Share retained stroke mix pass helper

src/legacy_canvas_stroke_execution_services.h

@@ -137,6 +137,35 @@ struct LegacyCanvasStrokePadExecutionResult {
     std::size_t padded_faces = 0;
 };
 
+struct LegacyCanvasStrokeMixPassPlane {
+    int index = 0;
+    bool visible = true;
+    bool has_target = false;
+    float opacity = 1.0f;
+    glm::mat4 mvp { 1.0f };
+};
+
+struct LegacyCanvasStrokeMixPassRequest {
+    std::string_view context;
+    glm::vec2 resolution {};
+    std::span<const LegacyCanvasStrokeMixPassPlane> planes;
+    std::function<void()> bind_mix_samplers;
+    std::function<void()> unbind_mix_samplers;
+    std::function<void(int, const glm::mat4&)> setup_plane_shader;
+    std::function<void(int)> bind_layer_texture;
+    std::function<void(int)> bind_stroke_texture;
+    std::function<void(int)> bind_mask_texture;
+    std::function<void()> draw_plane;
+    std::function<void(int)> unbind_mask_texture;
+    std::function<void(int)> unbind_stroke_texture;
+    std::function<void(int)> unbind_layer_texture;
+};
+
+struct LegacyCanvasStrokeMixPassResult {
+    bool ok = false;
+    std::size_t composed_planes = 0;
+};
+
 struct LegacyCanvasStrokeComputeRequest {
     StrokeSample previous_sample {};
     std::span<const StrokeSample> samples;
@@ -923,4 +952,49 @@ template <typename ExecuteSample, typename BeginFace, typename PrepareDirtyReque
         });
 }
 
+[[nodiscard]] inline LegacyCanvasStrokeMixPassResult execute_legacy_canvas_stroke_mix_pass(
+    const LegacyCanvasStrokeMixPassRequest& request)
+{
+    LegacyCanvasStrokeMixPassResult result;
+    if (request.resolution.x <= 0.0f ||
+        request.resolution.y <= 0.0f ||
+        !request.setup_plane_shader ||
+        !request.bind_layer_texture ||
+        !request.bind_stroke_texture ||
+        !request.bind_mask_texture ||
+        !request.draw_plane ||
+        !request.unbind_mask_texture ||
+        !request.unbind_stroke_texture ||
+        !request.unbind_layer_texture) {
+        return result;
+    }
+
+    if (request.bind_mix_samplers) {
+        request.bind_mix_samplers();
+    }
+
+    for (const auto& plane : request.planes) {
+        if (!plane.visible || plane.opacity <= 0.0f || !plane.has_target) {
+            continue;
+        }
+
+        request.setup_plane_shader(plane.index, plane.mvp);
+        request.bind_layer_texture(plane.index);
+        request.bind_stroke_texture(plane.index);
+        request.bind_mask_texture(plane.index);
+        request.draw_plane();
+        request.unbind_mask_texture(plane.index);
+        request.unbind_stroke_texture(plane.index);
+        request.unbind_layer_texture(plane.index);
+        ++result.composed_planes;
+    }
+
+    if (request.unbind_mix_samplers) {
+        request.unbind_mix_samplers();
+    }
+
+    result.ok = true;
+    return result;
+}
+
 } // namespace pp::panopainter