Cascade effects through a chain

renderChain runs a list of stages in order: stage 0 works on the loaded
sources, every later one on the output of the stage before it, so blur
after scanlines softens the scanlines rather than the photographs. The
effects themselves are untouched — none of them knows whether its input
came from disk. All stages share one Rng and run in sequence, so a seed
still reproduces the cascade byte for byte.

The sidebar grows an "Effect chain" list: add a stage from a menu built
from the registry, reorder it, remove it, and edit the selected stage's
params underneath. Each stage keeps its own values. The CLI takes the
same chain from repeated -e, with every flag going to the stage whose
effect declares it.

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

src/cli/options.cpp

@@ -4,6 +4,7 @@
 #include <cstring>
 #include <sstream>
 
+#include "core/chain.hpp"
 #include "core/effect.hpp"
 #include "core/registry.hpp"
 
@@ -48,7 +49,6 @@ std::string requireValue(int argc, char** argv, int& i, const std::string& flag)
 
 Options parseOptions(int argc, char** argv) {
     Options options;
-    options.effect = defaultEffectName();
 
     for (int i = 1; i < argc; ++i) {
         const std::string arg = argv[i];
@@ -58,8 +58,9 @@ Options parseOptions(int argc, char** argv) {
         } else if (arg == "--list-effects") {
             options.listEffects = true;
         } else if (arg == "-e" || arg == "--effect") {
-            options.effect = requireValue(argc, argv, i, arg);
-            effectByName(options.effect);  // validate now, fail early
+            const std::string name = requireValue(argc, argv, i, arg);
+            effectByName(name);  // validate now, fail early
+            options.effects.push_back(name);
         } else if (arg == "-o" || arg == "--output") {
             options.output = requireValue(argc, argv, i, arg);
         } else if (arg == "-s" || arg == "--size") {
@@ -119,6 +120,7 @@ Options parseOptions(int argc, char** argv) {
             options.images.push_back(arg);
         }
     }
+    if (options.effects.empty()) options.effects.push_back(defaultEffectName());
     return options;
 }
 
@@ -127,7 +129,7 @@ std::string usage(const char* programName) {
     out << "Usage: " << programName << " [options] [image ...]\n\n"
         << "Image effects app. Runs the GTK interface unless --output is given.\n\n"
         << "Options:\n"
-        << "  -e, --effect NAME     effect to apply (default: " << defaultEffectName() << ")\n"
+        << "  -e, --effect NAME     effect to apply, repeat to cascade (default: " << defaultEffectName() << ")\n"
         << "  -o, --output FILE     render headlessly to this file and exit\n"
         << "  -s, --size WxH        canvas size (default: 1920x1080)\n"
         << "      --seed N          random seed (default: random)\n"
@@ -147,4 +149,19 @@ std::string usage(const char* programName) {
     return out.str();
 }
 
+std::vector<ChainStage> buildChain(const Options& options) {
+    std::vector<ChainStage> stages;
+    for (const auto& name : options.effects) {
+        ChainStage stage;
+        stage.effect = effectByName(name);
+        stage.values = stage.effect->defaults();
+        for (const auto& spec : stage.effect->params()) {
+            const auto it = options.params.find(spec.key);
+            if (it != options.params.end()) stage.values[spec.key] = it->second;
+        }
+        stages.push_back(std::move(stage));
+    }
+    return stages;
+}
+
 }  // namespace effector