Let effects declare whether they need source images

Whether an effect works from the loaded photographs is now part of the
declarative plug-in contract, next to params(): Effect::needsSources()
defaults to true, and blur — which only softens what it is handed — says
no and blurs an empty canvas when given nothing.

Only the first stage of a cascade ever sees the sources, so
chainNeedsSources() just asks the first stage. Both front-ends gate on
that instead of on "are there any images": the CLI only refuses an empty
batch when the chain wants one, and the interface renders a blur-led
chain with no images loaded, saying whichever thing is actually missing.

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

src/gui/window.cpp

@@ -57,7 +57,13 @@ EffectorWindow::EffectorWindow(const Options& initial) {
     applyInitial(initial);
 
     suspendRender_ = false;
-    if (!sources_->empty()) startRender();
+    if (loading_) {
+        placeholder_.set_text(missingText());
+    } else if (canRender()) {
+        startRender();
+    } else {
+        showPlaceholder();
+    }
 }
 
 EffectorWindow::~EffectorWindow() {
@@ -569,15 +575,7 @@ void EffectorWindow::removeSource(Gtk::ListBoxRow* row) {
         }
     });
     sourceList_.remove(*row);
-
-    if (!sources_->empty()) {
-        requestRender();
-    } else {
-        result_ = Image();
-        saveButton_.set_sensitive(false);
-        previewStack_.set_visible_child("placeholder");
-        status_.set_text("");
-    }
+    requestRender();
 }
 
 void EffectorWindow::onAddImages() {
@@ -680,14 +678,6 @@ void EffectorWindow::removeStage(int index) {
     rebuildChainRows();
     buildParamWidgets();
 
-    if (stages_.empty()) {
-        result_ = Image();
-        saveButton_.set_sensitive(false);
-        previewStack_.set_visible_child("placeholder");
-        placeholder_.set_text("Add an effect to start.");
-        status_.set_text("");
-        return;
-    }
     requestRender();
 }
 
@@ -794,15 +784,40 @@ void EffectorWindow::onSeedEntered() {
 void EffectorWindow::onRecreate() {
     seed_ = Rng::randomSeed();
     seedEntry_.set_text(std::to_string(seed_));
-    if (sources_->empty()) {
-        showError("Add at least one source image first.");
+    if (!canRender()) {
+        showError(missingText());
         return;
     }
     requestRender();
 }
 
+// Whether the effect chain has everything it needs. Only the first stage is
+// 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;
+    return !sources_->empty() || !chainNeedsSources(stages_);
+}
+
+std::string EffectorWindow::missingText() const {
+    if (stages_.empty()) return "Add an effect to start.";
+    return "Add source images to start.";
+}
+
+void EffectorWindow::showPlaceholder() {
+    result_ = Image();
+    saveButton_.set_sensitive(false);
+    placeholder_.set_text(missingText());
+    previewStack_.set_visible_child("placeholder");
+    status_.set_text("");
+}
+
 void EffectorWindow::requestRender() {
-    if (suspendRender_ || sources_->empty() || stages_.empty()) return;
+    if (suspendRender_) return;
+    if (!canRender()) {
+        showPlaceholder();
+        return;
+    }
     // Spin buttons fire on every step while they are held down; coalesce those
     // into one render instead of queueing a job per tick.
     debounce_.disconnect();
@@ -815,7 +830,7 @@ void EffectorWindow::requestRender() {
 }
 
 void EffectorWindow::startRender() {
-    if (sources_->empty() || stages_.empty()) return;
+    if (!canRender()) return;
     if (rendering_) {
         staleResult_ = true;  // a fresh render follows this one
         return;
@@ -893,9 +908,11 @@ void EffectorWindow::onRenderFinished() {
     std::ostringstream status;
     status.setf(std::ios::fixed);
     status.precision(1);
-    status << chainTitle(stages_) << " · " << result_.width() << "×" << result_.height() << " · "
-           << sources_->size() << (sources_->size() == 1 ? " source" : " sources") << " · seed "
-           << seed_ << " · " << millis << " ms";
+    status << chainTitle(stages_) << " · " << result_.width() << "×" << result_.height() << " · ";
+    if (!sources_->empty()) {
+        status << sources_->size() << (sources_->size() == 1 ? " source" : " sources") << " · ";
+    }
+    status << "seed " << seed_ << " · " << millis << " ms";
     status_.set_text(status.str());
 }
 
@@ -970,8 +987,7 @@ void EffectorWindow::applyProject(const Project& project, const std::string& pat
     suspendRender_ = false;
 
     if (project.sources.empty()) {
-        placeholder_.set_text("Add source images to start.");
-        status_.set_text("");
+        requestRender();  // a chain that needs no sources still has a picture
     } else {
         addPaths(project.sources);  // renders once the images are decoded
     }

src/gui/window.hpp

@@ -33,6 +33,9 @@ private:
     void storeParams();     // widgets -> the selected stage's values
     void applyInitial(const Options& initial);
 
+    bool canRender() const;     // a chain, and sources if the chain needs them
+    std::string missingText() const;  // what is still missing before a render
+    void showPlaceholder();     // drops the preview and says what is missing
     void requestRender();   // coalesced: restarts the short debounce timer
     void startRender();     // starts a worker, or marks the result stale
     void onAddImages();