[Impeller] ensure that srcOver to src conversion takes stroke coverage into account. (#54817)

When computing whether or not an entity can be coerced into src blend from srcOver, take into account that stroke geometries may modulate with alpha to emulate stroke widths less than 1.0.

While we're at it, update several computations to use max basis XY (from flutter/flutter#153451 ) since that can lead to incorrect stroke width computations in the presence of canvas scaling.

Most of the changes update the APIs that used to take a ` const Entity& entity` to instead use the `const Matrix& transform`, since that is all we used the entity for.

See screeenshots in flutter/flutter#154178 (comment)

Fixes flutter/flutter#154178

Author: Jonah Williams

impeller/aiks/aiks_unittests.cc

@@ -314,7 +314,7 @@ TEST_P(AiksTest, OpaqueEntitiesGetCoercedToSource) {
   });
 
   ASSERT_TRUE(entity.size() >= 1);
-  ASSERT_TRUE(contents->IsOpaque());
+  ASSERT_TRUE(contents->IsOpaque({}));
   ASSERT_EQ(entity[0].GetBlendMode(), BlendMode::kSource);
 }
 

impeller/aiks/experimental_canvas.cc

@@ -673,7 +673,7 @@ void ExperimentalCanvas::AddRenderEntityToCurrentPass(Entity entity,
       entity.GetTransform());
   entity.SetInheritedOpacity(transform_stack_.back().distributed_opacity);
   if (entity.GetBlendMode() == BlendMode::kSourceOver &&
-      entity.GetContents()->IsOpaque()) {
+      entity.GetContents()->IsOpaque(entity.GetTransform())) {
     entity.SetBlendMode(BlendMode::kSource);
   }
 

impeller/entity/contents/color_source_contents.cc

@@ -54,4 +54,9 @@ void ColorSourceContents::SetInheritedOpacity(Scalar opacity) {
   inherited_opacity_ = opacity;
 }
 
+bool ColorSourceContents::AppliesAlphaForStrokeCoverage(
+    const Matrix& transform) const {
+  return GetGeometry() && GetGeometry()->ComputeAlphaCoverage(transform) < 1.0;
+}
+
 }  // namespace impeller

impeller/entity/contents/color_source_contents.h

@@ -126,6 +126,10 @@ class ColorSourceContents : public Contents {
     return geom.GetPositionBuffer(renderer, entity, pass);
   }
 
+  /// @brief Whether the entity should be treated as non-opaque due to stroke
+  ///        geometry requiring alpha for coverage.
+  bool AppliesAlphaForStrokeCoverage(const Matrix& transform) const;
+
   template <typename VertexShaderT>
   bool DrawGeometry(const ContentContext& renderer,
                     const Entity& entity,

impeller/entity/contents/conical_gradient_contents.cc

@@ -80,7 +80,8 @@ bool ConicalGradientContents::RenderSSBO(const ContentContext& renderer,
         frag_info.tile_mode = static_cast<Scalar>(tile_mode_);
         frag_info.decal_border_color = decal_border_color_;
         frag_info.alpha =
-            GetOpacityFactor() * GetGeometry()->ComputeAlphaCoverage(entity);
+            GetOpacityFactor() *
+            GetGeometry()->ComputeAlphaCoverage(entity.GetTransform());
         if (focus_) {
           frag_info.focus = focus_.value();
           frag_info.focus_radius = focus_radius_;
@@ -137,7 +138,8 @@ bool ConicalGradientContents::RenderTexture(const ContentContext& renderer,
         frag_info.texture_sampler_y_coord_scale =
             gradient_texture->GetYCoordScale();
         frag_info.alpha =
-            GetOpacityFactor() * GetGeometry()->ComputeAlphaCoverage(entity);
+            GetOpacityFactor() *
+            GetGeometry()->ComputeAlphaCoverage(entity.GetTransform());
         frag_info.half_texel =
             Vector2(0.5 / gradient_texture->GetSize().width,
                     0.5 / gradient_texture->GetSize().height);

impeller/entity/contents/contents.cc

@@ -49,7 +49,7 @@ Contents::Contents() = default;
 
 Contents::~Contents() = default;
 
-bool Contents::IsOpaque() const {
+bool Contents::IsOpaque(const Matrix& transform) const {
   return false;
 }
 

impeller/entity/contents/contents.h

@@ -91,7 +91,9 @@ class Contents {
   ///        properties (e.g. the blend mode), clips/visibility culling, or
   ///        inherited opacity.
   ///
-  virtual bool IsOpaque() const;
+  /// @param transform The current transform matrix of the entity that will
+  /// render this contents.
+  virtual bool IsOpaque(const Matrix& transform) const;
 
   //----------------------------------------------------------------------------
   /// @brief Given the current pass space bounding rectangle of the clip

impeller/entity/contents/linear_gradient_contents.cc

@@ -44,7 +44,7 @@ void LinearGradientContents::SetTileMode(Entity::TileMode tile_mode) {
   tile_mode_ = tile_mode;
 }
 
-bool LinearGradientContents::IsOpaque() const {
+bool LinearGradientContents::IsOpaque(const Matrix& transform) const {
   if (GetOpacityFactor() < 1 || tile_mode_ == Entity::TileMode::kDecal) {
     return false;
   }
@@ -53,7 +53,7 @@ bool LinearGradientContents::IsOpaque() const {
       return false;
     }
   }
-  return true;
+  return !AppliesAlphaForStrokeCoverage(transform);
 }
 
 bool LinearGradientContents::CanApplyFastGradient() const {
@@ -176,7 +176,8 @@ bool LinearGradientContents::FastLinearGradient(const ContentContext& renderer,
 
         FS::FragInfo frag_info;
         frag_info.alpha =
-            GetOpacityFactor() * GetGeometry()->ComputeAlphaCoverage(entity);
+            GetOpacityFactor() *
+            GetGeometry()->ComputeAlphaCoverage(entity.GetTransform());
 
         FS::BindFragInfo(pass, host_buffer.EmplaceUniform(frag_info));
 
@@ -231,7 +232,8 @@ bool LinearGradientContents::RenderTexture(const ContentContext& renderer,
         frag_info.texture_sampler_y_coord_scale =
             gradient_texture->GetYCoordScale();
         frag_info.alpha =
-            GetOpacityFactor() * GetGeometry()->ComputeAlphaCoverage(entity);
+            GetOpacityFactor() *
+            GetGeometry()->ComputeAlphaCoverage(entity.GetTransform());
         ;
         frag_info.half_texel =
             Vector2(0.5 / gradient_texture->GetSize().width,
@@ -284,7 +286,8 @@ bool LinearGradientContents::RenderSSBO(const ContentContext& renderer,
         frag_info.tile_mode = static_cast<Scalar>(tile_mode_);
         frag_info.decal_border_color = decal_border_color_;
         frag_info.alpha =
-            GetOpacityFactor() * GetGeometry()->ComputeAlphaCoverage(entity);
+            GetOpacityFactor() *
+            GetGeometry()->ComputeAlphaCoverage(entity.GetTransform());
         frag_info.start_to_end = end_point_ - start_point_;
         frag_info.inverse_dot_start_to_end =
             CalculateInverseDotStartToEnd(start_point_, end_point_);

impeller/entity/contents/linear_gradient_contents.h

@@ -21,7 +21,7 @@ class LinearGradientContents final : public ColorSourceContents {
   ~LinearGradientContents() override;
 
   // |Contents|
-  bool IsOpaque() const override;
+  bool IsOpaque(const Matrix& transform) const override;
 
   // |Contents|
   bool Render(const ContentContext& renderer,

impeller/entity/contents/radial_gradient_contents.cc

@@ -43,7 +43,7 @@ const std::vector<Scalar>& RadialGradientContents::GetStops() const {
   return stops_;
 }
 
-bool RadialGradientContents::IsOpaque() const {
+bool RadialGradientContents::IsOpaque(const Matrix& transform) const {
   if (GetOpacityFactor() < 1 || tile_mode_ == Entity::TileMode::kDecal) {
     return false;
   }
@@ -52,7 +52,7 @@ bool RadialGradientContents::IsOpaque() const {
       return false;
     }
   }
-  return true;
+  return !AppliesAlphaForStrokeCoverage(transform);
 }
 
 bool RadialGradientContents::Render(const ContentContext& renderer,
@@ -86,7 +86,8 @@ bool RadialGradientContents::RenderSSBO(const ContentContext& renderer,
         frag_info.tile_mode = static_cast<Scalar>(tile_mode_);
         frag_info.decal_border_color = decal_border_color_;
         frag_info.alpha =
-            GetOpacityFactor() * GetGeometry()->ComputeAlphaCoverage(entity);
+            GetOpacityFactor() *
+            GetGeometry()->ComputeAlphaCoverage(entity.GetTransform());
 
         auto& host_buffer = renderer.GetTransientsBuffer();
         auto colors = CreateGradientColors(colors_, stops_);
@@ -136,7 +137,8 @@ bool RadialGradientContents::RenderTexture(const ContentContext& renderer,
         frag_info.texture_sampler_y_coord_scale =
             gradient_texture->GetYCoordScale();
         frag_info.alpha =
-            GetOpacityFactor() * GetGeometry()->ComputeAlphaCoverage(entity);
+            GetOpacityFactor() *
+            GetGeometry()->ComputeAlphaCoverage(entity.GetTransform());
         frag_info.half_texel =
             Vector2(0.5 / gradient_texture->GetSize().width,
                     0.5 / gradient_texture->GetSize().height);