Use blend gate plan for canvas copy decisions

docs/modernization/capability-map.md

@@ -37,7 +37,7 @@ and validation command.
 | PPBR import/export | brush panel/dialog | `pp_assets`, `pp_panopainter_ui` | Round-trip fixture |
 | Stroke sampling | `Stroke`, `Canvas` | `pp_paint` | Property tests for spacing, pressure, jitter |
 | Dual brush/pattern behavior | `Brush`, shaders | `pp_paint`, `pp_paint_renderer` | Stroke-alpha CPU reference, dual/pattern feedback planning, GPU golden |
-| Blend modes | GLSL include files, layer rendering | `pp_paint`, `pp_paint_renderer` | Final RGBA and stroke-alpha CPU reference vectors, fixed-function/framebuffer-fetch/ping-pong stroke composite planning, live `Canvas`/`NodeCanvas` blend-gate coverage, and GPU parity |
+| Blend modes | GLSL include files, layer rendering | `pp_paint`, `pp_paint_renderer` | Final RGBA and stroke-alpha CPU reference vectors, fixed-function/framebuffer-fetch/ping-pong stroke composite planning, live `Canvas`/`NodeCanvas` blend-gate and destination-copy coverage, and GPU parity |
 | Erase/flood fill/masks | `Canvas`, modes, shaders | `pp_document`, `pp_paint_renderer` | Edge masks, alpha lock, dirty rects |
 
 ## Layers And Animation

docs/modernization/debt.md

@@ -53,7 +53,7 @@ agent or engineer to remove them without reconstructing context from chat.
 | DEBT-0033 | Open | Modernization | Tools menu planning and direct command execution dispatch now consume pure `pp_app_core` through `App::init_menu_tools`, `pano_cli plan-tools-menu`, `pano_cli plan-tools-panel`, and the `ToolsMenuServices` boundary, but live adapters still construct legacy `NodePanelFloating` panels, mutate legacy panel nodes, clear `CanvasModeGrid`, reset `NodeCanvas` camera state, open legacy shortcuts UI, and call the iOS SonarPen bridge directly | Preserve current Tools menu behavior while UI shell actions move toward app/UI/platform services | `pp_app_core_tools_menu_tests`; `pano_cli plan-tools-menu --command shortcuts`; `pano_cli plan-tools-panel --panel layers`; `pano_cli plan-tools-panel --panel animation --already-visible`; `ctest --preset desktop-fast --build-config Debug` | Tools panel creation, submenu routing, grid clear, camera reset, shortcuts dialog, and SonarPen dispatch are owned by injected app/UI/platform services with `App::init_menu_tools` acting only as a UI adapter and no legacy Tools adapter |
 | DEBT-0034 | Open | Modernization | About menu command planning and execution dispatch now consume pure `pp_app_core` through `App::init_menu_about`, `pano_cli plan-about-menu`, and the `AboutMenuServices` boundary, but the live adapter still opens legacy About/manual/what's-new dialogs, invokes the injected crash hook, and runs the legacy Canvas stroke performance test directly | Preserve About menu behavior while dialogs and diagnostics move toward app/UI/platform services | `pp_app_core_about_menu_tests`; `pano_cli plan-about-menu --command news --version-major 2 --version-minor 5 --version-fix 7`; `pano_cli plan-about-menu --command performance --no-canvas`; `ctest --preset desktop-fast --build-config Debug` | About/manual/what's-new dialog dispatch, crash-test dispatch, and performance-test execution are owned by injected app/UI/platform services with `App::init_menu_about` acting only as a UI adapter and no legacy About adapter |
 | DEBT-0035 | Open | Modernization | Main toolbar/status command planning and execution dispatch now consume pure `pp_app_core` through `App::init_toolbar_main`, `pano_cli plan-main-toolbar`, and the `MainToolbarServices` boundary, and history/canvas commands now hand off through `HistoryUiServices` and `DocumentCanvasClearServices`, but the live adapter still opens legacy open/save/settings/message-box dialogs and delegates to legacy history/canvas adapters | Preserve reachable toolbar/status behavior while app shell commands move toward app/document/UI services | `pp_app_core_main_toolbar_tests`; `pano_cli plan-main-toolbar --command undo --undo-count 2`; `pano_cli plan-main-toolbar --command clear-canvas --no-canvas`; `ctest --preset desktop-fast --build-config Debug` | Open/save/settings/message-box routing, undo/redo/clear-history execution, and canvas-clear execution are owned by injected app/document/UI services with `App::init_toolbar_main` acting only as a UI adapter and no legacy toolbar adapter |
-| DEBT-0036 | Open | Modernization | `pp_renderer_api`, `pp_paint_renderer`, `pano_cli plan-paint-feedback`, and `pano_cli plan-stroke-composite` can choose backend-neutral complex paint feedback strategies for fixed-function blending, framebuffer-fetch-capable renderers, or ping-pong render targets. OpenGL extension detection now stores `pp::renderer::RenderDeviceFeatures` through `ShaderManager`, using `pp_renderer_gl::render_device_features` as the backend conversion point. `pp_paint_renderer::plan_canvas_blend_gate` owns the compatibility mapping from persisted layer/brush blend indices to the extracted stroke-composite planner, and live `Canvas::draw_merge` plus `NodeCanvas` panorama rendering both call it with the stored renderer-neutral feature set for their existing shader-blend gates. Actual live stroke compositing, dual-brush feedback, and pattern feedback still use the legacy OpenGL canvas path | Preserve current painting behavior while the renderer boundary matures for OpenGL parity and later Vulkan/Metal experiments | `pp_renderer_api_tests`; `pp_renderer_gl_capabilities_tests`; `pp_paint_renderer_compositor_tests`; `pano_cli plan-paint-feedback --framebuffer-fetch --explicit-transitions --render-only`; `pano_cli plan-paint-feedback --texture-copy`; `pano_cli plan-stroke-composite --stroke-blend 10 --framebuffer-fetch --explicit-transitions --render-only`; `pano_cli plan-stroke-composite --layer-blend 4 --dual-blend --texture-copy`; `ctest --preset desktop-fast --build-config Debug`; `cmake --build --preset windows-msvc-default --config Debug --target PanoPainter` | Live stroke/layer compositing chooses its feedback path through `pp_paint_renderer` and renderer services, with OpenGL golden parity and Vulkan/Metal lab tests covering framebuffer-fetch and ping-pong behavior |
+| DEBT-0036 | Open | Modernization | `pp_renderer_api`, `pp_paint_renderer`, `pano_cli plan-paint-feedback`, and `pano_cli plan-stroke-composite` can choose backend-neutral complex paint feedback strategies for fixed-function blending, framebuffer-fetch-capable renderers, or ping-pong render targets. OpenGL extension detection now stores `pp::renderer::RenderDeviceFeatures` through `ShaderManager`, using `pp_renderer_gl::render_device_features` as the backend conversion point. `pp_paint_renderer::plan_canvas_blend_gate` owns the compatibility mapping from persisted layer/brush blend indices to the extracted stroke-composite planner, and live `Canvas::draw_merge` plus `NodeCanvas` panorama rendering both call it with the stored renderer-neutral feature set for their existing shader-blend gates and destination-copy versus framebuffer-fetch decisions. Actual live stroke compositing, dual-brush feedback, and pattern feedback still use the legacy OpenGL canvas path | Preserve current painting behavior while the renderer boundary matures for OpenGL parity and later Vulkan/Metal experiments | `pp_renderer_api_tests`; `pp_renderer_gl_capabilities_tests`; `pp_paint_renderer_compositor_tests`; `pano_cli plan-paint-feedback --framebuffer-fetch --explicit-transitions --render-only`; `pano_cli plan-paint-feedback --texture-copy`; `pano_cli plan-stroke-composite --stroke-blend 10 --framebuffer-fetch --explicit-transitions --render-only`; `pano_cli plan-stroke-composite --layer-blend 4 --dual-blend --texture-copy`; `ctest --preset desktop-fast --build-config Debug`; `cmake --build --preset windows-msvc-default --config Debug --target PanoPainter` | Live stroke/layer compositing chooses its feedback path through `pp_paint_renderer` and renderer services, with OpenGL golden parity and Vulkan/Metal lab tests covering framebuffer-fetch and ping-pong behavior |
 
 ## Closed Debt
 

docs/modernization/roadmap.md

@@ -882,7 +882,8 @@ the app calls through renderer services for the whole compositing path.
 mapping from persisted layer and brush blend indices to that planner, including
 fallback behavior for unknown nonzero indices. Both `Canvas::draw_merge` and
 `NodeCanvas` panorama rendering consume that shared gate, so the live app no
-longer has duplicate local blend-trigger logic.
+longer has duplicate local blend-trigger logic or duplicate destination-copy
+versus framebuffer-fetch decisions in those paths.
 The OpenGL shader initialization path now stores a renderer-neutral
 `RenderDeviceFeatures` snapshot converted by `pp_renderer_gl`, and those live
 canvas gates consume that snapshot instead of rebuilding feature flags from
@@ -1618,6 +1619,10 @@ Results:
   feature snapshot through the legacy shader manager, and live canvas blend
   gates consume that `RenderDeviceFeatures` value instead of hand-built
   framebuffer-fetch/texture-copy flags.
+- Canvas draw-merge and `NodeCanvas` panorama shader-blend paths now use the
+  shared canvas blend-gate plan to decide whether they can read destination
+  color through framebuffer fetch or must copy the destination texture before
+  the legacy OpenGL blend draw.
 - Canvas equirectangular import drawing and depth export rendering now route
   depth/blend state and active texture units through the renderer GL backend
   mapping.

src/canvas.cpp

@@ -49,7 +49,7 @@ pp::renderer::RenderDeviceFeatures canvas_stroke_composite_features() noexcept
     return ShaderManager::render_device_features();
 }
 
-bool draw_merge_needs_shader_blend(
+pp::paint_renderer::CanvasBlendGatePlan draw_merge_blend_gate_plan(
     int width,
     int height,
     const std::vector<std::shared_ptr<Layer>>& layers,
@@ -75,7 +75,15 @@ bool draw_merge_needs_shader_blend(
             .has_stroke_blend_mode = brush != nullptr,
             .stroke_blend_mode = brush ? brush->m_blend_mode : 0,
         });
-    return plan ? plan.value().shader_blend : true;
+    if (plan) {
+        return plan.value();
+    }
+
+    pp::paint_renderer::CanvasBlendGatePlan fallback;
+    fallback.shader_blend = true;
+    fallback.complex_blend = true;
+    fallback.compatibility_fallback = true;
+    return fallback;
 }
 
 GLenum unsigned_byte_component_type()
@@ -1121,11 +1129,13 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
     auto ortho = glm::ortho<float>(-0.5f, 0.5f, -0.5f, 0.5f, -1.f, 1.f);
     const auto& b = m_current_stroke->m_brush;
 
-    const bool use_blend = draw_merge_needs_shader_blend(
+    const auto blend_gate = draw_merge_blend_gate_plan(
         m_width,
         m_height,
         m_layers,
         m_current_stroke ? m_current_stroke->m_brush.get() : nullptr);
+    const bool use_blend = blend_gate.shader_blend;
+    const bool copy_blend_destination = use_blend && !blend_gate.reads_destination_color;
 
     // if not using shader blend, use gl rasterizer blend
     glDisable(depth_test_state());
@@ -1289,7 +1299,7 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
                 ShaderManager::u_int(kShaderUniform::BlendMode, m_layers[layer_index]->m_blend_mode);
                 ShaderManager::u_float(kShaderUniform::Alpha, 1.f);
                 ShaderManager::u_mat4(kShaderUniform::MVP, ortho);
-                if (!ShaderManager::ext_framebuffer_fetch)
+                if (copy_blend_destination)
                 {
                     m_sampler.bind(2);
                     ShaderManager::u_int(kShaderUniform::TexBG, 2);
@@ -1297,7 +1307,7 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
 
                 set_active_texture_unit(0);
                 m_merge_rtt.bindTexture();
-                if (!ShaderManager::ext_framebuffer_fetch)
+                if (copy_blend_destination)
                 {
                     set_active_texture_unit(2);
                     m_merge_tex.bind();
@@ -1306,7 +1316,7 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
 
                 m_plane.draw_fill();
 
-                if (!ShaderManager::ext_framebuffer_fetch)
+                if (copy_blend_destination)
                 {
                     set_active_texture_unit(2);
                     m_merge_tex.unbind();

src/node_canvas.cpp

@@ -32,7 +32,7 @@ pp::renderer::RenderDeviceFeatures node_canvas_stroke_composite_features() noexc
     return ShaderManager::render_device_features();
 }
 
-bool node_canvas_needs_shader_blend(
+pp::paint_renderer::CanvasBlendGatePlan node_canvas_blend_gate_plan(
     int width,
     int height,
     const std::vector<std::shared_ptr<Layer>>& layers,
@@ -58,7 +58,15 @@ bool node_canvas_needs_shader_blend(
             .has_stroke_blend_mode = brush != nullptr,
             .stroke_blend_mode = brush ? brush->m_blend_mode : 0,
         });
-    return plan ? plan.value().shader_blend : true;
+    if (plan) {
+        return plan.value();
+    }
+
+    pp::paint_renderer::CanvasBlendGatePlan fallback;
+    fallback.shader_blend = true;
+    fallback.complex_blend = true;
+    fallback.compatibility_fallback = true;
+    return fallback;
 }
 
 void run_history_undo_if_available()
@@ -290,11 +298,13 @@ void NodeCanvas::draw()
     }
     else
     {
-        const bool use_blend = node_canvas_needs_shader_blend(
+        const auto blend_gate = node_canvas_blend_gate_plan(
             m_cache_rtt.getWidth(),
             m_cache_rtt.getHeight(),
             m_canvas->m_layers,
             m_canvas->m_current_stroke ? m_canvas->m_current_stroke->m_brush.get() : nullptr);
+        const bool use_blend = blend_gate.shader_blend;
+        const bool copy_blend_destination = use_blend && !blend_gate.reads_destination_color;
 
         if (use_blend)
         {
@@ -485,7 +495,7 @@ void NodeCanvas::draw()
 
                     ShaderManager::use(kShader::TextureBlend);
                     ShaderManager::u_int(kShaderUniform::Tex, 0);
-                    if (!ShaderManager::ext_framebuffer_fetch)
+                    if (copy_blend_destination)
                         ShaderManager::u_int(kShaderUniform::TexBG, 2);
                     ShaderManager::u_int(kShaderUniform::BlendMode, m_canvas->m_layers[layer_index]->m_blend_mode);
                     ShaderManager::u_float(kShaderUniform::Alpha, 1.f);
@@ -493,7 +503,7 @@ void NodeCanvas::draw()
                 
                     set_active_texture_unit(0);
                     m_blender_rtt.bindTexture();
-                    if (!ShaderManager::ext_framebuffer_fetch)
+                    if (copy_blend_destination)
                     {
                         set_active_texture_unit(2);
                         m_blender_bg.bind();
@@ -503,7 +513,7 @@ void NodeCanvas::draw()
 
                     m_face_plane.draw_fill();
 
-                    if (!ShaderManager::ext_framebuffer_fetch)
+                    if (copy_blend_destination)
                     {
                         set_active_texture_unit(2);
                         m_blender_bg.unbind();

src/paint_renderer/compositor.cpp

@@ -116,11 +116,22 @@ void apply_stroke_plan(CanvasBlendGatePlan& gate, const StrokeCompositePlan& str
     gate.requires_render_target_blit = stroke.requires_render_target_blit;
 }
 
-void mark_shader_blend_fallback(CanvasBlendGatePlan& gate) noexcept
+void mark_shader_blend_fallback(
+    CanvasBlendGatePlan& gate,
+    pp::renderer::RenderDeviceFeatures features) noexcept
 {
     gate.shader_blend = true;
     gate.complex_blend = true;
     gate.compatibility_fallback = true;
+    if (features.framebuffer_fetch) {
+        gate.path = StrokeCompositePath::framebuffer_fetch;
+        gate.reads_destination_color = true;
+    } else if (features.texture_copy || features.render_target_blit) {
+        gate.path = StrokeCompositePath::ping_pong_textures;
+        gate.requires_auxiliary_texture = true;
+        gate.requires_texture_copy = features.texture_copy;
+        gate.requires_render_target_blit = !features.texture_copy && features.render_target_blit;
+    }
 }
 
 }
@@ -227,7 +238,7 @@ pp::foundation::Result<CanvasBlendGatePlan> plan_canvas_blend_gate(
         if (!paint_blend_mode_from_persisted_index(request.layer_blend_modes[i], layer_blend)) {
             if (request.layer_blend_modes[i] != 0) {
                 gate.first_complex_layer_index = static_cast<int>(i);
-                mark_shader_blend_fallback(gate);
+                mark_shader_blend_fallback(gate, features);
                 return pp::foundation::Result<CanvasBlendGatePlan>::success(gate);
             }
             continue;
@@ -259,7 +270,7 @@ pp::foundation::Result<CanvasBlendGatePlan> plan_canvas_blend_gate(
         if (!stroke_blend_mode_from_persisted_index(request.stroke_blend_mode, stroke_blend)) {
             if (request.stroke_blend_mode != 0) {
                 gate.stroke_complex = true;
-                mark_shader_blend_fallback(gate);
+                mark_shader_blend_fallback(gate, features);
                 return pp::foundation::Result<CanvasBlendGatePlan>::success(gate);
             }
         } else if (stroke_blend != pp::paint::StrokeBlendMode::normal) {

tests/paint_renderer/compositor_tests.cpp

@@ -317,7 +317,7 @@ void canvas_blend_gate_preserves_legacy_fallbacks(pp::tests::Harness& h)
 {
     const std::vector<int> unknown_layer { 0, 99 };
     const auto unknown = plan_canvas_blend_gate(
-        RenderDeviceFeatures {},
+        RenderDeviceFeatures { .texture_copy = true },
         CanvasBlendGateRequest {
             .extent = Extent2D { .width = 32, .height = 16 },
             .layer_blend_modes = unknown_layer,
@@ -328,6 +328,9 @@ void canvas_blend_gate_preserves_legacy_fallbacks(pp::tests::Harness& h)
         PP_EXPECT(h, unknown.value().complex_blend);
         PP_EXPECT(h, unknown.value().compatibility_fallback);
         PP_EXPECT(h, unknown.value().first_complex_layer_index == 1);
+        PP_EXPECT(h, unknown.value().path == StrokeCompositePath::ping_pong_textures);
+        PP_EXPECT(h, unknown.value().requires_auxiliary_texture);
+        PP_EXPECT(h, unknown.value().requires_texture_copy);
     }
 
     const std::vector<int> normal_layers { 0 };
@@ -344,6 +347,21 @@ void canvas_blend_gate_preserves_legacy_fallbacks(pp::tests::Harness& h)
         PP_EXPECT(h, unsupported.value().shader_blend);
         PP_EXPECT(h, unsupported.value().stroke_complex);
         PP_EXPECT(h, unsupported.value().compatibility_fallback);
+        PP_EXPECT(h, !unsupported.value().requires_texture_copy);
+    }
+
+    const auto unknown_fetch = plan_canvas_blend_gate(
+        RenderDeviceFeatures { .framebuffer_fetch = true },
+        CanvasBlendGateRequest {
+            .extent = Extent2D { .width = 32, .height = 16 },
+            .layer_blend_modes = unknown_layer,
+        });
+    PP_EXPECT(h, unknown_fetch);
+    if (unknown_fetch) {
+        PP_EXPECT(h, unknown_fetch.value().compatibility_fallback);
+        PP_EXPECT(h, unknown_fetch.value().path == StrokeCompositePath::framebuffer_fetch);
+        PP_EXPECT(h, unknown_fetch.value().reads_destination_color);
+        PP_EXPECT(h, !unknown_fetch.value().requires_texture_copy);
     }
 
     const auto dual_pattern = plan_canvas_blend_gate(