diff --git a/CMakeLists.txt b/CMakeLists.txt
index 57540ef..f47c058 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -28,6 +28,8 @@ add_library(effector_core STATIC
     src/core/registry.cpp
     src/core/rng.cpp
     src/effects/blur.cpp
+    src/effects/grain.cpp
+    src/effects/monochrome.cpp
     src/effects/scanlines.cpp
 )
 target_include_directories(effector_core PUBLIC src)
diff --git a/src/cli/options.cpp b/src/cli/options.cpp
index 504a159..6b24f41 100644
--- a/src/cli/options.cpp
+++ b/src/cli/options.cpp
@@ -24,7 +24,10 @@ 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;
@@ -66,7 +69,9 @@ Options parseOptions(int argc, char** argv) {
         } 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 == "--seed") {
             const std::string value = requireValue(argc, argv, i, arg);
@@ -131,7 +136,8 @@ std::string usage(const char* programName) {
         << "Options:\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"
         << "      --seed N          random seed (default: random)\n"
         << "      --list-effects    list available effects and exit\n"
         << "  -h, --help            show this help and exit\n";
diff --git a/src/core/chain.hpp b/src/core/chain.hpp
index 72d95ef..420470b 100644
--- a/src/core/chain.hpp
+++ b/src/core/chain.hpp
@@ -10,6 +10,13 @@
 
 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;
diff --git a/src/core/project.cpp b/src/core/project.cpp
index d0de8c7..2712ab5 100644
--- a/src/core/project.cpp
+++ b/src/core/project.cpp
@@ -283,6 +283,21 @@ std::string relativeTo(const std::string& path, const std::string& directory) {
     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);
+}
+
 std::string resolve(const std::string& path, const std::string& directory) {
     std::filesystem::path file(path);
     if (file.is_absolute() || directory.empty()) return path;
@@ -350,12 +365,11 @@ Project projectFromJson(const std::string& text, const std::string& directory) {
     Project project;
 
     if (const auto* canvas = root.member("canvas")) {
-        if (const auto* width = canvas->member("width")) project.width = static_cast<int>(width->number);
+        if (const auto* width = canvas->member("width")) project.width = canvasSide(*width, "width");
         if (const auto* height = canvas->member("height")) {
-            project.height = static_cast<int>(height->number);
+            project.height = canvasSide(*height, "height");
         }
     }
-    if (project.width <= 0 || project.height <= 0) throw EffectError("the canvas size is not positive");
 
     if (const auto* seed = root.member("seed")) {
         if (seed->kind == Json::Kind::Number) {
diff --git a/src/core/registry.cpp b/src/core/registry.cpp
index b540896..efd0358 100644
--- a/src/core/registry.cpp
+++ b/src/core/registry.cpp
@@ -1,6 +1,8 @@
 #include "core/registry.hpp"
 
 #include "effects/blur.hpp"
+#include "effects/grain.hpp"
+#include "effects/monochrome.hpp"
 #include "effects/scanlines.hpp"
 
 namespace effector {
@@ -10,6 +12,8 @@ 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>(),
     };
     return registry;
 }
diff --git a/src/effects/blur.cpp b/src/effects/blur.cpp
index e69f0ef..99f2f28 100644
--- a/src/effects/blur.cpp
+++ b/src/effects/blur.cpp
@@ -5,6 +5,7 @@
 #include <vector>
 
 #include "core/parallel.hpp"
+#include "effects/source_fill.hpp"
 
 namespace effector {
 
@@ -24,31 +25,6 @@ const std::vector<ParamSpec>& BlurEffect::params() const {
 
 namespace {
 
-// 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.
-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;
-            }
-        }
-    });
-}
-
 // 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.
diff --git a/src/effects/grain.cpp b/src/effects/grain.cpp
new file mode 100644
index 0000000..0166557
--- /dev/null
+++ b/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
diff --git a/src/effects/grain.hpp b/src/effects/grain.hpp
new file mode 100644
index 0000000..3cf1826
--- /dev/null
+++ b/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
diff --git a/src/effects/monochrome.cpp b/src/effects/monochrome.cpp
new file mode 100644
index 0000000..dd291a4
--- /dev/null
+++ b/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
diff --git a/src/effects/monochrome.hpp b/src/effects/monochrome.hpp
new file mode 100644
index 0000000..d116b02
--- /dev/null
+++ b/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
diff --git a/src/effects/source_fill.hpp b/src/effects/source_fill.hpp
new file mode 100644
index 0000000..9a96059
--- /dev/null
+++ b/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
diff --git a/src/gui/window.cpp b/src/gui/window.cpp
index 0c01216..c46a393 100644
--- a/src/gui/window.cpp
+++ b/src/gui/window.cpp
@@ -85,13 +85,6 @@ void EffectorWindow::buildUi() {
     menuButton_.set_popover(static_cast<Gtk::PopoverMenu&>(buildMainMenu()));
     header->pack_end(menuButton_);
 
-    auto* addButton = Gtk::make_managed<Gtk::Button>();
-    addButton->set_icon_name("list-add-symbolic");
-    addButton->set_tooltip_text("Add images");
-    addButton->signal_clicked().connect(sigc::mem_fun(*this, &EffectorWindow::onAddImages));
-    header->pack_start(*addButton);
-    addButton_ = addButton;
-
     saveButton_.set_sensitive(false);
     saveButton_.signal_clicked().connect(sigc::mem_fun(*this, &EffectorWindow::onSave));
     header->pack_end(saveButton_);
@@ -120,33 +113,73 @@ void EffectorWindow::buildUi() {
     root->append(*paned);
 }
 
+// The chain is what the interface is about, so it comes first and the params
+// of the selected stage sit right below it. The sources are a prerequisite,
+// not the subject: they live at the bottom in a collapsed expander.
 Gtk::Widget& EffectorWindow::buildSidebar() {
     auto* sidebar = Gtk::make_managed<Gtk::Box>(Gtk::Orientation::VERTICAL, 12);
     sidebar->set_margin(12);
 
-    sidebar->append(*sectionLabel("Source images"));
+    sidebar->append(buildChainSection());
+
+    paramHeading_.set_xalign(0.0F);
+    paramHeading_.add_css_class("heading");
+    sidebar->append(paramHeading_);
+    paramGrid_.set_column_spacing(8);
+    paramGrid_.set_row_spacing(6);
+    auto* paramScroller = Gtk::make_managed<Gtk::ScrolledWindow>();
+    paramScroller->set_policy(Gtk::PolicyType::NEVER, Gtk::PolicyType::AUTOMATIC);
+    paramScroller->set_child(paramGrid_);
+    paramScroller->set_vexpand(true);
+    sidebar->append(*paramScroller);
+    rebuildChainRows();
+    buildParamWidgets();
+
+    sidebar->append(*Gtk::make_managed<Gtk::Separator>(Gtk::Orientation::HORIZONTAL));
+
     sourceList_.set_selection_mode(Gtk::SelectionMode::NONE);
     sourceList_.add_css_class("boxed-list");
     auto* empty = Gtk::make_managed<Gtk::Label>("No images yet");
-    empty->set_margin_top(18);
-    empty->set_margin_bottom(18);
+    empty->set_margin_top(12);
+    empty->set_margin_bottom(12);
     sourceList_.set_placeholder(*empty);
 
     auto* scroller = Gtk::make_managed<Gtk::ScrolledWindow>();
     scroller->set_policy(Gtk::PolicyType::NEVER, Gtk::PolicyType::AUTOMATIC);
     scroller->set_child(sourceList_);
-    scroller->set_size_request(-1, 170);
-    scroller->set_vexpand(true);
-    sidebar->append(*scroller);
+    scroller->set_size_request(-1, 140);
+    scroller->set_margin_top(6);
+
+    auto* sourcesBox = Gtk::make_managed<Gtk::Box>(Gtk::Orientation::VERTICAL, 6);
+    sourcesBox->append(*scroller);
+
+    // Same shape as "Add effect": the icon and the text are put together by
+    // hand because set_label() and set_icon_name() replace one another.
+    auto* addBox = Gtk::make_managed<Gtk::Box>(Gtk::Orientation::HORIZONTAL, 6);
+    addBox->append(*Gtk::make_managed<Gtk::Image>(
+        Gio::ThemedIcon::create("list-add-symbolic")));
+    addBox->append(*Gtk::make_managed<Gtk::Label>("Add images"));
+    auto* addButton = Gtk::make_managed<Gtk::Button>();
+    addButton->set_child(*addBox);
+    addButton->set_halign(Gtk::Align::START);
+    addButton->set_tooltip_text("Add source images");
+    addButton->signal_clicked().connect(sigc::mem_fun(*this, &EffectorWindow::onAddImages));
+    sourcesBox->append(*addButton);
+    addButton_ = addButton;
+
+    sourcesExpander_.set_child(*sourcesBox);
+    sourcesExpander_.set_expanded(false);
+    updateSourcesLabel();
+    sidebar->append(sourcesExpander_);
 
     sidebar->append(*sectionLabel("Canvas"));
     auto* grid = Gtk::make_managed<Gtk::Grid>();
     grid->set_column_spacing(8);
     grid->set_row_spacing(6);
-    widthSpin_.set_range(1, 20000);
+    widthSpin_.set_range(kMinCanvasSide, kMaxCanvasSide);
     widthSpin_.set_increments(1, 100);
     widthSpin_.set_value(1920);
-    heightSpin_.set_range(1, 20000);
+    heightSpin_.set_range(kMinCanvasSide, kMaxCanvasSide);
     heightSpin_.set_increments(1, 100);
     heightSpin_.set_value(1080);
     for (auto* spin : {&widthSpin_, &heightSpin_}) {
@@ -163,17 +196,6 @@ Gtk::Widget& EffectorWindow::buildSidebar() {
     grid->attach(heightSpin_, 1, 1);
     sidebar->append(*grid);
 
-    sidebar->append(buildChainSection());
-
-    paramHeading_.set_xalign(0.0F);
-    paramHeading_.add_css_class("heading");
-    sidebar->append(paramHeading_);
-    paramGrid_.set_column_spacing(8);
-    paramGrid_.set_row_spacing(6);
-    sidebar->append(paramGrid_);
-    rebuildChainRows();
-    buildParamWidgets();
-
     auto* seedBox = Gtk::make_managed<Gtk::Box>(Gtk::Orientation::HORIZONTAL, 8);
     auto* seedLabel = Gtk::make_managed<Gtk::Label>("Seed");
     seedLabel->set_xalign(0.0F);
@@ -187,6 +209,15 @@ Gtk::Widget& EffectorWindow::buildSidebar() {
     return *sidebar;
 }
 
+// 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() {
+    int count = 0;
+    while (sourceList_.get_row_at_index(count) != nullptr) ++count;
+    sourcesExpander_.set_label(count == 0 ? "Source images"
+                                          : "Source images (" + std::to_string(count) + ")");
+}
+
 Gtk::Widget& EffectorWindow::buildPreview() {
     picture_.set_can_shrink(true);
     picture_.set_content_fit(Gtk::ContentFit::CONTAIN);
@@ -565,6 +596,7 @@ void EffectorWindow::appendSourceRow(const std::string& path) {
 
     row->set_child(*box);
     sourceList_.append(*row);
+    updateSourcesLabel();
 }
 
 void EffectorWindow::removeSource(Gtk::ListBoxRow* row) {
@@ -575,6 +607,7 @@ void EffectorWindow::removeSource(Gtk::ListBoxRow* row) {
         }
     });
     sourceList_.remove(*row);
+    updateSourcesLabel();
     requestRender();
 }
 
@@ -622,7 +655,12 @@ Gtk::Widget& EffectorWindow::buildChainSection() {
     chainList_.set_placeholder(*empty);
     chainList_.signal_row_selected().connect(
         [this](Gtk::ListBoxRow*) { onStageSelected(); });
-    box->append(chainList_);
+
+    auto* scroller = Gtk::make_managed<Gtk::ScrolledWindow>();
+    scroller->set_policy(Gtk::PolicyType::NEVER, Gtk::PolicyType::AUTOMATIC);
+    scroller->set_child(chainList_);
+    scroller->set_size_request(-1, 150);
+    box->append(*scroller);
 
     // set_label() and set_icon_name() replace one another, so the icon and the
     // text are put together by hand.
@@ -995,6 +1033,7 @@ void EffectorWindow::applyProject(const Project& project, const std::string& pat
 
 void EffectorWindow::clearSources() {
     while (auto* row = sourceList_.get_row_at_index(0)) sourceList_.remove(*row);
+    updateSourcesLabel();
     mutateSources([](SourceList& list) { list.clear(); });
     loadQueue_.clear();
     if (loading_) dropLoadedImages_ = true;
diff --git a/src/gui/window.hpp b/src/gui/window.hpp
index 5c70fa5..99731e5 100644
--- a/src/gui/window.hpp
+++ b/src/gui/window.hpp
@@ -48,6 +48,7 @@ private:
     void closeLoadWindow();
     void cancelLoad();
     void appendSourceRow(const std::string& path);
+    void updateSourcesLabel();  // the collapsed expander shows the count
     void removeSource(Gtk::ListBoxRow* row);
 
     Gtk::Widget& buildChainSection();
@@ -139,6 +140,7 @@ private:
     std::unique_ptr<Gtk::Window> loadWindow_;
     Gtk::ProgressBar* loadBar_ = nullptr;
     Gtk::Label* loadFileLabel_ = nullptr;
+    Gtk::Expander sourcesExpander_;
     Gtk::ListBox sourceList_;
     Gtk::SpinButton widthSpin_;
     Gtk::SpinButton heightSpin_;
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 5fcf2aa..e98d7bd 100644
--- a/tests/CMakeLists.txt
+++ b/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)
 
diff --git a/tests/core_tests.cpp b/tests/core_tests.cpp
index dd1ce27..ea4ed87 100644
--- a/tests/core_tests.cpp
+++ b/tests/core_tests.cpp
@@ -7,14 +7,19 @@
 #include <fstream>
 #include <iostream>
 #include <mutex>
+#include <optional>
 #include <string>
+#include <utility>
 #include <vector>
 
+#include "cli/options.hpp"
 #include "core/chain.hpp"
 #include "core/project.hpp"
 #include "core/registry.hpp"
 #include "core/rng.hpp"
 #include "effects/blur.hpp"
+#include "effects/grain.hpp"
+#include "effects/monochrome.hpp"
 #include "effects/scanlines.hpp"
 
 namespace {
@@ -43,6 +48,28 @@ effector::Image gradient(int width, int height, int shade) {
     return image;
 }
 
+// A canvas taken from a project file, written by hand rather than by
+// projectToJson, so a size no writer would produce can be fed to the reader.
+std::string projectWithCanvas(const std::string& width, const std::string& height) {
+    return "{\"canvas\": {\"width\": " + width + ", \"height\": " + height +
+           "},\n \"chain\": [{\"effect\": \"blur\"}]}";
+}
+
+// The canvas a `-s` value ends up as, or nothing when the parser refuses it.
+std::optional<std::pair<int, int>> canvasFromSize(const std::string& size) {
+    char program[] = "effector";
+    char flag[] = "-s";
+    std::vector<char> value(size.begin(), size.end());
+    value.push_back('\0');
+    char* argv[] = {program, flag, value.data()};
+    try {
+        const auto options = effector::parseOptions(3, argv);
+        return std::pair<int, int>{options.width, options.height};
+    } catch (const effector::EffectError&) {
+        return std::nullopt;
+    }
+}
+
 bool sameBytes(const effector::Image& a, const effector::Image& b) {
     if (a.width() != b.width() || a.height() != b.height()) return false;
     return std::equal(a.data(), a.data() + a.byteSize(), b.data());
@@ -71,6 +98,45 @@ effector::Image renderBlur(const std::vector<effector::Image>& sources, int widt
     return effect.render(sources, width, height, rng, merged);
 }
 
+effector::Image renderGrain(const std::vector<effector::Image>& sources, int width, int height,
+                            std::uint64_t seed, const effector::ParamValues& values) {
+    effector::GrainEffect effect;
+    effector::Rng rng(seed);
+    auto merged = effect.defaults();
+    for (const auto& [key, value] : values) merged[key] = value;
+    return effect.render(sources, width, height, rng, merged);
+}
+
+effector::Image renderMono(const std::vector<effector::Image>& sources, int width, int height,
+                           std::uint64_t seed, const effector::ParamValues& values) {
+    effector::MonochromeEffect effect;
+    effector::Rng rng(seed);
+    auto merged = effect.defaults();
+    for (const auto& [key, value] : values) merged[key] = value;
+    return effect.render(sources, width, height, rng, merged);
+}
+
+// A flat field of one colour, to read the channel weighting off directly.
+effector::Image solid(int width, int height, std::uint8_t r, std::uint8_t g, std::uint8_t b) {
+    effector::Image image(width, height);
+    for (int y = 0; y < height; ++y) {
+        auto* row = image.row(y);
+        for (int x = 0; x < width; ++x) {
+            row[x * 3 + 0] = r;
+            row[x * 3 + 1] = g;
+            row[x * 3 + 2] = b;
+        }
+    }
+    return image;
+}
+
+// A flat field of one shade, to measure what the grain does to it.
+effector::Image flat(int width, int height, std::uint8_t shade) {
+    effector::Image image(width, height);
+    std::fill(image.data(), image.data() + image.byteSize(), shade);
+    return image;
+}
+
 effector::Image render(const std::vector<effector::Image>& sources, int width, int height,
                        std::uint64_t seed, const effector::ParamValues& values) {
     effector::ScanlinesEffect effect;
@@ -202,6 +268,142 @@ int main() {
         check(allBlack, "without sources blur renders an empty canvas");
     }
 
+    std::cout << "grain\n";
+    {
+        std::vector<Image> sources = {gradient(1200, 800, 10), gradient(640, 480, 90)};
+
+        const auto first = renderGrain(sources, 400, 300, 4, {});
+        const auto again = renderGrain(sources, 400, 300, 4, {});
+        const auto other = renderGrain(sources, 400, 300, 11, {});
+        check(first.width() == 400 && first.height() == 300, "canvas has the requested size");
+        check(sameBytes(first, again), "same seed reproduces the image");
+        check(!sameBytes(first, other), "a different seed changes the image");
+
+        // Mid grey is where the grain is strongest, so it must move, and stay
+        // grey while the channels are drawn together.
+        std::vector<Image> grey = {flat(200, 100, 128)};
+        const auto grained = renderGrain(grey, 200, 100, 3, {{"grain_amount", 0.5}});
+        bool moved = false;
+        bool neutral = true;
+        for (int x = 0; x < 200; ++x) {
+            const auto* px = grained.row(50) + x * 3;
+            moved = moved || px[0] != 128;
+            neutral = neutral && px[0] == px[1] && px[1] == px[2];
+        }
+        check(moved, "grain moves a flat mid grey");
+        check(neutral, "monochrome grain keeps the channels equal");
+
+        const auto coloured = renderGrain(grey, 200, 100, 3, {{"grain_colour", true}});
+        bool split = false;
+        for (int x = 0; x < 200; ++x) {
+            const auto* px = coloured.row(50) + x * 3;
+            split = split || px[0] != px[1] || px[1] != px[2];
+        }
+        check(split, "coloured grain pulls the channels apart");
+
+        // With the midtone weighting on, black and white are left alone; with
+        // it off the grain lands everywhere.
+        const auto blackWeighted = renderGrain({flat(64, 64, 0)}, 64, 64, 2, {});
+        bool untouched = true;
+        for (std::size_t i = 0; i < blackWeighted.byteSize(); ++i) {
+            untouched = untouched && blackWeighted.data()[i] == 0;
+        }
+        check(untouched, "the midtone weighting leaves black alone");
+
+        const auto blackFlat =
+            renderGrain({flat(64, 64, 0)}, 64, 64, 2, {{"grain_midtones", false}});
+        bool lifted = false;
+        for (std::size_t i = 0; i < blackFlat.byteSize(); ++i) {
+            lifted = lifted || blackFlat.data()[i] != 0;
+        }
+        check(lifted, "without the weighting grain reaches the shadows");
+
+        // Zero amount is a pass through, and a grain wider than the canvas must
+        // still only read the one cell it has.
+        check(sameBytes(renderGrain({flat(32, 32, 128)}, 32, 32, 1, {{"grain_amount", 0.0}}),
+                        flat(32, 32, 128)),
+              "zero amount leaves the image untouched");
+        const auto coarse = renderGrain(grey, 40, 30, 7, {{"grain_size", 64L}});
+        check(coarse.width() == 40 && coarse.height() == 30,
+              "a grain wider than the canvas is clamped");
+
+        // Like blur, grain needs no sources: with none it grains the empty
+        // canvas it starts from instead of refusing to run.
+        const auto blank = renderGrain({}, 100, 100, 1, {{"grain_midtones", false}});
+        bool speckled = blank.width() == 100 && blank.height() == 100;
+        bool any = false;
+        for (std::size_t i = 0; i < blank.byteSize(); ++i) any = any || blank.data()[i] != 0;
+        check(speckled && any, "without sources grain works on an empty canvas");
+    }
+
+    std::cout << "monochrome\n";
+    {
+        std::vector<Image> sources = {gradient(1200, 800, 10), gradient(640, 480, 90)};
+
+        const auto first = renderMono(sources, 400, 300, 4, {});
+        const auto again = renderMono(sources, 400, 300, 4, {});
+        check(first.width() == 400 && first.height() == 300, "canvas has the requested size");
+        check(sameBytes(first, again), "same seed reproduces the image");
+
+        bool grey = true;
+        for (std::size_t i = 0; i + 2 < first.byteSize(); i += 3) {
+            grey = grey && first.data()[i] == first.data()[i + 1] &&
+                   first.data()[i + 1] == first.data()[i + 2];
+        }
+        check(grey, "every pixel comes out neutral grey");
+
+        // A pure red field: the luminance weighting keeps little of it, the red
+        // weighting nearly all, and the blue weighting almost none.
+        std::vector<Image> red = {solid(64, 64, 255, 0, 0)};
+        const auto viaLuma = renderMono(red, 64, 64, 1, {});
+        const auto viaRed = renderMono(red, 64, 64, 1, {{"mono_weighting", std::string("red")}});
+        const auto viaBlue = renderMono(red, 64, 64, 1, {{"mono_weighting", std::string("blue")}});
+        check(viaLuma.data()[0] > 40 && viaLuma.data()[0] < 70, "luminance darkens a pure red");
+        check(viaRed.data()[0] > 190, "the red weighting keeps a pure red light");
+        check(viaBlue.data()[0] < 50, "the blue weighting renders a pure red dark");
+
+        // Contrast works about mid grey: it leaves that alone and pushes the
+        // shades either side of it apart.
+        const auto midHard =
+            renderMono({solid(32, 32, 128, 128, 128)}, 32, 32, 1, {{"mono_contrast", 2.0}});
+        check(midHard.data()[0] >= 127 && midHard.data()[0] <= 129, "contrast pivots on mid grey");
+        const auto darkFlat = renderMono({solid(32, 32, 80, 80, 80)}, 32, 32, 1, {});
+        const auto darkHard =
+            renderMono({solid(32, 32, 80, 80, 80)}, 32, 32, 1, {{"mono_contrast", 2.0}});
+        check(darkHard.data()[0] < darkFlat.data()[0], "contrast deepens the shadows");
+
+        // The toners tint the greys, each in its own direction.
+        const auto sepia = renderMono({solid(32, 32, 128, 128, 128)}, 32, 32, 1,
+                                      {{"mono_tone", std::string("sepia")}});
+        check(sepia.data()[0] > sepia.data()[1] && sepia.data()[1] > sepia.data()[2],
+              "sepia warms the greys");
+        const auto cyan = renderMono({solid(32, 32, 128, 128, 128)}, 32, 32, 1,
+                                     {{"mono_tone", std::string("cyanotype")}});
+        check(cyan.data()[2] > cyan.data()[1] && cyan.data()[1] > cyan.data()[0],
+              "the cyanotype toner cools them");
+
+        // Strength mixes back towards the colour input: zero is a pass through,
+        // half sits between the two.
+        check(sameBytes(
+                  renderMono({solid(32, 32, 200, 40, 10)}, 32, 32, 1, {{"mono_strength", 0.0}}),
+                  solid(32, 32, 200, 40, 10)),
+              "zero strength leaves the colours untouched");
+        const auto half =
+            renderMono({solid(32, 32, 200, 40, 10)}, 32, 32, 1, {{"mono_strength", 0.5}});
+        const auto full = renderMono({solid(32, 32, 200, 40, 10)}, 32, 32, 1, {});
+        check(half.data()[0] > full.data()[0] && half.data()[0] < 200,
+              "half strength sits between the grey and the colour");
+
+        // Like blur and grain it needs no sources: with none it converts the
+        // empty canvas, which stays black.
+        const auto blank = renderMono({}, 50, 50, 1, {});
+        bool allBlack = blank.width() == 50 && blank.height() == 50;
+        for (std::size_t i = 0; i < blank.byteSize(); ++i) {
+            allBlack = allBlack && blank.data()[i] == 0;
+        }
+        check(allBlack, "without sources monochrome renders an empty canvas");
+    }
+
     std::cout << "chain\n";
     {
         std::vector<Image> sources = {gradient(1200, 800, 10), gradient(640, 480, 90)};
@@ -345,6 +547,72 @@ int main() {
         std::filesystem::remove_all(dir);
     }
 
+    std::cout << "canvas size\n";
+    {
+        // The sizes the front-ends actually offer make the round trip whole.
+        int kept = 0;
+        for (const auto [w, h] : {std::pair<int, int>{1, 1},
+                                  {640, 400},
+                                  {1920, 1080},
+                                  {kMaxCanvasSide, kMaxCanvasSide}}) {
+            const auto back = projectFromJson(
+                projectWithCanvas(std::to_string(w), std::to_string(h)));
+            if (back.width == w && back.height == h) ++kept;
+        }
+        check(kept == 4, "every canvas in range is read back exactly");
+
+        Project project;
+        project.stages.push_back({effectByName("blur"), {}});
+        project.width = kMaxCanvasSide;
+        project.height = 3;
+        const auto written = projectFromJson(projectToJson(project));
+        check(written.width == kMaxCanvasSide && written.height == 3,
+              "the largest canvas survives a write and a read");
+
+        // A side that does not fit in an int must not be narrowed into one
+        // that does: 4294968296 truncates to a perfectly plausible 1000.
+        int refusals = 0;
+        for (const char* bad : {"4294968296", "2147483648", "99999999999999999999", "-5", "0",
+                                "20001", "800.5", "\"800\"", "true"}) {
+            try {
+                projectFromJson(projectWithCanvas(bad, "600"));
+            } catch (const EffectError&) {
+                ++refusals;
+            }
+        }
+        check(refusals == 9, "a canvas width an int cannot hold is refused, not wrapped");
+
+        refusals = 0;
+        for (const char* bad : {"4294968296", "-5", "0", "20001", "800.5"}) {
+            try {
+                projectFromJson(projectWithCanvas("800", bad));
+            } catch (const EffectError&) {
+                ++refusals;
+            }
+        }
+        check(refusals == 5, "the same bounds apply to the canvas height");
+
+        // A canvas the file does not mention keeps the defaults rather than
+        // failing: an old project is still readable.
+        const auto bare = projectFromJson("{\"chain\": [{\"effect\": \"blur\"}]}");
+        check(bare.width == 1920 && bare.height == 1080, "a project with no canvas keeps 1920x1080");
+
+        // The CLI narrows the same way, from a long rather than a double.
+        const auto plain = canvasFromSize("800x600");
+        check(plain && plain->first == 800 && plain->second == 600, "-s 800x600 is stored as given");
+        const auto big = canvasFromSize(std::to_string(kMaxCanvasSide) + "x" +
+                                        std::to_string(kMaxCanvasSide));
+        check(big && big->first == kMaxCanvasSide, "-s takes the largest canvas");
+        check(!canvasFromSize("4294968296x600"), "-s refuses a width that does not fit in an int");
+        check(!canvasFromSize("2147483648x600"), "-s refuses a width just past INT_MAX");
+        check(!canvasFromSize("99999999999999999999x600"), "-s refuses a width past a long");
+        check(!canvasFromSize("800x4294968296"), "-s refuses a height that does not fit in an int");
+        check(!canvasFromSize("20001x600") && !canvasFromSize("800x20001"),
+              "-s refuses a side past the canvas limit");
+        check(!canvasFromSize("0x600") && !canvasFromSize("-5x600"),
+              "-s refuses a side that is not positive");
+    }
+
     std::cout << "image io\n";
     {
         const auto path = std::filesystem::temp_directory_path() / "effector_test_roundtrip.png";