Plan canvas stroke feedback copies

2026-06-03 18:52:37 +02:00
parent b576143afb
commit 2ac2c45b11
8 changed files with 174 additions and 14 deletions
--- a/docs/modernization/capability-map.md
+++ b/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 and destination-copy 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 coverage, live canvas stroke-feedback 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
--- a/docs/modernization/debt.md
+++ b/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 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 |
+| 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. `pp_paint_renderer::plan_canvas_stroke_feedback` also owns the current stroke shader feedback decision, and live `Canvas::stroke_draw` uses it for main-brush, dual-brush, and stroke-pad destination-copy decisions. Actual live stroke rasterization, dual-brush compositing, pattern feedback math, and thumbnail layer blending still use legacy OpenGL canvas execution | 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

--- a/docs/modernization/roadmap.md
+++ b/docs/modernization/roadmap.md
@@ -888,6 +888,12 @@ 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
 individual `ShaderManager` extension booleans.
+`pp_paint_renderer::plan_canvas_stroke_feedback` now models the current stroke
+shader's required destination feedback without changing the legacy shader math.
+Live `Canvas::stroke_draw` consumes that plan for main-brush, dual-brush, and
+stroke-pad destination-copy versus framebuffer-fetch decisions; thumbnail layer
+blending is the remaining direct canvas feedback branch before a fuller live
+paint-renderer execution boundary can take over.
 The existing renderer classes are not yet fully
 behind the renderer interfaces.

@@ -1623,6 +1629,10 @@ Results:
  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 main-brush, dual-brush, and stroke-pad draw paths now use the tested
+  `pp_paint_renderer` stroke-feedback plan to decide whether framebuffer fetch
+  supplies destination color or the legacy OpenGL path must copy the target
+  texture before drawing.
 - Canvas equirectangular import drawing and depth export rendering now route
  depth/blend state and active texture units through the renderer GL backend
  mapping.
--- a/src/canvas.cpp
+++ b/src/canvas.cpp
@@ -49,6 +49,27 @@ pp::renderer::RenderDeviceFeatures canvas_stroke_composite_features() noexcept
    return ShaderManager::render_device_features();
 }

+pp::paint_renderer::CanvasStrokeFeedbackPlan canvas_stroke_feedback_plan(
+    int width,
+    int height) noexcept
+{
+    const auto plan = pp::paint_renderer::plan_canvas_stroke_feedback(
+        canvas_stroke_composite_features(),
+        pp::renderer::Extent2D {
+            .width = static_cast<std::uint32_t>(std::max(width, 0)),
+            .height = static_cast<std::uint32_t>(std::max(height, 0)),
+        });
+    if (plan) {
+        return plan.value();
+    }
+
+    pp::paint_renderer::CanvasStrokeFeedbackPlan fallback;
+    fallback.compatibility_fallback = true;
+    fallback.path = pp::paint_renderer::StrokeCompositePath::ping_pong_textures;
+    fallback.requires_auxiliary_texture = true;
+    return fallback;
+}
+
 pp::paint_renderer::CanvasBlendGatePlan draw_merge_blend_gate_plan(
    int width,
    int height,
@@ -464,9 +485,12 @@ std::array<std::vector<vertex_t>, 6> Canvas::stroke_draw_project(std::array<vert
    return ret;
 }

-glm::vec4 Canvas::stroke_draw_samples(int i, std::vector<vertex_t>& P)
+glm::vec4 Canvas::stroke_draw_samples(
+    int i,
+    std::vector<vertex_t>& P,
+    bool copy_stroke_destination)
 {
-    if (!ShaderManager::ext_framebuffer_fetch)
+    if (copy_stroke_destination)
    {
        set_active_texture_unit(1);
        m_tex[i].bind(); // bg, copy of framebuffer (copied before drawing)
@@ -484,7 +508,7 @@ glm::vec4 Canvas::stroke_draw_samples(int i, std::vector<vertex_t>& P)
    glm::vec2 pad(1);
    glm::ivec2 tex_pos = glm::clamp(glm::floor(bb_min) - pad, { 0, 0 }, { m_width, m_height });
    glm::ivec2 tex_sz = glm::clamp(glm::ceil(bb_sz) + pad * 2.f, { 0, 0 }, (glm::vec2)(glm::ivec2(m_width, m_height) - tex_pos));
-    if (!ShaderManager::ext_framebuffer_fetch)
+    if (copy_stroke_destination)
    {
        glCopyTexSubImage2D(texture_2d_target(), 0, tex_pos.x, tex_pos.y,
            tex_pos.x, tex_pos.y, tex_sz.x, tex_sz.y);
@@ -512,7 +536,7 @@ glm::vec4 Canvas::stroke_draw_samples(int i, std::vector<vertex_t>& P)
    }
    m_brush_shape.draw_fill();

-    if (!ShaderManager::ext_framebuffer_fetch)
+    if (copy_stroke_destination)
    {
        set_active_texture_unit(1);
        m_tex[i].unbind();
@@ -630,10 +654,13 @@ void Canvas::stroke_draw()
    if (brush->m_pattern_flipx) patt_scale.x *= -1.f;
    if (brush->m_pattern_flipy) patt_scale.y *= -1.f;

+    const auto stroke_feedback = canvas_stroke_feedback_plan(m_width, m_height);
+    const bool copy_stroke_destination = !stroke_feedback.reads_destination_color;
+
    glDisable(blend_state());
    ShaderManager::use(kShader::Stroke);
    ShaderManager::u_int(kShaderUniform::Tex, 0); // brush
-    if (!ShaderManager::ext_framebuffer_fetch)
+    if (copy_stroke_destination)
        ShaderManager::u_int(kShaderUniform::TexBG, 1); // bg
    ShaderManager::u_int(kShaderUniform::TexPattern, 2); // pattern
    ShaderManager::u_int(kShaderUniform::TexMix, 3); // mixer
@@ -691,7 +718,7 @@ void Canvas::stroke_draw()
            ShaderManager::u_vec4(kShaderUniform::Col, f.col);
            ShaderManager::u_float(kShaderUniform::Alpha, f.flow);
            ShaderManager::u_float(kShaderUniform::Opacity, f.opacity);
-            auto box_sample = stroke_draw_samples(i, P);
+            auto box_sample = stroke_draw_samples(i, P, copy_stroke_destination);

            m_tmp[i].unbindFramebuffer();
            
@@ -718,7 +745,7 @@ void Canvas::stroke_draw()
    // work on documents that doesn't have the padding, so on document loading.
    ShaderManager::use(kShader::StrokePad);
    ShaderManager::u_vec4(kShaderUniform::Col, pad_color);
-    if (!ShaderManager::ext_framebuffer_fetch)
+    if (copy_stroke_destination)
    {
        set_active_texture_unit(1);
        ShaderManager::u_int(kShaderUniform::TexBG, 1);
@@ -748,7 +775,7 @@ void Canvas::stroke_draw()
        m_brush_shape.update_vertices(pad_quad.data(), pad_quad.size());

        m_tmp[i].bindFramebuffer();
-        if (!ShaderManager::ext_framebuffer_fetch)
+        if (copy_stroke_destination)
        {
            glm::vec2 o = glm::max({0, 0}, xy(b) - pad);
            glm::vec2 sz = glm::min(m_size, zw(b) + pad) - o;
@@ -759,7 +786,7 @@ void Canvas::stroke_draw()
        m_brush_shape.draw_fill();
        m_tmp[i].unbindFramebuffer();
    }
-    if (!ShaderManager::ext_framebuffer_fetch)
+    if (copy_stroke_destination)
    {
        unbind_texture_2d();
    }
@@ -790,7 +817,7 @@ void Canvas::stroke_draw()
                if (P.size() < 3)
                    continue;
                m_tmp_dual[i].bindFramebuffer();
-                auto box_sample = stroke_draw_samples(i, P);
+                auto box_sample = stroke_draw_samples(i, P, copy_stroke_destination);
                m_tmp_dual[i].unbindFramebuffer();

                // this mode overflows the main brush boundries
--- a/src/canvas.h
+++ b/src/canvas.h
@@ -205,7 +205,7 @@ public:
    void stroke_draw_mix(const glm::vec2& bb_min, const glm::vec2& bb_sz);
    std::array<std::vector<vertex_t>, 6> stroke_draw_project(std::array<vertex_t, 4>& B, bool project_3d = false, glm::mat4 mv = glm::mat4(1)) const;
    // return rect {origin, size}
-    glm::vec4 stroke_draw_samples(int i, std::vector<vertex_t>& P);
+    glm::vec4 stroke_draw_samples(int i, std::vector<vertex_t>& P, bool copy_stroke_destination);
    std::vector<StrokeFrame> stroke_draw_compute(Stroke& stroke) const;
    void stroke_draw();
    void stroke_end();
--- a/src/paint_renderer/compositor.cpp
+++ b/src/paint_renderer/compositor.cpp
@@ -110,12 +110,21 @@ namespace {
 void apply_stroke_plan(CanvasBlendGatePlan& gate, const StrokeCompositePlan& stroke) noexcept
 {
    gate.path = stroke.path;
-    gate.reads_destination_color = stroke.reads_destination_color;
+    gate.reads_destination_color = stroke.path == StrokeCompositePath::framebuffer_fetch;
    gate.requires_auxiliary_texture = stroke.requires_auxiliary_texture;
    gate.requires_texture_copy = stroke.requires_texture_copy;
    gate.requires_render_target_blit = stroke.requires_render_target_blit;
 }

+void apply_feedback_plan(CanvasStrokeFeedbackPlan& plan, const pp::renderer::PaintFeedbackPlan& feedback) noexcept
+{
+    plan.path = composite_path_from_feedback(feedback.path);
+    plan.reads_destination_color = plan.path == StrokeCompositePath::framebuffer_fetch;
+    plan.requires_auxiliary_texture = feedback.requires_auxiliary_texture;
+    plan.requires_texture_copy = feedback.requires_texture_copy;
+    plan.requires_render_target_blit = feedback.requires_render_target_blit;
+}
+
 void mark_shader_blend_fallback(
    CanvasBlendGatePlan& gate,
    pp::renderer::RenderDeviceFeatures features) noexcept
@@ -303,6 +312,45 @@ pp::foundation::Result<CanvasBlendGatePlan> plan_canvas_blend_gate(
    return pp::foundation::Result<CanvasBlendGatePlan>::success(gate);
 }

+pp::foundation::Result<CanvasStrokeFeedbackPlan> plan_canvas_stroke_feedback(
+    pp::renderer::RenderDeviceFeatures features,
+    pp::renderer::Extent2D extent) noexcept
+{
+    const auto extent_status = pp::renderer::validate_extent(extent);
+    if (!extent_status.ok()) {
+        return pp::foundation::Result<CanvasStrokeFeedbackPlan>::failure(extent_status);
+    }
+
+    const pp::renderer::TextureDesc target_desc {
+        .extent = extent,
+        .format = pp::renderer::TextureFormat::rgba8,
+        .usage = pp::renderer::TextureUsage::render_target
+            | pp::renderer::TextureUsage::sampled
+            | pp::renderer::TextureUsage::copy_source
+            | pp::renderer::TextureUsage::copy_destination,
+        .debug_name = "canvas-stroke-feedback-target",
+    };
+    const auto feedback = pp::renderer::plan_paint_feedback(features, target_desc, true);
+    if (feedback) {
+        CanvasStrokeFeedbackPlan plan;
+        apply_feedback_plan(plan, feedback.value());
+        return pp::foundation::Result<CanvasStrokeFeedbackPlan>::success(plan);
+    }
+
+    CanvasStrokeFeedbackPlan fallback;
+    fallback.compatibility_fallback = true;
+    if (features.framebuffer_fetch) {
+        fallback.path = StrokeCompositePath::framebuffer_fetch;
+        fallback.reads_destination_color = true;
+    } else {
+        fallback.path = StrokeCompositePath::ping_pong_textures;
+        fallback.requires_auxiliary_texture = true;
+        fallback.requires_texture_copy = features.texture_copy;
+        fallback.requires_render_target_blit = !features.texture_copy && features.render_target_blit;
+    }
+    return pp::foundation::Result<CanvasStrokeFeedbackPlan>::success(fallback);
+}
+
 const char* stroke_composite_path_name(StrokeCompositePath path) noexcept
 {
    switch (path) {
--- a/src/paint_renderer/compositor.h
+++ b/src/paint_renderer/compositor.h
@@ -74,6 +74,15 @@ struct CanvasBlendGatePlan {
    bool requires_render_target_blit = false;
 };

+struct CanvasStrokeFeedbackPlan {
+    StrokeCompositePath path = StrokeCompositePath::fixed_function_blend;
+    bool reads_destination_color = false;
+    bool requires_auxiliary_texture = false;
+    bool requires_texture_copy = false;
+    bool requires_render_target_blit = false;
+    bool compatibility_fallback = false;
+};
+
 [[nodiscard]] pp::foundation::Status composite_layer(
    std::span<pp::paint::Rgba> destination,
    pp::renderer::Extent2D extent,
@@ -93,6 +102,10 @@ struct CanvasBlendGatePlan {
    pp::renderer::RenderDeviceFeatures features,
    CanvasBlendGateRequest request) noexcept;

+[[nodiscard]] pp::foundation::Result<CanvasStrokeFeedbackPlan> plan_canvas_stroke_feedback(
+    pp::renderer::RenderDeviceFeatures features,
+    pp::renderer::Extent2D extent) noexcept;
+
 [[nodiscard]] const char* stroke_composite_path_name(StrokeCompositePath path) noexcept;

 }
--- a/tests/paint_renderer/compositor_tests.cpp
+++ b/tests/paint_renderer/compositor_tests.cpp
@@ -15,6 +15,7 @@ using pp::paint_renderer::StrokeCompositePath;
 using pp::paint_renderer::StrokeCompositeRequest;
 using pp::paint_renderer::composite_layer;
 using pp::paint_renderer::plan_canvas_blend_gate;
+using pp::paint_renderer::plan_canvas_stroke_feedback;
 using pp::paint_renderer::plan_stroke_composite;
 using pp::paint_renderer::stroke_composite_path_name;
 using pp::paint_renderer::stroke_composite_requires_feedback;
@@ -309,6 +310,7 @@ void plans_canvas_blend_gate_from_persisted_indices(pp::tests::Harness& h)
        PP_EXPECT(h, stroke.value().stroke_complex);
        PP_EXPECT(h, stroke.value().first_complex_layer_index == -1);
        PP_EXPECT(h, stroke.value().path == StrokeCompositePath::ping_pong_textures);
+        PP_EXPECT(h, !stroke.value().reads_destination_color);
        PP_EXPECT(h, stroke.value().requires_texture_copy);
    }
 }
@@ -381,6 +383,64 @@ void canvas_blend_gate_preserves_legacy_fallbacks(pp::tests::Harness& h)
    }
 }

+void plans_canvas_stroke_feedback_paths(pp::tests::Harness& h)
+{
+    const Extent2D extent { .width = 32, .height = 16 };
+    const auto fetch = plan_canvas_stroke_feedback(
+        RenderDeviceFeatures { .framebuffer_fetch = true },
+        extent);
+    PP_EXPECT(h, fetch);
+    if (fetch) {
+        PP_EXPECT(h, fetch.value().path == StrokeCompositePath::framebuffer_fetch);
+        PP_EXPECT(h, fetch.value().reads_destination_color);
+        PP_EXPECT(h, !fetch.value().requires_auxiliary_texture);
+        PP_EXPECT(h, !fetch.value().compatibility_fallback);
+    }
+
+    const auto copy = plan_canvas_stroke_feedback(
+        RenderDeviceFeatures { .texture_copy = true },
+        extent);
+    PP_EXPECT(h, copy);
+    if (copy) {
+        PP_EXPECT(h, copy.value().path == StrokeCompositePath::ping_pong_textures);
+        PP_EXPECT(h, !copy.value().reads_destination_color);
+        PP_EXPECT(h, copy.value().requires_auxiliary_texture);
+        PP_EXPECT(h, copy.value().requires_texture_copy);
+        PP_EXPECT(h, !copy.value().requires_render_target_blit);
+    }
+
+    const auto blit = plan_canvas_stroke_feedback(
+        RenderDeviceFeatures { .render_target_blit = true },
+        extent);
+    PP_EXPECT(h, blit);
+    if (blit) {
+        PP_EXPECT(h, blit.value().path == StrokeCompositePath::ping_pong_textures);
+        PP_EXPECT(h, blit.value().requires_auxiliary_texture);
+        PP_EXPECT(h, !blit.value().requires_texture_copy);
+        PP_EXPECT(h, blit.value().requires_render_target_blit);
+    }
+}
+
+void canvas_stroke_feedback_preserves_legacy_fallback(pp::tests::Harness& h)
+{
+    const auto fallback = plan_canvas_stroke_feedback(
+        RenderDeviceFeatures {},
+        Extent2D { .width = 32, .height = 16 });
+    PP_EXPECT(h, fallback);
+    if (fallback) {
+        PP_EXPECT(h, fallback.value().path == StrokeCompositePath::ping_pong_textures);
+        PP_EXPECT(h, fallback.value().requires_auxiliary_texture);
+        PP_EXPECT(h, !fallback.value().requires_texture_copy);
+        PP_EXPECT(h, fallback.value().compatibility_fallback);
+    }
+
+    const auto invalid = plan_canvas_stroke_feedback(
+        RenderDeviceFeatures { .texture_copy = true },
+        Extent2D { .width = 0, .height = 16 });
+    PP_EXPECT(h, !invalid.ok());
+    PP_EXPECT(h, invalid.status().code == StatusCode::invalid_argument);
+}
+
 }

 int main()
@@ -394,5 +454,7 @@ int main()
    harness.run("rejects_bad_stroke_composite_plans", rejects_bad_stroke_composite_plans);
    harness.run("plans_canvas_blend_gate_from_persisted_indices", plans_canvas_blend_gate_from_persisted_indices);
    harness.run("canvas_blend_gate_preserves_legacy_fallbacks", canvas_blend_gate_preserves_legacy_fallbacks);
+    harness.run("plans_canvas_stroke_feedback_paths", plans_canvas_stroke_feedback_paths);
+    harness.run("canvas_stroke_feedback_preserves_legacy_fallback", canvas_stroke_feedback_preserves_legacy_fallback);
    return harness.finish();
 }