Use the stencil buffer to avoid overdraw when rendering non-opaque solid strokes.

Fixes flutter/flutter#101330

Author: bdero

aiks/aiks_unittests.cc

@@ -3,6 +3,7 @@
 // found in the LICENSE file.
 
 #include <array>
+
 #include "flutter/testing/testing.h"
 #include "impeller/aiks/aiks_playground.h"
 #include "impeller/aiks/canvas.h"
@@ -483,40 +484,77 @@ TEST_F(AiksTest, TransformMultipliesCorrectly) {
   // clang-format on
 }
 
-TEST_F(AiksTest, PathsShouldHaveUniformAlpha) {
+TEST_F(AiksTest, SolidStrokesRenderCorrectly) {
   // Compare with https://fiddle.skia.org/c/027392122bec8ac2b5d5de00a4b9bbe2
-  Canvas canvas;
-  Paint paint;
+  bool first_frame = true;
+  auto callback = [&](AiksContext& renderer, RenderPass& pass) {
+    if (first_frame) {
+      first_frame = false;
+      ImGui::SetNextWindowSize({480, 100});
+      ImGui::SetNextWindowPos({100, 550});
+    }
 
-  paint.color = Color::White();
-  canvas.DrawPaint(paint);
+    static Color color = Color::Black().WithAlpha(0.5);
+    static float scale = 3;
+    static bool add_circle_clip = true;
 
-  paint.color = Color::Black().WithAlpha(0.5);
-  paint.style = Paint::Style::kStroke;
-  paint.stroke_width = 10;
+    ImGui::Begin("Controls");
+    ImGui::ColorEdit4("Color", reinterpret_cast<float*>(&color));
+    ImGui::SliderFloat("Scale", &scale, 0, 6);
+    ImGui::Checkbox("Circle clip", &add_circle_clip);
+    ImGui::End();
+
+    Canvas canvas;
+    Paint paint;
+
+    paint.color = Color::White();
+    canvas.DrawPaint(paint);
+
+    paint.color = color;
+    paint.style = Paint::Style::kStroke;
+    paint.stroke_width = 10;
+
+    Path path = PathBuilder{}
+                    .MoveTo({20, 20})
+                    .QuadraticCurveTo({60, 20}, {60, 60})
+                    .Close()
+                    .MoveTo({60, 20})
+                    .QuadraticCurveTo({60, 60}, {20, 60})
+                    .TakePath();
 
-  Path path = PathBuilder{}
-                  .MoveTo({20, 20})
-                  .QuadraticCurveTo({60, 20}, {60, 60})
-                  .Close()
-                  .MoveTo({60, 20})
-                  .QuadraticCurveTo({60, 60}, {20, 60})
-                  .TakePath();
-
-  canvas.Scale({3, 3});
-  for (auto join :
-       {SolidStrokeContents::Join::kBevel, SolidStrokeContents::Join::kRound,
-        SolidStrokeContents::Join::kMiter}) {
-    paint.stroke_join = join;
-    for (auto cap :
-         {SolidStrokeContents::Cap::kButt, SolidStrokeContents::Cap::kSquare,
-          SolidStrokeContents::Cap::kRound}) {
-      paint.stroke_cap = cap;
-      canvas.DrawPath(path, paint);
-      canvas.Translate({80, 0});
+    canvas.Scale(Vector2(scale, scale));
+
+    if (add_circle_clip) {
+      auto [handle_a, handle_b] = IMPELLER_PLAYGROUND_LINE(
+          Point(60, 300), Point(600, 300), 20, Color::Red(), Color::Red());
+
+      auto screen_to_canvas = canvas.GetCurrentTransformation().Invert();
+      Point point_a = screen_to_canvas * handle_a;
+      Point point_b = screen_to_canvas * handle_b;
+
+      Point middle = (point_a + point_b) / 2;
+      auto radius = point_a.GetDistance(middle);
+      canvas.ClipPath(PathBuilder{}.AddCircle(middle, radius).TakePath());
     }
-    canvas.Translate({-240, 60});
-  }
+
+    for (auto join :
+         {SolidStrokeContents::Join::kBevel, SolidStrokeContents::Join::kRound,
+          SolidStrokeContents::Join::kMiter}) {
+      paint.stroke_join = join;
+      for (auto cap :
+           {SolidStrokeContents::Cap::kButt, SolidStrokeContents::Cap::kSquare,
+            SolidStrokeContents::Cap::kRound}) {
+        paint.stroke_cap = cap;
+        canvas.DrawPath(path, paint);
+        canvas.Translate({80, 0});
+      }
+      canvas.Translate({-240, 60});
+    }
+
+    return renderer.Render(canvas.EndRecordingAsPicture(), pass);
+  };
+
+  ASSERT_TRUE(OpenPlaygroundHere(callback));
 }
 
 TEST_F(AiksTest, CoverageOriginShouldBeAccountedForInSubpasses) {

entity/contents/solid_color_contents.cc

@@ -53,7 +53,7 @@ bool SolidColorContents::Render(const ContentContext& renderer,
   using VS = SolidFillPipeline::VertexShader;
 
   Command cmd;
-  cmd.label = "SolidFill";
+  cmd.label = "Solid Fill";
   cmd.pipeline =
       renderer.GetSolidFillPipeline(OptionsFromPassAndEntity(pass, entity));
   cmd.stencil_reference = entity.GetStencilDepth();
@@ -63,7 +63,7 @@ bool SolidColorContents::Render(const ContentContext& renderer,
   VS::FrameInfo frame_info;
   frame_info.mvp = Matrix::MakeOrthographic(pass.GetRenderTargetSize()) *
                    entity.GetTransformation();
-  frame_info.color = color_;
+  frame_info.color = color_.Premultiply();
   VS::BindFrameInfo(cmd, pass.GetTransientsBuffer().EmplaceUniform(frame_info));
 
   cmd.primitive_type = PrimitiveType::kTriangle;

entity/contents/solid_stroke_contents.cc

@@ -4,6 +4,7 @@
 
 #include "solid_stroke_contents.h"
 
+#include "impeller/entity/contents/clip_contents.h"
 #include "impeller/entity/contents/content_context.h"
 #include "impeller/entity/entity.h"
 #include "impeller/geometry/path_builder.h"
@@ -78,12 +79,19 @@ static VertexBuffer CreateSolidStrokeVertices(
       vtx.vertex_position = polyline.points[contour_start_point_i - 1];
       vtx.vertex_normal = {};
       vtx.pen_down = 0.0;
+      // Append two transparent vertices when "picking up" the pen so that the
+      // triangle drawn when moving to the beginning of the new contour will
+      // have zero volume. This is necessary because strokes with a transparent
+      // color affect the stencil buffer to prevent overdraw.
+      vtx_builder.AppendVertex(vtx);
       vtx_builder.AppendVertex(vtx);
 
       vtx.vertex_position = polyline.points[contour_start_point_i];
-      // Append two transparent vertices at the beginning of the new contour
-      // because it's a triangle strip.
+      // Append two vertices at the beginning of the new contour
+      // so that the next appended vertex will create a triangle with zero
+      // volume.
       vtx_builder.AppendVertex(vtx);
+      vtx.pen_down = 1.0;
       vtx_builder.AppendVertex(vtx);
     }
 
@@ -147,14 +155,18 @@ bool SolidStrokeContents::Render(const ContentContext& renderer,
                    entity.GetTransformation();
 
   VS::StrokeInfo stroke_info;
-  stroke_info.color = color_;
+  stroke_info.color = color_.Premultiply();
   stroke_info.size = stroke_size_;
 
   Command cmd;
   cmd.primitive_type = PrimitiveType::kTriangleStrip;
-  cmd.label = "SolidStroke";
-  cmd.pipeline =
-      renderer.GetSolidStrokePipeline(OptionsFromPassAndEntity(pass, entity));
+  cmd.label = "Solid Stroke";
+  auto options = OptionsFromPassAndEntity(pass, entity);
+  if (!color_.IsOpaque()) {
+    options.stencil_compare = CompareFunction::kEqual;
+    options.stencil_operation = StencilOperation::kIncrementClamp;
+  }
+  cmd.pipeline = renderer.GetSolidStrokePipeline(options);
   cmd.stencil_reference = entity.GetStencilDepth();
 
   auto smoothing = SmoothingApproximation(
@@ -167,7 +179,11 @@ bool SolidStrokeContents::Render(const ContentContext& renderer,
   VS::BindStrokeInfo(cmd,
                      pass.GetTransientsBuffer().EmplaceUniform(stroke_info));
 
-  pass.AddCommand(std::move(cmd));
+  pass.AddCommand(cmd);
+
+  if (!color_.IsOpaque()) {
+    return ClipRestoreContents().Render(renderer, entity, pass);
+  }
 
   return true;
 }

entity/entity_unittests.cc

@@ -752,8 +752,7 @@ TEST_F(EntityTest, GaussianBlurFilter) {
     // unfiltered input.
     Entity cover_entity;
     cover_entity.SetPath(PathBuilder{}.AddRect(rect).TakePath());
-    cover_entity.SetContents(
-        SolidColorContents::Make(cover_color.Premultiply()));
+    cover_entity.SetContents(SolidColorContents::Make(cover_color));
     cover_entity.SetTransformation(ctm);
 
     cover_entity.Render(context, pass);
@@ -764,8 +763,7 @@ TEST_F(EntityTest, GaussianBlurFilter) {
         PathBuilder{}
             .AddRect(target_contents->GetCoverage(entity).value())
             .TakePath());
-    bounds_entity.SetContents(
-        SolidColorContents::Make(bounds_color.Premultiply()));
+    bounds_entity.SetContents(SolidColorContents::Make(bounds_color));
     bounds_entity.SetTransformation(Matrix());
 
     bounds_entity.Render(context, pass);