style; `"text"`
+for the text effect), and effects answering the same kind share one pool, so a
+second text-ish effect would slot into the styles that already exist. The
+front-ends know nothing more than that: the interface builds the style row and
+the project file writes a style's params from `styleSpecs(kind)`
+(`src/core/style.cpp`), which is to a style exactly what `params()` is to a
+stage.
+
 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`)
@@ -73,10 +84,32 @@ 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.
 
+A stage carries a `ChainStage::enabled` flag, and `enabledStages`
+(`src/core/chain.cpp`) is the one place that reads it: a disabled stage is
+skipped whole — handed no image and drawing nothing from the shared rng — so
+switching one off renders exactly what the chain rendered before it was added.
+`renderChain`, `chainNeedsSources`, `chainTitle` and `chainSlug` all go through
+it, which is why the status line and the file name never mention a stage that
+did not run. A chain with every stage disabled throws rather than rendering
+nothing.
+
+### A style is a look several stages share
+
+`ChainStage::style` names a style, and `stageValues` (`src/core/chain.cpp`) is
+the one place that resolves one: an effect's defaults, then the stage's own
+values, then the style's values for the params it carries. A stage always keeps
+a full set of values of its own beside the style, so switching it to "Custom",
+dropping the style, or opening the project in a build that does not know the
+style renders exactly the same picture — a style moves stages together, it
+never holds the only copy of anything. The interface writes an edited param to
+both the style and the stage for that reason. `renderChain` takes the pool as a
+trailing argument; the command line builds no styles and passes none.
+
 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`
+menu, reorder, remove, switch off with the check on the row, 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`, with `--disable NAME`
+leaving a stage in place but skipping it. 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.
 
@@ -94,11 +127,15 @@ 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
+named styles, the chain with its params (and its `"enabled": false` and
+`"style"`, each written only when there is something to say), 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.
+project keeps the defaults for params that did not exist when it was saved. The
+same leniency covers styles: one of a kind this build does not know, one with
+no name or a repeated one, and a stage naming a style the file does not hold
+are each dropped on their own rather than refusing the project.
 Source paths under the project's own folder are written relative, so a folder
 can be moved or shared whole.
 

CMakeLists.txt

@@ -31,6 +31,7 @@ add_library(effector_core STATIC
     src/core/project.cpp
     src/core/registry.cpp
     src/core/rng.cpp
+    src/core/style.cpp
     src/effects/blur.cpp
     src/effects/grain.cpp
     src/effects/monochrome.cpp

src/cli/options.cpp

@@ -1,5 +1,6 @@
 #include "cli/options.hpp"
 
+#include <algorithm>
 #include <charconv>
 #include <cstdint>
 #include <cstring>
@@ -89,6 +90,10 @@ Options parseOptions(int argc, char** argv) {
                                   std::to_string(kMinDpi) + "-" + std::to_string(kMaxDpi));
             }
             options.dpi = static_cast<int>(dpi);
+        } else if (arg == "--disable") {
+            const std::string name = requireValue(argc, argv, i, arg);
+            effectByName(name);  // validate now, exactly as -e does
+            options.disabled.push_back(name);
         } else if (arg == "--seed") {
             const std::string value = requireValue(argc, argv, i, arg);
             long seed = 0;
@@ -160,7 +165,6 @@ Options parseOptions(int argc, char** argv) {
     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;
 }
 
@@ -169,13 +173,15 @@ 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, repeat to cascade (default: " << defaultEffectName() << ")\n"
+        << "  -e, --effect NAME     effect to apply, repeat to cascade (--output"
+           " defaults to " << defaultEffectName() << ")\n"
         << "  -o, --output FILE     render headlessly to this file and exit\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"
+        << "      --disable NAME    keep this effect in the chain but skip it\n"
         << "      --seed N          random seed (default: random)\n"
         << "      --list-effects    list available effects and exit\n"
         << "  -h, --help            show this help and exit\n";
@@ -207,6 +213,8 @@ std::vector<ChainStage> buildChain(const Options& options) {
         ChainStage stage;
         stage.effect = effectByName(name);
         stage.values = stage.effect->defaults();
+        stage.enabled = std::find(options.disabled.begin(), options.disabled.end(), name) ==
+                        options.disabled.end();
         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;

src/cli/options.hpp

@@ -14,6 +14,10 @@ namespace effector {
 struct Options {
     std::vector<std::string> images;
     std::vector<std::string> effects;  // one per cascade stage, in order
+    // The effects named by --disable: their stages stay in the chain, in
+    // place, but are skipped. Named rather than numbered, exactly as the
+    // params are, so two stages of one effect are disabled together.
+    std::vector<std::string> disabled;
     std::string output;              // empty -> launch the interface
     int width = 1920;
     int height = 1080;
@@ -33,7 +37,8 @@ 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`.
+// together for `-e scanlines -e blur`. A stage named by --disable is built
+// with all its settings and marked to be skipped.
 std::vector<ChainStage> buildChain(const Options& options);
 
 }  // namespace effector

src/core/chain.cpp

@@ -2,17 +2,51 @@
 
 namespace effector {
 
+std::vector<const ChainStage*> enabledStages(const std::vector<ChainStage>& stages) {
+    std::vector<const ChainStage*> running;
+    running.reserve(stages.size());
+    for (const auto& stage : stages) {
+        if (stage.enabled && stage.effect) running.push_back(&stage);
+    }
+    return running;
+}
+
+ParamValues stageValues(const ChainStage& stage, const StyleSet& styles) {
+    ParamValues values = stage.effect->defaults();
+    for (const auto& [key, value] : stage.values) values[key] = value;
+
+    const Style* style = findStyle(styles, stage.effect->styleKind(), stage.style);
+    if (style != nullptr) {
+        // Only the params the effect marked styled(): the words and the
+        // placement stay the stage's own, which is what lets two stages share
+        // one look without sharing a position.
+        for (const auto& spec : stage.effect->params()) {
+            if (!spec.inStyle) continue;
+            const auto found = style->values.find(spec.key);
+            if (found != style->values.end()) values[spec.key] = found->second;
+        }
+    }
+    return values;
+}
+
 Image renderChain(const std::vector<ChainStage>& stages, const std::vector<Image>& sources,
-                  int width, int height, Rng& rng) {
+                  int width, int height, Rng& rng, const StyleSet& styles) {
     if (stages.empty()) throw EffectError("The effect chain is empty.");
+    for (const auto& stage : stages) {
+        if (!stage.effect) throw EffectError("The effect chain has an empty stage.");
+    }
+
+    // A disabled stage never reaches the render at all: it is not handed an
+    // image and draws nothing from the shared rng, so switching one off gives
+    // back exactly the picture the chain made before it was added.
+    const auto running = enabledStages(stages);
+    if (running.empty()) throw EffectError("Every effect in the chain is disabled.");
 
     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.");
+    for (std::size_t i = 0; i < running.size(); ++i) {
+        const auto& stage = *running[i];
 
-        auto values = stage.effect->defaults();
-        for (const auto& [key, value] : stage.values) values[key] = value;
+        const auto values = stageValues(stage, styles);
 
         if (i == 0) {
             image = stage.effect->render(sources, width, height, rng, values);
@@ -27,26 +61,25 @@ Image renderChain(const std::vector<ChainStage>& stages, const std::vector<Image
 }
 
 bool chainNeedsSources(const std::vector<ChainStage>& stages) {
-    if (stages.empty() || !stages.front().effect) return false;
-    return stages.front().effect->needsSources();
+    const auto running = enabledStages(stages);
+    if (running.empty()) return false;
+    return running.front()->effect->needsSources();
 }
 
 std::string chainTitle(const std::vector<ChainStage>& stages) {
     std::string text;
-    for (const auto& stage : stages) {
-        if (!stage.effect) continue;
+    for (const auto* stage : enabledStages(stages)) {
         if (!text.empty()) text += " → ";
-        text += stage.effect->title();
+        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;
+    for (const auto* stage : enabledStages(stages)) {
         if (!text.empty()) text += "-";
-        text += stage.effect->name();
+        text += stage->effect->name();
     }
     return text;
 }

src/core/chain.hpp

@@ -7,6 +7,7 @@
 #include "core/image.hpp"
 #include "core/params.hpp"
 #include "core/rng.hpp"
+#include "core/style.hpp"
 
 namespace effector {
 
@@ -21,23 +22,46 @@ inline constexpr int kMaxCanvasSide = 20000;
 struct ChainStage {
     EffectPtr effect;
     ParamValues values;  // missing keys fall back to the effect's defaults
+    // A disabled stage is skipped whole, keeping its settings and its place in
+    // the chain, so a stage can be switched off and back on while trying
+    // things out without being taken apart and rebuilt.
+    bool enabled = true;
+    // The style this stage follows, by name, in the pool its effect names with
+    // styleKind(); empty when the stage stands on its own. A stage keeps a
+    // full set of values either way, so a style that is dropped or renamed
+    // away leaves the stage rendering exactly what it rendered before.
+    std::string style = {};
 };
 
-// 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.
+// The values a stage actually runs with: its effect's defaults, then the
+// stage's own values, then the values of the style it follows for the params
+// that style carries. The one place a style is resolved.
+ParamValues stageValues(const ChainStage& stage, const StyleSet& styles);
+
+// The stages that actually run, in order, as pointers into `stages`. A
+// disabled stage draws nothing from the rng and is handed no image, so a chain
+// renders exactly as it would if that stage had never been added -- this is
+// the one place the rest of the code asks what "disabled" means.
+std::vector<const ChainStage*> enabledStages(const std::vector<ChainStage>& stages);
+
+// Renders the enabled 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.
+// reproduces the cascade byte for byte. Throws when no stage is left to run.
+// `styles` is the project's pool; a chain built without one -- the command
+// line's -- passes none and renders from the stages alone.
 Image renderChain(const std::vector<ChainStage>& stages, const std::vector<Image>& sources,
-                  int width, int height, Rng& rng);
+                  int width, int height, Rng& rng, const StyleSet& styles = {});
 
-// 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.
+// Whether the chain has to be given source images. Only the first stage that
+// runs ever sees them — every later one works on the stage before it — so this
+// is simply whether that effect declares that it needs them.
 bool chainNeedsSources(const std::vector<ChainStage>& stages);
 
-// "Scanlines → Blur", for window titles and status lines.
+// "Scanlines → Blur", for window titles and status lines. Both of these name
+// what was rendered, so a disabled stage is left out of them.
 std::string chainTitle(const std::vector<ChainStage>& stages);
 
 // "scanlines-blur", for file names.

src/core/effect.cpp

@@ -4,25 +4,7 @@ namespace effector {
 
 ParamValues Effect::defaults() const {
     ParamValues values;
-    for (const auto& spec : params()) {
-        switch (spec.type) {
-            case ParamSpec::Type::Int:
-                values[spec.key] = static_cast<long>(spec.defaultValue);
-                break;
-            case ParamSpec::Type::Double:
-                values[spec.key] = spec.defaultValue;
-                break;
-            case ParamSpec::Type::Bool:
-                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;
-        }
-    }
+    for (const auto& spec : params()) values[spec.key] = paramDefault(spec);
     return values;
 }
 

src/core/effect.hpp

@@ -32,6 +32,12 @@ public:
     // chain that starts with one of them needs no sources at all.
     virtual bool needsSources() const { return true; }
 
+    // The pool of named styles this effect's styled params are drawn from, or
+    // empty when it takes no style. Effects that answer the same kind share
+    // one pool, so a stage can be pointed at a look its neighbours already
+    // use. An effect with no kind must declare no styled params.
+    virtual std::string styleKind() const { return {}; }
+
     virtual Image render(const std::vector<Image>& sources, int width, int height, Rng& rng,
                          const ParamValues& values) const = 0;
 

src/core/params.cpp

@@ -10,6 +10,12 @@ std::string ParamSpec::flag() const {
     return flag;
 }
 
+ParamSpec ParamSpec::styled() const {
+    ParamSpec copy = *this;
+    copy.inStyle = true;
+    return copy;
+}
+
 ParamSpec ParamSpec::makeInt(std::string key, std::string label, long defaultValue, long minimum,
                              long maximum, std::string help, long step) {
     ParamSpec spec;
@@ -103,6 +109,23 @@ const ParamValue* find(const ParamValues& values, const std::string& key) {
 
 }  // namespace
 
+ParamValue paramDefault(const ParamSpec& spec) {
+    switch (spec.type) {
+        case ParamSpec::Type::Int:
+            return static_cast<long>(spec.defaultValue);
+        case ParamSpec::Type::Double:
+            return spec.defaultValue;
+        case ParamSpec::Type::Bool:
+            return spec.defaultValue != 0.0;
+        case ParamSpec::Type::Choice:
+        case ParamSpec::Type::Path:
+        case ParamSpec::Type::Text:
+        case ParamSpec::Type::Color:
+            break;
+    }
+    return spec.defaultChoice;
+}
+
 long paramInt(const ParamValues& values, const std::string& key, long fallback) {
     if (const auto* value = find(values, key)) {
         if (const auto* v = std::get_if<long>(value)) return *v;

src/core/params.hpp

@@ -24,9 +24,17 @@ struct ParamSpec {
     int digits = 2;
     std::string defaultChoice;
     std::vector<std::string> choices;
+    // Whether a named style carries this param rather than the stage it sits
+    // on: see core/style.hpp.
+    bool inStyle = false;
 
     std::string flag() const;  // "strip_height" -> "--strip-height"
 
+    // A copy of this spec marked as carried by a style, so that a declaration
+    // reads makeDouble("font_size", ...).styled() and the factories keep the
+    // signatures they have.
+    ParamSpec styled() const;
+
     static ParamSpec makeInt(std::string key, std::string label, long defaultValue, long minimum,
                              long maximum, std::string help = {}, long step = 1);
     static ParamSpec makeDouble(std::string key, std::string label, double defaultValue,
@@ -53,6 +61,10 @@ struct ParamSpec {
 using ParamValue = std::variant<long, double, bool, std::string>;
 using ParamValues = std::map<std::string, ParamValue>;
 
+// The value a spec stands for when nothing has been set: one place, so that
+// an effect's defaults and a style's are filled the same way.
+ParamValue paramDefault(const ParamSpec& spec);
+
 long paramInt(const ParamValues& values, const std::string& key, long fallback);
 double paramDouble(const ParamValues& values, const std::string& key, double fallback);
 bool paramBool(const ParamValues& values, const std::string& key, bool fallback);

src/core/project.cpp

@@ -325,6 +325,96 @@ std::string resolve(const std::string& path, const std::string& directory) {
     return (std::filesystem::path(directory) / file).lexically_normal().string();
 }
 
+// The params of a stage or of a style, written from the specs rather than from
+// whatever the values happen to hold: a key the effect does not declare is
+// never written, so it can never be read back either.
+void writeParams(std::ostream& out, const std::vector<ParamSpec>& specs, const ParamValues& values,
+                 const std::string& directory) {
+    out << "{";
+    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(values, spec.key, spec.defaultValue != 0.0) ? "true" : "false");
+                break;
+            case ParamSpec::Type::Int:
+                out << paramInt(values, spec.key, static_cast<long>(spec.defaultValue));
+                break;
+            case ParamSpec::Type::Double:
+                out << number(paramDouble(values, spec.key, spec.defaultValue), spec.digits);
+                break;
+            case ParamSpec::Type::Choice:
+            case ParamSpec::Type::Text:
+                out << quote(paramText(values, spec.key, spec.defaultChoice));
+                break;
+            case ParamSpec::Type::Color: {
+                std::uint8_t rgb[3];
+                paramColor(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(values, spec.key, "");
+                out << quote(path.empty() ? path : relativeTo(path, directory));
+                break;
+            }
+        }
+    }
+    out << (specs.empty() ? "}" : "\n      }");
+}
+
+// The mirror image: only declared keys are read, each behind the gate its type
+// asks for, so anything else in the file leaves the default standing.
+void readParams(const std::vector<ParamSpec>& specs, const Json& params,
+                const std::string& directory, ParamValues& values) {
+    for (const auto& spec : specs) {
+        const auto* value = params.member(spec.key);
+        if (value == nullptr) continue;
+        switch (spec.type) {
+            case ParamSpec::Type::Bool:
+                if (value->kind == Json::Kind::Bool) values[spec.key] = value->boolean;
+                break;
+            case ParamSpec::Type::Int:
+                if (value->kind == Json::Kind::Number) {
+                    values[spec.key] = static_cast<long>(std::llround(value->number));
+                }
+                break;
+            case ParamSpec::Type::Double:
+                if (value->kind == Json::Kind::Number) values[spec.key] = value->number;
+                break;
+            case ParamSpec::Type::Path:
+                if (value->kind == Json::Kind::String) {
+                    values[spec.key] =
+                        value->text.empty() ? value->text : resolve(value->text, directory);
+                }
+                break;
+            case ParamSpec::Type::Text:
+                if (value->kind == Json::Kind::String) 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)) {
+                    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) {
+                        values[spec.key] = choice;
+                    }
+                }
+                break;
+        }
+    }
+}
+
 }  // namespace
 
 std::string projectToJson(const Project& project, const std::string& directory) {
@@ -353,51 +443,37 @@ std::string projectToJson(const Project& project, const std::string& directory)
     }
     out << (project.sources.empty() ? "" : "\n  ") << "],\n";
 
+    // Before the chain, because a stage's "style" is a name from this list.
+    if (!project.styles.empty()) {
+        out << "  \"styles\": [";
+        for (std::size_t i = 0; i < project.styles.size(); ++i) {
+            const auto& style = project.styles[i];
+            out << (i == 0 ? "\n" : ",\n");
+            out << "    {\n      \"kind\": " << quote(style.kind) << ",\n";
+            out << "      \"name\": " << quote(style.name) << ",\n";
+            out << "      \"params\": ";
+            writeParams(out, styleSpecs(style.kind), style.values, directory);
+            out << "\n    }";
+        }
+        out << "\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    }";
+        // Only written when it is false: a stage is enabled unless the file
+        // says otherwise, so an ordinary project stays as uncluttered as it
+        // was before a stage could be switched off at all.
+        if (!stage.enabled) out << "      \"enabled\": false,\n";
+        // Likewise only when there is one: a stage that stands on its own
+        // values says nothing about styles at all.
+        if (!stage.style.empty()) out << "      \"style\": " << quote(stage.style) << ",\n";
+        out << "      \"params\": ";
+        writeParams(out, stage.effect->params(), stage.values, directory);
+        out << "\n    }";
     }
     out << (project.stages.empty() ? "" : "\n  ") << "]\n";
     out << "}\n";
@@ -453,6 +529,33 @@ Project projectFromJson(const std::string& text, const std::string& directory) {
         }
     }
 
+    // Read before the chain, so that a stage's "style" can be checked against
+    // the pool as it is read.
+    if (const auto* styles = root.member("styles")) {
+        for (const auto& item : styles->items) {
+            const auto* kind = item.member("kind");
+            const auto* name = item.member("name");
+            if (kind == nullptr || kind->kind != Json::Kind::String) continue;
+            if (name == nullptr || name->kind != Json::Kind::String || name->text.empty()) continue;
+
+            // A kind no effect in this build claims carries no params, so
+            // there would be nothing to keep; a repeated name would make
+            // "style": "Title" ambiguous, so the first one wins.
+            const auto specs = styleSpecs(kind->text);
+            if (specs.empty()) continue;
+            if (findStyle(project.styles, kind->text, name->text) != nullptr) continue;
+
+            Style style;
+            style.kind = kind->text;
+            style.name = name->text;
+            style.values = styleDefaults(style.kind);
+            if (const auto* params = item.member("params")) {
+                readParams(specs, *params, directory, style.values);
+            }
+            project.styles.push_back(std::move(style));
+        }
+    }
+
     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");
@@ -466,53 +569,26 @@ Project projectFromJson(const std::string& text, const std::string& directory) {
         ChainStage stage;
         stage.effect = effectByName(name->text);  // throws on an unknown effect
         stage.values = stage.effect->defaults();
+        if (const auto* enabled = item.member("enabled");
+            enabled != nullptr && enabled->kind == Json::Kind::Bool) {
+            stage.enabled = enabled->boolean;
+        }
 
         // 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;
-                }
+            readParams(stage.effect->params(), *params, directory, stage.values);
         }
+
+        // A style is kept only when the file actually holds one of that name
+        // for this effect's pool: a stage pointing at a style that is not
+        // there falls back to the values it wrote beside it, which are the
+        // ones it last rendered with.
+        if (const auto* style = item.member("style");
+            style != nullptr && style->kind == Json::Kind::String &&
+            findStyle(project.styles, stage.effect->styleKind(), style->text) != nullptr) {
+            stage.style = style->text;
         }
         project.stages.push_back(std::move(stage));
     }

src/core/project.hpp

@@ -8,6 +8,7 @@
 #include "core/canvas.hpp"
 #include "core/chain.hpp"
 #include "core/params.hpp"
+#include "core/style.hpp"
 
 namespace effector {
 
@@ -17,6 +18,10 @@ namespace effector {
 struct Project {
     std::vector<std::string> sources;  // absolute paths
     std::vector<ChainStage> stages;
+    // The named looks the stages draw their formatting from. Kept beside the
+    // chain rather than inside it because that is the whole point of a style:
+    // several stages share one, and editing it moves all of them at once.
+    StyleSet styles;
     int width = 1920;
     int height = 1080;
     // The named format the canvas came from, empty for a custom pixel size.
@@ -36,7 +41,9 @@ 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.
+// does not declare are dropped, and missing ones keep the effect's default. A
+// style of an unknown kind, or a stage naming a style the file does not hold,
+// is dropped the same way: the stage keeps its own values and still renders.
 Project projectFromJson(const std::string& text, const std::string& directory = {});
 
 // Both throw EffectError: the file is the user's, so failures are reported

src/core/style.cpp

@@ -0,0 +1,39 @@
+#include "core/style.hpp"
+
+#include <algorithm>
+
+#include "core/registry.hpp"
+
+namespace effector {
+
+std::vector<ParamSpec> styleSpecs(const std::string& kind) {
+    std::vector<ParamSpec> specs;
+    if (kind.empty()) return specs;
+    for (const auto& effect : effects()) {
+        if (effect->styleKind() != kind) continue;
+        for (const auto& spec : effect->params()) {
+            if (!spec.inStyle) continue;
+            const bool known = std::any_of(specs.begin(), specs.end(), [&](const ParamSpec& seen) {
+                return seen.key == spec.key;
+            });
+            if (!known) specs.push_back(spec);
+        }
+    }
+    return specs;
+}
+
+ParamValues styleDefaults(const std::string& kind) {
+    ParamValues values;
+    for (const auto& spec : styleSpecs(kind)) values[spec.key] = paramDefault(spec);
+    return values;
+}
+
+const Style* findStyle(const StyleSet& styles, const std::string& kind, const std::string& name) {
+    if (kind.empty() || name.empty()) return nullptr;
+    for (const auto& style : styles) {
+        if (style.kind == kind && style.name == name) return &style;
+    }
+    return nullptr;
+}
+
+}  // namespace effector

src/core/style.hpp

@@ -0,0 +1,39 @@
+#pragma once
+
+#include <string>
+#include <vector>
+
+#include "core/params.hpp"
+
+namespace effector {
+
+// A named set of formatting values, shared by every stage that follows it.
+//
+// A style carries only the params an effect marked styled() -- the look, never
+// the placement -- so several text stages can share one typography while each
+// keeps its own words and its own corner of the canvas. Changing the style is
+// the point: it changes every stage pointing at it at once.
+struct Style {
+    std::string kind;    // the pool, as Effect::styleKind() names it
+    std::string name;    // unique within a kind, never empty
+    ParamValues values;  // the styled params only
+};
+
+using StyleSet = std::vector<Style>;
+
+// The styled specs of a kind: every registered effect answering that kind
+// contributes its styled params, the first declaration of a key winning. This
+// is to a style what Effect::params() is to a stage -- the only thing the
+// interface and the project file know about what a style holds -- so an empty
+// or unknown kind gives an empty list.
+std::vector<ParamSpec> styleSpecs(const std::string& kind);
+
+// A fresh set of values for a style of that kind: every styled param at the
+// default its effect declares, the way Effect::defaults() fills a stage.
+ParamValues styleDefaults(const std::string& kind);
+
+// The style of that name in that pool, or nullptr. A stage naming a style that
+// is not there simply stands on its own values.
+const Style* findStyle(const StyleSet& styles, const std::string& kind, const std::string& name);
+
+}  // namespace effector

src/effects/text.cpp

@@ -17,31 +17,39 @@ const std::vector<ParamSpec>& TextEffect::params() const {
                             "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\", ..."),
+                            "\"DejaVu Serif\", ...")
+            .styled(),
         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."),
+                              "rendered in the nearest one it has.")
+            .styled(),
+        ParamSpec::makeBool("font_italic", "Italic", false, "Slants the glyphs.").styled(),
         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."),
+                              "Height of the glyphs in pixels.", 1.0, 1)
+            .styled(),
+        ParamSpec::makeColor("text_color", "Colour", "#ffffff", "The colour of the glyphs.").styled(),
         ParamSpec::makeChoice("text_align", "Alignment", "center", {"left", "center", "right"},
-                              "How several lines line up against each other."),
+                              "How several lines line up against each other.")
+            .styled(),
         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."),
+                              "from disk cropped to the rectangle.")
+            .styled(),
         ParamSpec::makeColor("box_color", "Box colour", "#000000",
-                            "The colour the box is filled with."),
+                            "The colour the box is filled with.")
+            .styled(),
         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."),
+                           "How far the box reaches past the text on every side.")
+            .styled(),
         ParamSpec::makeDouble("box_opacity", "Box opacity", 1.0, 0.0, 1.0,
-                              "How strongly the box is mixed over the input."),
+                              "How strongly the box is mixed over the input.")
+            .styled(),
         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."),
@@ -50,7 +58,8 @@ const std::vector<ParamSpec>& TextEffect::params() const {
         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."),
+                              "How strongly the glyphs are mixed over what is behind them.")
+            .styled(),
     };
     return specs;
 }

src/effects/text.hpp

@@ -28,6 +28,7 @@ public:
                "box filled with a colour or with a picture.";
     }
     const std::vector<ParamSpec>& params() const override;
+    std::string styleKind() const override { return "text"; }
     bool needsSources() const override { return false; }
 
     Image render(const std::vector<Image>& sources, int width, int height, Rng& rng,

src/gui/window.cpp

@@ -6,6 +6,7 @@
 #include <cstdint>
 #include <filesystem>
 #include <sstream>
+#include <utility>
 
 #include "core/canvas.hpp"
 #include "core/chain.hpp"
@@ -49,7 +50,8 @@ EffectorWindow::EffectorWindow(const Options& initial) {
     set_default_size(1180, 760);
 
     effects_ = effects();
-    stages_.push_back({effects_.front(), effects_.front()->defaults()});
+    // The chain starts empty: the interface opens on "No effects yet" and the
+    // user picks the first stage, rather than being handed one at random.
     seed_ = Rng::randomSeed();
 
     renderDone_.connect(sigc::mem_fun(*this, &EffectorWindow::onRenderFinished));
@@ -191,6 +193,15 @@ Gtk::Widget& EffectorWindow::buildSidebar() {
     return *sidebar;
 }
 
+// Only the first stage is ever handed the sources, so whether the chain wants
+// them at all is `chainNeedsSources`. An effect that starts from nothing (blur,
+// text) makes the whole sources section beside the point, so it is hidden
+// rather than left there to be filled in vain. This is the same declarative
+// answer the render gate uses -- no effect is named here.
+void EffectorWindow::updateSourcesVisible() {
+    sourcesExpander_.set_visible(chainNeedsSources(stages_));
+}
+
 // The expander is collapsed by default, so its label carries the count that
 // used to be plain to see in the list.
 void EffectorWindow::updateSourcesLabel() {
@@ -350,27 +361,62 @@ void EffectorWindow::buildParamWidgets() {
     while (auto* child = paramGrid_.get_first_child()) paramGrid_.remove(*child);
     paramWidgets_.clear();
     ownedWidgets_.clear();
+    styleDrop_ = nullptr;
 
     if (stages_.empty()) {
         paramHeading_.set_text("No effects");
         return;
     }
     const auto& stage = stages_[static_cast<std::size_t>(selectedStage_)];
+    // The settings of a stage that is switched off stay editable: it is still
+    // being worked on, it just is not in the picture for the moment.
     paramHeading_.set_text(std::to_string(selectedStage_ + 1) + ". " + stage.effect->title() +
-                           " settings");
+                           (stage.enabled ? " settings" : " settings (off)"));
 
     int row = 0;
+    // The style the stage follows, when its effect takes one at all: the
+    // params it carries are then shown and edited on the style rather than on
+    // the stage, which is what makes one change move every stage using it.
+    const Style* style = selectedStyle();
+    if (!stage.effect->styleKind().empty()) {
+        auto label = std::make_unique<Gtk::Label>("Style");
+        label->set_xalign(0.0F);
+        label->set_tooltip_text(
+            "A named set of formatting shared by several stages: the font, the colour and the "
+            "box, but never the words or the placement.");
+        auto widget = buildStyleRow(stage);
+        widget->set_hexpand(true);
+        paramGrid_.attach(*label, 0, row);
+        paramGrid_.attach(*widget, 1, row);
+        ownedWidgets_.push_back(std::move(label));
+        ownedWidgets_.push_back(std::move(widget));
+        ++row;
+    }
+
     for (const auto& spec : stage.effect->params()) {
+        // A styled param is read from the style while one is followed, so the
+        // widgets show what the stage will actually render with.
+        const bool shared = style != nullptr && spec.inStyle;
+        const ParamValues& values = shared ? style->values : stage.values;
+
         auto label = std::make_unique<Gtk::Label>(spec.label);
         label->set_xalign(0.0F);
-        if (!spec.help.empty()) label->set_tooltip_text(spec.help);
+        std::string help = spec.help;
+        if (shared) {
+            label->set_markup(Glib::Markup::escape_text(spec.label) +
+                              " <span alpha='55%'>(style)</span>");
+            if (!help.empty()) help += "\n\n";
+            help += "Part of the style \"" + style->name +
+                    "\": changing it changes every stage that follows it.";
+        }
+        if (!help.empty()) label->set_tooltip_text(help);
 
         std::unique_ptr<Gtk::Widget> widget;
         switch (spec.type) {
             case ParamSpec::Type::Bool: {
                 auto toggle = std::make_unique<Gtk::Switch>();
                 toggle->set_halign(Gtk::Align::START);
-                toggle->set_active(paramBool(stage.values, spec.key, spec.defaultValue != 0.0));
+                toggle->set_active(paramBool(values, spec.key, spec.defaultValue != 0.0));
                 toggle->property_active().signal_changed().connect(
                     sigc::mem_fun(*this, &EffectorWindow::onParamChanged));
                 widget = std::move(toggle);
@@ -383,9 +429,9 @@ void EffectorWindow::buildParamWidgets() {
                 spin->set_increments(spec.step, spec.step * 10);
                 spin->set_digits(spec.type == ParamSpec::Type::Int ? 0 : spec.digits);
                 spin->set_value(spec.type == ParamSpec::Type::Int
-                                    ? static_cast<double>(paramInt(stage.values, spec.key,
+                                    ? static_cast<double>(paramInt(values, spec.key,
                                                                   static_cast<long>(spec.defaultValue)))
-                                    : paramDouble(stage.values, spec.key, spec.defaultValue));
+                                    : paramDouble(values, spec.key, spec.defaultValue));
                 spin->signal_value_changed().connect(
                     sigc::mem_fun(*this, &EffectorWindow::onParamChanged));
                 widget = std::move(spin);
@@ -397,7 +443,7 @@ void EffectorWindow::buildParamWidgets() {
                 auto box = std::make_unique<Gtk::Box>(Gtk::Orientation::HORIZONTAL, 6);
                 auto* entry = Gtk::make_managed<Gtk::Entry>();
                 entry->set_hexpand(true);
-                entry->set_text(paramText(stage.values, spec.key, ""));
+                entry->set_text(paramText(values, spec.key, ""));
                 entry->set_placeholder_text("No image chosen");
                 entry->signal_activate().connect(
                     sigc::mem_fun(*this, &EffectorWindow::onParamChanged));
@@ -415,7 +461,7 @@ void EffectorWindow::buildParamWidgets() {
             }
             case ParamSpec::Type::Text: {
                 auto entry = std::make_unique<Gtk::Entry>();
-                entry->set_text(paramText(stage.values, spec.key, spec.defaultChoice));
+                entry->set_text(paramText(values, spec.key, spec.defaultChoice));
                 // Re-rendering on every keystroke would start a render per
                 // letter typed, so the text is taken when the entry is left or
                 // Enter is pressed.
@@ -432,7 +478,7 @@ void EffectorWindow::buildParamWidgets() {
                 auto button = std::make_unique<Gtk::ColorDialogButton>(Gtk::ColorDialog::create());
                 button->set_halign(Gtk::Align::START);
                 std::uint8_t rgb[3];
-                paramColor(stage.values, spec.key, spec.defaultChoice, rgb);
+                paramColor(values, spec.key, spec.defaultChoice, rgb);
                 Gdk::RGBA colour;
                 colour.set_rgba(rgb[0] / 255.0, rgb[1] / 255.0, rgb[2] / 255.0, 1.0);
                 button->set_rgba(colour);
@@ -445,7 +491,7 @@ void EffectorWindow::buildParamWidgets() {
                 std::vector<Glib::ustring> choices(spec.choices.begin(), spec.choices.end());
                 auto dropdown = std::make_unique<Gtk::DropDown>();
                 dropdown->set_model(Gtk::StringList::create(choices));
-                const auto current = paramChoice(stage.values, spec.key, spec.defaultChoice);
+                const auto current = paramChoice(values, spec.key, spec.defaultChoice);
                 for (std::size_t i = 0; i < spec.choices.size(); ++i) {
                     if (spec.choices[i] == current) dropdown->set_selected(i);
                 }
@@ -511,7 +557,282 @@ ParamValues EffectorWindow::paramValues() const {
 // whenever they change; the other stages keep the settings they were given.
 void EffectorWindow::storeParams() {
     if (stages_.empty()) return;
-    stages_[static_cast<std::size_t>(selectedStage_)].values = paramValues();
+    auto& stage = stages_[static_cast<std::size_t>(selectedStage_)];
+    const ParamValues values = paramValues();
+    stage.values = values;
+
+    // A param the style carries is written to the style too -- that is how the
+    // look of every stage following it is changed from here. The stage keeps
+    // its own copy of the same value, so leaving the style, or losing it, does
+    // not move the picture.
+    if (Style* style = selectedStyle()) {
+        for (const auto& spec : stage.effect->params()) {
+            if (!spec.inStyle) continue;
+            const auto found = values.find(spec.key);
+            if (found != values.end()) style->values[spec.key] = found->second;
+        }
+    }
+}
+
+// ---------------------------------------------------------- text styles ---
+
+std::vector<std::string> EffectorWindow::styleNames(const std::string& kind) const {
+    std::vector<std::string> names;
+    for (const auto& style : styles_) {
+        if (style.kind == kind) names.push_back(style.name);
+    }
+    return names;
+}
+
+Style* EffectorWindow::selectedStyle() {
+    return const_cast<Style*>(std::as_const(*this).selectedStyle());
+}
+
+const Style* EffectorWindow::selectedStyle() const {
+    if (stages_.empty()) return nullptr;
+    const auto& stage = stages_[static_cast<std::size_t>(selectedStage_)];
+    return findStyle(styles_, stage.effect->styleKind(), stage.style);
+}
+
+bool EffectorWindow::styleNameTaken(const std::string& kind, const std::string& name) const {
+    return findStyle(styles_, kind, name) != nullptr;
+}
+
+std::unique_ptr<Gtk::Widget> EffectorWindow::buildStyleRow(const ChainStage& stage) {
+    auto box = std::make_unique<Gtk::Box>(Gtk::Orientation::HORIZONTAL, 6);
+
+    const auto names = styleNames(stage.effect->styleKind());
+    // "Custom" first so that the stage standing on its own is the plain case,
+    // and "New style…" last so that the names in between keep their indices.
+    std::vector<Glib::ustring> entries{"Custom"};
+    for (const auto& name : names) entries.emplace_back(name);
+    entries.emplace_back("New style…");
+
+    auto* drop = Gtk::make_managed<Gtk::DropDown>();
+    drop->set_model(Gtk::StringList::create(entries));
+    drop->set_hexpand(true);
+    suspendStyleSelect_ = true;
+    drop->set_selected(0);
+    for (std::size_t i = 0; i < names.size(); ++i) {
+        if (names[i] == stage.style) drop->set_selected(i + 1);
+    }
+    suspendStyleSelect_ = false;
+    drop->property_selected().signal_changed().connect(
+        sigc::mem_fun(*this, &EffectorWindow::onStyleSelected));
+    box->append(*drop);
+    styleDrop_ = drop;
+
+    // Renaming and removing only make sense for a style that is being followed.
+    auto* rename = Gtk::make_managed<Gtk::Button>("Rename…");
+    rename->set_sensitive(!stage.style.empty());
+    rename->signal_clicked().connect(sigc::mem_fun(*this, &EffectorWindow::onRenameStyle));
+    box->append(*rename);
+
+    auto* remove = Gtk::make_managed<Gtk::Button>("Remove");
+    remove->set_sensitive(!stage.style.empty());
+    remove->set_tooltip_text(
+        "Drops the style. Every stage following it keeps the look it has and goes back to its own "
+        "settings.");
+    remove->signal_clicked().connect(sigc::mem_fun(*this, &EffectorWindow::onRemoveStyle));
+    box->append(*remove);
+
+    return box;
+}
+
+void EffectorWindow::onStyleSelected() {
+    if (suspendStyleSelect_ || styleDrop_ == nullptr || stages_.empty()) return;
+    const auto& stage = stages_[static_cast<std::size_t>(selectedStage_)];
+    const auto names = styleNames(stage.effect->styleKind());
+    const auto index = styleDrop_->get_selected();
+
+    if (index == 0) {
+        adoptStyle("");
+    } else if (index <= names.size()) {
+        adoptStyle(names[index - 1]);
+    } else {
+        onNewStyle();  // the last entry
+    }
+}
+
+// Pointing the stage at a style, or at nothing. The style's values are copied
+// onto the stage as well, so the project file always says what the stage would
+// render if the style went away -- which is the same picture.
+void EffectorWindow::adoptStyle(const std::string& name) {
+    if (stages_.empty()) return;
+    auto& stage = stages_[static_cast<std::size_t>(selectedStage_)];
+    stage.style = name;
+    if (const Style* style = selectedStyle()) {
+        for (const auto& spec : stage.effect->params()) {
+            if (!spec.inStyle) continue;
+            const auto found = style->values.find(spec.key);
+            if (found != style->values.end()) stage.values[spec.key] = found->second;
+        }
+    }
+    buildParamWidgets();
+    requestRender();
+}
+
+void EffectorWindow::onNewStyle() {
+    if (stages_.empty()) return;
+    const auto& stage = stages_[static_cast<std::size_t>(selectedStage_)];
+    const std::string kind = stage.effect->styleKind();
+    if (kind.empty()) return;
+
+    askStyleName("New style", "Style " + std::to_string(styleNames(kind).size() + 1),
+                 [this, kind](const std::string& name) {
+                     // Seeded from the stage it was made on, so the style
+                     // starts as the look that is already on screen.
+                     Style style;
+                     style.kind = kind;
+                     style.name = name;
+                     style.values = styleDefaults(kind);
+                     const auto& current = stages_[static_cast<std::size_t>(selectedStage_)];
+                     for (const auto& spec : current.effect->params()) {
+                         if (!spec.inStyle) continue;
+                         const auto found = current.values.find(spec.key);
+                         if (found != current.values.end()) style.values[spec.key] = found->second;
+                     }
+                     styles_.push_back(std::move(style));
+                     adoptStyle(name);
+                 });
+}
+
+void EffectorWindow::onRenameStyle() {
+    const Style* style = selectedStyle();
+    if (style == nullptr) return;
+    const std::string kind = style->kind;
+    const std::string was = style->name;
+
+    askStyleName("Rename style", was, [this, kind, was](const std::string& name) {
+        if (name == was) return;
+        for (auto& candidate : styles_) {
+            if (candidate.kind == kind && candidate.name == was) candidate.name = name;
+        }
+        // Every stage that followed the old name follows the new one: a rename
+        // must not quietly cut a stage loose from its look.
+        for (auto& stage : stages_) {
+            if (stage.effect->styleKind() == kind && stage.style == was) stage.style = name;
+        }
+        buildParamWidgets();
+    });
+}
+
+void EffectorWindow::onRemoveStyle() {
+    const Style* style = selectedStyle();
+    if (style == nullptr) return;
+    const std::string kind = style->kind;
+    const std::string name = style->name;
+
+    // The stages keep the values they were rendering with, so dropping a style
+    // changes nothing in the picture -- it only stops the stages moving
+    // together.
+    for (auto& stage : stages_) {
+        if (stage.effect->styleKind() == kind && stage.style == name) stage.style.clear();
+    }
+    styles_.erase(std::remove_if(styles_.begin(), styles_.end(),
+                                 [&](const Style& candidate) {
+                                     return candidate.kind == kind && candidate.name == name;
+                                 }),
+                  styles_.end());
+    buildParamWidgets();
+    requestRender();
+}
+
+// Built from scratch each time it is needed, like the progress window, so the
+// entry it holds is a raw pointer that closeStyleWindow() nulls.
+void EffectorWindow::askStyleName(const std::string& title, const std::string& initial,
+                                  const std::function<void(const std::string&)>& accept) {
+    closeStyleWindow();
+    if (stages_.empty()) return;
+    const std::string kind = stages_[static_cast<std::size_t>(selectedStage_)].effect->styleKind();
+
+    styleWindow_ = std::make_shared<Gtk::Window>();
+    styleWindow_->set_title(title);
+    styleWindow_->set_transient_for(*this);
+    styleWindow_->set_modal(true);
+    styleWindow_->set_resizable(false);
+    styleWindow_->set_default_size(360, -1);
+
+    auto* box = Gtk::make_managed<Gtk::Box>(Gtk::Orientation::VERTICAL, 12);
+    box->set_margin(18);
+
+    auto* entry = Gtk::make_managed<Gtk::Entry>();
+    entry->set_text(initial);
+    entry->set_activates_default(false);
+    box->append(*entry);
+    styleNameEntry_ = entry;
+
+    auto* error = Gtk::make_managed<Gtk::Label>("");
+    error->set_xalign(0.0F);
+    error->add_css_class("dim-label");
+    error->set_visible(false);
+    box->append(*error);
+    styleNameError_ = error;
+
+    auto* buttons = Gtk::make_managed<Gtk::Box>(Gtk::Orientation::HORIZONTAL, 6);
+    buttons->set_halign(Gtk::Align::END);
+
+    auto* cancel = Gtk::make_managed<Gtk::Button>("Cancel");
+    cancel->signal_clicked().connect([this] {
+        closeStyleWindow();
+        // The dropdown may be sitting on "New style…"; put it back on what the
+        // stage actually follows.
+        buildParamWidgets();
+    });
+    buttons->append(*cancel);
+
+    auto* confirm = Gtk::make_managed<Gtk::Button>("OK");
+    confirm->add_css_class("suggested-action");
+    // A name that is empty or already in the pool is refused in place rather
+    // than silently changed, so two styles can never share a name.
+    const auto submit = [this, kind, accept] {
+        if (styleNameEntry_ == nullptr) return;
+        const std::string name = styleNameEntry_->get_text();
+        const char* complaint = nullptr;
+        if (name.empty()) {
+            complaint = "A style needs a name.";
+        } else if (styleNameTaken(kind, name)) {
+            complaint = "There is already a style with that name.";
+        }
+        if (complaint != nullptr) {
+            if (styleNameError_ != nullptr) {
+                styleNameError_->set_text(complaint);
+                styleNameError_->set_visible(true);
+            }
+            return;
+        }
+        closeStyleWindow();
+        accept(name);
+    };
+    confirm->signal_clicked().connect(submit);
+    entry->signal_activate().connect(submit);
+    buttons->append(*confirm);
+    box->append(*buttons);
+
+    styleWindow_->signal_close_request().connect(
+        [this] {
+            closeStyleWindow();
+            buildParamWidgets();
+            return true;
+        },
+        false);
+
+    styleWindow_->set_child(*box);
+    styleWindow_->present();
+    entry->grab_focus();
+}
+
+void EffectorWindow::closeStyleWindow() {
+    styleNameEntry_ = nullptr;
+    styleNameError_ = nullptr;
+    if (!styleWindow_) return;
+    styleWindow_->set_hide_on_close(false);
+    styleWindow_->hide();
+    // Closing is asked for by the window's own buttons and by its close
+    // request, so it is torn down once that signal has finished running rather
+    // than underneath it.
+    Glib::signal_idle().connect_once([window = styleWindow_] {});
+    styleWindow_.reset();
 }
 
 void EffectorWindow::applyInitial(const Options& initial) {
@@ -903,6 +1224,12 @@ void EffectorWindow::rebuildChainRows() {
     // removing it would leave the empty list blank.
     while (auto* row = chainList_.get_row_at_index(0)) chainList_.remove(*row);
 
+    // The first stage that actually runs is the one handed the sources, which
+    // is not stage 1 once stage 1 is switched off.
+    const auto running = enabledStages(stages_);
+    const int firstRunning =
+        running.empty() ? -1 : static_cast<int>(running.front() - stages_.data());
+
     for (std::size_t i = 0; i < stages_.size(); ++i) {
         const int index = static_cast<int>(i);
         auto* row = Gtk::make_managed<Gtk::ListBoxRow>();
@@ -912,13 +1239,28 @@ void EffectorWindow::rebuildChainRows() {
         box->set_margin_start(8);
         box->set_margin_end(4);
 
+        // The check is what switches a stage off: it keeps its settings and
+        // its place in the chain, and the render simply skips it.
+        auto* check = Gtk::make_managed<Gtk::CheckButton>();
+        check->set_active(stages_[i].enabled);
+        check->set_tooltip_text("Apply this effect");
+        check->set_valign(Gtk::Align::CENTER);
+        // Connected after set_active, so filling in the rebuilt rows does not
+        // look like the user reaching for the check.
+        check->signal_toggled().connect([this, index, check] {
+            setStageEnabled(index, check->get_active());
+        });
+        box->append(*check);
+
         auto* label = Gtk::make_managed<Gtk::Label>(std::to_string(index + 1) + ". " +
                                                     stages_[i].effect->title());
         label->set_xalign(0.0F);
         label->set_hexpand(true);
         label->set_ellipsize(Pango::EllipsizeMode::END);
-        label->set_tooltip_text(index == 0 ? "Applied to the source images"
-                                           : "Applied to the output of the stage above");
+        if (!stages_[i].enabled) label->add_css_class("dim-label");
+        label->set_tooltip_text(!stages_[i].enabled  ? "Skipped while it is switched off"
+                                : index == firstRunning ? "Applied to the source images"
+                                                        : "Applied to the output so far");
         box->append(*label);
 
         struct Action {
@@ -952,6 +1294,20 @@ void EffectorWindow::rebuildChainRows() {
 
     selectStage(selectedStage_);
     suspendStageSelect_ = false;
+    updateSourcesVisible();
+}
+
+// Switching a stage off leaves everything about it alone -- its settings, its
+// place in the chain, the stage the params below are showing -- so the only
+// visible change is the dimmed row and the picture the render comes back with.
+void EffectorWindow::setStageEnabled(int index, bool enabled) {
+    if (index < 0 || static_cast<std::size_t>(index) >= stages_.size()) return;
+    auto& stage = stages_[static_cast<std::size_t>(index)];
+    if (stage.enabled == enabled) return;
+    stage.enabled = enabled;
+    rebuildChainRows();  // the row dims, and another stage may now lead the chain
+    if (index == selectedStage_) buildParamWidgets();  // the heading says "off"
+    requestRender();
 }
 
 void EffectorWindow::selectStage(int index) {
@@ -1028,12 +1384,13 @@ void EffectorWindow::onRecreate() {
 // ever handed the sources, so an effect that does not want them (blur) renders
 // happily with none loaded.
 bool EffectorWindow::canRender() const {
-    if (stages_.empty()) return false;
+    if (enabledStages(stages_).empty()) return false;
     return !sources_->empty() || !chainNeedsSources(stages_);
 }
 
 std::string EffectorWindow::missingText() const {
     if (stages_.empty()) return "Add an effect to start.";
+    if (enabledStages(stages_).empty()) return "Every effect is switched off.";
     return "Add source images to start.";
 }
 
@@ -1072,6 +1429,7 @@ void EffectorWindow::startRender() {
 
     const auto sources = sources_;
     const auto stages = stages_;  // a snapshot, like the sources
+    const auto styles = styles_;  // the same, so the worker reads nothing live
     const int width = static_cast<int>(widthSpin_.get_value());
     const int height = static_cast<int>(heightSpin_.get_value());
     const std::uint64_t seed = seed_;
@@ -1079,13 +1437,13 @@ void EffectorWindow::startRender() {
     spinner_.start();
     recreateButton_.set_sensitive(false);
 
-    worker_ = std::thread([this, sources, stages, width, height, seed] {
+    worker_ = std::thread([this, sources, stages, styles, width, height, seed] {
         Image image;
         std::string error;
         const auto start = std::chrono::steady_clock::now();
         try {
             Rng rng(seed);
-            image = renderChain(stages, *sources, width, height, rng);
+            image = renderChain(stages, *sources, width, height, rng, styles);
         } catch (const std::exception& exception) {
             error = exception.what();
         }
@@ -1193,6 +1551,7 @@ Project EffectorWindow::currentProject() const {
     Project project;
     for (const auto& image : *sources_) project.sources.push_back(image.path());
     project.stages = stages_;
+    project.styles = styles_;
     project.width = static_cast<int>(widthSpin_.get_value());
     project.height = static_cast<int>(heightSpin_.get_value());
     project.format = canvasFormatKey();
@@ -1204,9 +1563,12 @@ Project EffectorWindow::currentProject() const {
 void EffectorWindow::applyProject(const Project& project, const std::string& path) {
     // One render at the end rather than one per piece of the project.
     suspendRender_ = true;
+    closeStyleWindow();  // it names a style of the project being replaced
     clearSources();
 
     stages_ = project.stages;
+    // Before the rows and the params: both ask which style a stage follows.
+    styles_ = project.styles;
     selectedStage_ = 0;
     rebuildChainRows();
     buildParamWidgets();
@@ -1242,9 +1604,7 @@ void EffectorWindow::clearSources() {
 }
 
 void EffectorWindow::onNewProject() {
-    Project fresh;
-    fresh.stages.push_back({effects_.front(), effects_.front()->defaults()});
-    applyProject(fresh, "");
+    applyProject(Project(), "");  // same empty chain the window opens with
 }
 
 void EffectorWindow::onOpenProject() {

src/gui/window.hpp

@@ -7,6 +7,7 @@
 #include <memory>
 #include <mutex>
 #include <optional>
+#include <functional>
 #include <string>
 #include <thread>
 #include <vector>
@@ -16,6 +17,7 @@
 #include "core/effect.hpp"
 #include "core/image.hpp"
 #include "core/project.hpp"
+#include "core/style.hpp"
 
 namespace effector {
 
@@ -30,7 +32,29 @@ private:
     Gtk::Widget& buildPreview();
     void buildParamWidgets();
     ParamValues paramValues() const;
-    void storeParams();     // widgets -> the selected stage's values
+    void storeParams();     // widgets -> the selected stage, and its style
+
+    // The style row above the params of a stage whose effect takes one.
+    std::unique_ptr<Gtk::Widget> buildStyleRow(const ChainStage& stage);
+    // The names of the pool the selected stage draws from, in the order the
+    // dropdown shows them.
+    std::vector<std::string> styleNames(const std::string& kind) const;
+    // The style the selected stage follows, or nullptr when it stands alone.
+    Style* selectedStyle();
+    const Style* selectedStyle() const;
+    void onStyleSelected();   // the dropdown: adopt, drop, or make a new one
+    void adoptStyle(const std::string& name);
+    void onNewStyle();
+    void onRenameStyle();
+    void onRemoveStyle();
+    // The little modal that asks for a style name, used by both New and
+    // Rename. Built from scratch each time, like the progress window.
+    void askStyleName(const std::string& title, const std::string& initial,
+                      const std::function<void(const std::string&)>& accept);
+    void closeStyleWindow();
+    // A name that is empty, or already used in the pool, is refused in place.
+    bool styleNameTaken(const std::string& kind, const std::string& name) const;
+
     void applyInitial(const Options& initial);
 
     bool canRender() const;     // a chain, and sources if the chain needs them
@@ -49,6 +73,8 @@ private:
     void cancelLoad();
     void appendSourceRow(const std::string& path);
     void updateSourcesLabel();  // the collapsed expander shows the count
+    // The whole sources section only shows while the chain asks for sources.
+    void updateSourcesVisible();
 
     Gtk::Widget& buildCanvasSection();
     // "" while "Custom" is picked, otherwise the format's key.
@@ -64,6 +90,8 @@ private:
     void appendStage(const EffectPtr& effect);
     void removeStage(int index);
     void moveStage(int index, int delta);
+    // The check on a chain row: the stage stays where it is, and is skipped.
+    void setStageEnabled(int index, bool enabled);
     void rebuildChainRows();
     void selectStage(int index);
     void onStageSelected();
@@ -110,6 +138,9 @@ private:
     // The cascade: stage 0 works on the sources, every later stage on the
     // output of the one before it.
     std::vector<ChainStage> stages_;
+    // The named looks the stages share. Edited through the params of whichever
+    // stage follows one, and saved with the project.
+    StyleSet styles_;
     int selectedStage_ = 0;
     bool suspendStageSelect_ = false;  // guards the rebuild of the chain rows
     Image result_;
@@ -162,6 +193,11 @@ private:
     // 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::DropDown* styleDrop_ = nullptr;   // null while the stage takes no style
+    bool suspendStyleSelect_ = false;      // guards filling the dropdown
+    std::shared_ptr<Gtk::Window> styleWindow_;
+    Gtk::Entry* styleNameEntry_ = nullptr;  // nulled with the window
+    Gtk::Label* styleNameError_ = nullptr;
     Gtk::Grid paramGrid_;
     Gtk::Label paramHeading_;
     Gtk::Entry seedEntry_;

src/main.cpp

@@ -18,7 +18,14 @@
 namespace {
 
 int renderHeadless(const effector::Options& options) {
-    const auto stages = effector::buildChain(options);
+    auto stages = effector::buildChain(options);
+    // A headless run always renders something, so with no -e the default
+    // effect stands in. The interface has no such need: it opens on an empty
+    // chain and waits for the user to add the first stage.
+    if (stages.empty()) {
+        const auto effect = effector::effectByName(effector::defaultEffectName());
+        stages.push_back({effect, effect->defaults()});
+    }
 
     std::vector<effector::LoadFailure> failures;
 

tests/core_tests.cpp

@@ -1,5 +1,6 @@
 // Small self-contained checks for the pieces that have no GUI dependency.
 
+#include <algorithm>
 #include <array>
 #include <atomic>
 #include <clocale>
@@ -21,6 +22,7 @@
 #include "core/project.hpp"
 #include "core/registry.hpp"
 #include "core/rng.hpp"
+#include "core/style.hpp"
 #include "effects/blur.hpp"
 #include "effects/grain.hpp"
 #include "effects/monochrome.hpp"
@@ -828,6 +830,109 @@ int main() {
         std::filesystem::remove_all(dir);
     }
 
+    std::cout << "text styles\n";
+    {
+        const std::vector<Image> blue = {solid(200, 120, 0, 0, 255)};
+        const auto countRed = [](const Image& image) {
+            int found = 0;
+            for (int y = 0; y < image.height(); ++y) {
+                for (int x = 0; x < image.width(); ++x) {
+                    if (isRed(pixel(image, x, y))) ++found;
+                }
+            }
+            return found;
+        };
+
+        const auto specs = styleSpecs("text");
+        const auto carries = [&specs](const std::string& key) {
+            return std::any_of(specs.begin(), specs.end(),
+                               [&key](const ParamSpec& spec) { return spec.key == key; });
+        };
+        check(specs.size() == 11, "a text style carries eleven params");
+        check(carries("font_family") && carries("font_weight") && carries("font_italic") &&
+                  carries("font_size") && carries("text_color") && carries("text_align") &&
+                  carries("text_opacity") && carries("text_box") && carries("box_color") &&
+                  carries("box_padding") && carries("box_opacity"),
+              "the look is what it carries");
+        check(!carries("text") && !carries("text_wrap") && !carries("box_image") &&
+                  !carries("text_center") && !carries("text_offset_x") && !carries("text_offset_y"),
+              "the words and the placement stay on the stage");
+        check(styleSpecs("").empty() && styleSpecs("nosuchkind").empty(),
+              "a kind no effect claims carries nothing");
+        check(paramDouble(styleDefaults("text"), "font_size", 0.0) == 96.0 &&
+                  styleDefaults("text").size() == specs.size(),
+              "a fresh style starts at the defaults the effect declares");
+
+        StyleSet styles = {{"text", "Title", styleDefaults("text")}};
+        styles[0].values["font_size"] = 40.0;
+        styles[0].values["text_color"] = std::string("#ff0000");
+        check(findStyle(styles, "text", "Title") != nullptr &&
+                  findStyle(styles, "text", "Caption") == nullptr &&
+                  findStyle(styles, "other", "Title") == nullptr,
+              "a style is found by its kind and its name");
+
+        const ChainStage own{effectByName("text"),
+                             {{"text", std::string("Hi")},
+                              {"font_size", 12.0},
+                              {"text_color", std::string("#ff0000")}},
+                             true,
+                             ""};
+        ChainStage follows = own;
+        follows.style = "Title";
+
+        check(paramDouble(stageValues(own, styles), "font_size", 0.0) == 12.0,
+              "a stage on its own keeps the settings it was given");
+        check(paramDouble(stageValues(follows, styles), "font_size", 0.0) == 40.0,
+              "a stage following a style takes the style's");
+        check(paramText(stageValues(follows, styles), "text", "") == "Hi",
+              "and keeps its own words either way");
+
+        Rng ownRng(3);
+        Rng followRng(3);
+        const auto small = renderChain({own}, blue, 200, 120, ownRng, styles);
+        const auto large = renderChain({follows}, blue, 200, 120, followRng, styles);
+        check(countRed(small) > 0 && countRed(large) > countRed(small),
+              "the style is what actually reaches the render");
+
+        // The whole point: two stages, one style, one look in two places.
+        ChainStage left = follows;
+        left.values["text_offset_x"] = -40L;
+        ChainStage right = follows;
+        right.values["text_offset_x"] = 40L;
+        Rng leftRng(3);
+        Rng rightRng(3);
+        const auto leftImage = renderChain({left}, blue, 200, 120, leftRng, styles);
+        const auto rightImage = renderChain({right}, blue, 200, 120, rightRng, styles);
+        check(countRed(leftImage) == countRed(large) && countRed(rightImage) == countRed(large),
+              "two stages following one style ink the same glyphs");
+        check(!sameBytes(leftImage, rightImage), "each in its own place");
+
+        StyleSet edited = styles;
+        edited[0].values["font_size"] = 20.0;
+        Rng editedLeft(3);
+        Rng editedRight(3);
+        const auto afterLeft = renderChain({left}, blue, 200, 120, editedLeft, edited);
+        const auto afterRight = renderChain({right}, blue, 200, 120, editedRight, edited);
+        check(countRed(afterLeft) < countRed(leftImage) &&
+                  countRed(afterRight) < countRed(rightImage) &&
+                  countRed(afterLeft) == countRed(afterRight),
+              "editing the style moves every stage that follows it");
+
+        ChainStage missing = follows;
+        missing.style = "Nowhere";
+        Rng missingRng(3);
+        Rng bareRng(3);
+        check(sameBytes(renderChain({missing}, blue, 200, 120, missingRng, styles),
+                        renderChain({own}, blue, 200, 120, bareRng)),
+              "naming a style that is not there renders as standing alone");
+
+        Rng noStyleRng(3);
+        Rng noPoolRng(3);
+        check(sameBytes(renderChain({follows}, blue, 200, 120, noStyleRng, {}),
+                        renderChain({own}, blue, 200, 120, noPoolRng)),
+              "and so does a chain handed no styles at all");
+    }
+
     std::cout << "chain\n";
     {
         std::vector<Image> sources = {gradient(1200, 800, 10), gradient(640, 480, 90)};
@@ -889,6 +994,50 @@ int main() {
             scanlinesThrew = true;
         }
         check(scanlinesThrew, "scanlines still refuses to run without sources");
+
+        // A stage switched off is skipped whole: it is handed no image and
+        // draws nothing from the shared rng, so the cascade comes out exactly
+        // as it would have without that stage in it at all.
+        ChainStage blurOff = blur;
+        blurOff.enabled = false;
+        ChainStage scanlinesOff = scanlines;
+        scanlinesOff.enabled = false;
+
+        Rng skipping(5);
+        const auto skipped = renderChain({scanlines, blurOff}, sources, 400, 300, skipping);
+        check(sameBytes(skipped, direct), "a disabled stage is skipped, its rng draws with it");
+        check(chainTitle({scanlines, blurOff}) == "Scanlines", "the title names only what ran");
+        check(chainSlug({scanlines, blurOff}) == "scanlines", "and so does the slug");
+
+        // Whoever leads the chain is handed the sources, and that is the first
+        // stage still switched on.
+        check(!chainNeedsSources({scanlinesOff, blur}),
+              "a disabled leading stage leaves the question to the stage behind it");
+        Rng leading(5);
+        const auto ledByBlur = renderChain({scanlinesOff, blur}, {}, 200, 150, leading);
+        check(sameBytes(ledByBlur, sourceless), "the stage behind it leads instead");
+
+        bool allOffThrew = false;
+        try {
+            Rng rng(1);
+            renderChain({scanlinesOff, blurOff}, sources, 100, 100, rng);
+        } catch (const EffectError&) {
+            allOffThrew = true;
+        }
+        check(allOffThrew, "a chain with every stage disabled throws");
+
+        // The command line switches a stage off by effect name, the same way
+        // the params pick out the stage they belong to.
+        const auto cli =
+            parsedOptions({"-e", "scanlines", "-e", "blur", "--disable", "blur", "--radius", "4"});
+        check(cli.has_value(), "--disable is accepted");
+        const auto cliStages = buildChain(*cli);
+        check(cliStages.size() == 2 && cliStages[0].enabled && !cliStages[1].enabled,
+              "--disable keeps the stage in the chain and switches it off");
+        check(paramInt(cliStages[1].values, "radius", 0) == 4,
+              "a switched-off stage still takes its flags");
+        check(!parsedOptions({"-e", "blur", "--disable", "nope"}).has_value(),
+              "--disable refuses an unknown effect");
     }
 
     std::cout << "project\n";
@@ -949,6 +1098,86 @@ int main() {
         check(stripped.stages[0].values.find("radius") == stripped.stages[0].values.end(),
               "a stage only keeps the params its effect declares");
 
+        // A stage switched off keeps its place and its settings in the file and
+        // comes back switched off; nothing is written for the ordinary case.
+        Project toggled = project;
+        toggled.stages[1].enabled = false;
+        const auto offFile = dir / "disabled.json";
+        saveProject(offFile.string(), toggled);
+        const auto offBack = loadProject(offFile.string());
+        check(offBack.stages.size() == 2 && offBack.stages[0].enabled && !offBack.stages[1].enabled,
+              "a switched-off stage comes back switched off");
+        check(paramInt(offBack.stages[1].values, "radius", 0) == 3,
+              "and with the settings it was switched off with");
+        check(projectToJson(toggled).find("\"enabled\"") != std::string::npos &&
+                  projectToJson(project).find("\"enabled\"") == std::string::npos,
+              "only a switched-off stage is written out as one");
+
+        // Styles ride along with the chain, and the stages come back pointing
+        // at the ones they followed.
+        Project stylish;
+        stylish.styles = {{"text", "Title", styleDefaults("text")},
+                          {"text", "Caption", styleDefaults("text")}};
+        stylish.styles[0].values["font_size"] = 120.0;
+        stylish.styles[0].values["text_color"] = std::string("#ff8800");
+        stylish.styles[1].values["font_family"] = std::string("Serif");
+        stylish.stages.push_back(
+            {effectByName("text"), {{"text", std::string("One")}}, true, "Title"});
+        stylish.stages.push_back(
+            {effectByName("text"), {{"text", std::string("Two")}}, true, "Title"});
+        stylish.stages.push_back(
+            {effectByName("text"), {{"text", std::string("Three")}}, true, ""});
+
+        const auto styleFile = dir / "styles.json";
+        saveProject(styleFile.string(), stylish);
+        const auto styleBack = loadProject(styleFile.string());
+        check(styleBack.styles.size() == 2 && styleBack.styles[0].name == "Title" &&
+                  styleBack.styles[1].name == "Caption" && styleBack.styles[0].kind == "text",
+              "the styles survive a round trip");
+        check(paramDouble(styleBack.styles[0].values, "font_size", 0.0) == 120.0 &&
+                  paramText(styleBack.styles[0].values, "text_color", "") == "#ff8800" &&
+                  paramText(styleBack.styles[1].values, "font_family", "") == "Serif",
+              "and so do the values they carry");
+        check(styleBack.stages[0].style == "Title" && styleBack.stages[1].style == "Title" &&
+                  styleBack.stages[2].style.empty(),
+              "and which stage follows which");
+        check(paramDouble(stageValues(styleBack.stages[0], styleBack.styles), "font_size", 0.0) ==
+                      120.0 &&
+                  paramDouble(stageValues(styleBack.stages[2], styleBack.styles), "font_size",
+                              0.0) == 96.0,
+              "a reloaded project resolves its styles as it did");
+        check(projectToJson(stylish).find("\"styles\"") != std::string::npos &&
+                  projectToJson(project).find("\"styles\"") == std::string::npos &&
+                  projectToJson(project).find("\"style\"") == std::string::npos,
+              "a project with no styles writes nothing about them");
+
+        // Everything that can be wrong with a style is dropped on its own,
+        // leaving a project that still opens and still renders.
+        const auto lenient = projectFromJson(R"({
+          "styles": [
+            {"kind": "nosuchkind", "name": "Ghost", "params": {}},
+            {"kind": "text", "name": "", "params": {}},
+            {"kind": "text", "name": "Title", "params": {"font_size": 60, "nonsense": 3}},
+            {"kind": "text", "name": "Title", "params": {"font_size": 12}}
+          ],
+          "chain": [
+            {"effect": "text", "style": "Title", "params": {"text": "Kept"}},
+            {"effect": "text", "style": "Gone", "params": {"font_size": 30}},
+            {"effect": "blur", "style": "Title", "params": {}}
+          ]
+        })");
+        check(lenient.styles.size() == 1 && lenient.styles[0].name == "Title",
+              "an unknown kind, a nameless style and a repeated name are dropped");
+        check(paramDouble(lenient.styles[0].values, "font_size", 0.0) == 60.0 &&
+                  lenient.styles[0].values.find("nonsense") == lenient.styles[0].values.end(),
+              "a style only keeps the params its kind declares, and the first of a name wins");
+        check(lenient.stages[0].style == "Title" && lenient.stages[1].style.empty(),
+              "a stage naming a style that is not there is cut loose");
+        check(lenient.stages[2].style.empty(),
+              "and so is one whose effect takes no style at all");
+        check(paramDouble(stageValues(lenient.stages[1], lenient.styles), "font_size", 0.0) == 30.0,
+              "which leaves it rendering the values written beside it");
+
         int refusals = 0;
         for (const char* bad : {"{\"chain\": [{\"effect\": \"nope\"}]}", "{\"chain\": []}",
                                 "{\"chain\": [{\"effect\": \"blur\"},]}", "not json at all"}) {