Compute text coverage; use blend mode in savelayer; conservative pass…

… collapse/elision behavior (flutter#129)

impeller/aiks/aiks_unittests.cc

@@ -418,6 +418,27 @@ TEST_F(AiksTest, CanRenderEmojiTextFrame) {
   ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
 }
 
+TEST_F(AiksTest, CanRenderTextInSaveLayer) {
+  Canvas canvas;
+  canvas.DrawPaint({.color = Color::White()});
+  canvas.Translate({100, 100});
+  canvas.Scale(Vector2{0.5, 0.5});
+
+  // Blend the layer with the parent pass using kClear to expose the coverage.
+  canvas.SaveLayer({.blend_mode = Entity::BlendMode::kClear});
+  ASSERT_TRUE(RenderTextInCanvas(
+      GetContext(), canvas, "the quick brown fox jumped over the lazy dog!.?",
+      "Roboto-Regular.ttf"));
+  canvas.Restore();
+
+  // Render the text again over the cleared coverage rect.
+  ASSERT_TRUE(RenderTextInCanvas(
+      GetContext(), canvas, "the quick brown fox jumped over the lazy dog!.?",
+      "Roboto-Regular.ttf"));
+
+  ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
+}
+
 TEST_F(AiksTest, CanDrawPaint) {
   Paint paint;
   paint.color = Color::MediumTurquoise();

impeller/aiks/canvas.cc

@@ -141,9 +141,10 @@ void Canvas::DrawCircle(Point center, Scalar radius, Paint paint) {
 
 void Canvas::SaveLayer(Paint paint, std::optional<Rect> bounds) {
   GetCurrentPass().SetDelegate(
-      std::make_unique<PaintPassDelegate>(std::move(paint), bounds));
+      std::make_unique<PaintPassDelegate>(paint, bounds));
 
   Save(true);
+  GetCurrentPass().SetBlendMode(paint.blend_mode);
 
   if (bounds.has_value()) {
     // Render target switches due to a save layer can be elided. In such cases

impeller/aiks/paint_pass_delegate.cc

@@ -22,12 +22,12 @@ std::optional<Rect> PaintPassDelegate::GetCoverageRect() {
 
 // |EntityPassDelgate|
 bool PaintPassDelegate::CanElide() {
-  return paint_.color.IsTransparent();
+  return paint_.blend_mode == Entity::BlendMode::kDestination;
 }
 
 // |EntityPassDelgate|
 bool PaintPassDelegate::CanCollapseIntoParentPass() {
-  return paint_.color.IsOpaque();
+  return false;
 }
 
 // |EntityPassDelgate|

impeller/entity/contents/text_contents.cc

@@ -4,6 +4,8 @@
 
 #include "impeller/entity/contents/text_contents.h"
 
+#include <optional>
+
 #include "impeller/entity/contents/content_context.h"
 #include "impeller/entity/entity.h"
 #include "impeller/geometry/path_builder.h"
@@ -48,6 +50,14 @@ void TextContents::SetColor(Color color) {
   color_ = color;
 }
 
+std::optional<Rect> TextContents::GetCoverage(const Entity& entity) const {
+  auto bounds = frame_.GetBounds();
+  if (!bounds.has_value()) {
+    return std::nullopt;
+  }
+  return bounds->TransformBounds(entity.GetTransformation());
+}
+
 bool TextContents::Render(const ContentContext& renderer,
                           const Entity& entity,
                           RenderPass& pass) const {

impeller/entity/contents/text_contents.h

@@ -34,6 +34,9 @@ class TextContents final : public Contents {
 
   void SetColor(Color color);
 
+  // |Contents|
+  std::optional<Rect> GetCoverage(const Entity& entity) const override;
+
   // |Contents|
   bool Render(const ContentContext& renderer,
               const Entity& entity,

impeller/entity/entity_pass.cc

@@ -210,6 +210,7 @@ bool EntityPass::Render(ContentContext& renderer,
                        .TakePath());
     entity.SetContents(std::move(offscreen_texture_contents));
     entity.SetStencilDepth(stencil_depth_);
+    entity.SetBlendMode(subpass->blend_mode_);
     // Once we have filters being applied for SaveLayer, some special sauce
     // may be needed here (or in PaintPassDelegate) to ensure the filter
     // parameters are transformed by the `xformation_` matrix, while continuing
@@ -257,4 +258,8 @@ void EntityPass::SetStencilDepth(size_t stencil_depth) {
   stencil_depth_ = stencil_depth;
 }
 
+void EntityPass::SetBlendMode(Entity::BlendMode blend_mode) {
+  blend_mode_ = blend_mode;
+}
+
 }  // namespace impeller

impeller/entity/entity_pass.h

@@ -58,12 +58,15 @@ class EntityPass {
 
   void SetStencilDepth(size_t stencil_depth);
 
+  void SetBlendMode(Entity::BlendMode blend_mode);
+
  private:
   Entities entities_;
   Subpasses subpasses_;
   EntityPass* superpass_ = nullptr;
   Matrix xformation_;
   size_t stencil_depth_ = 0u;
+  Entity::BlendMode blend_mode_ = Entity::BlendMode::kSourceOver;
   std::unique_ptr<EntityPassDelegate> delegate_ =
       EntityPassDelegate::MakeDefault();
   std::shared_ptr<LazyGlyphAtlas> lazy_glyph_atlas_ =

impeller/typographer/backends/skia/text_frame_skia.cc

@@ -29,8 +29,7 @@ static Font ToFont(const SkFont& font, Scalar scale) {
   return Font{std::move(typeface), std::move(metrics)};
 }
 
-TextFrame TextFrameFromTextBlob(sk_sp<SkTextBlob> blob,
-                                Scalar scale) {
+TextFrame TextFrameFromTextBlob(sk_sp<SkTextBlob> blob, Scalar scale) {
   if (!blob) {
     return {};
   }

impeller/typographer/text_frame.cc

@@ -10,6 +10,21 @@ TextFrame::TextFrame() = default;
 
 TextFrame::~TextFrame() = default;
 
+std::optional<Rect> TextFrame::GetBounds() const {
+  std::optional<Rect> result;
+
+  for (const auto& run : runs_) {
+    const auto glyph_bounds = run.GetFont().GetMetrics().GetBoundingBox();
+    for (const auto& glyph_position : run.GetGlyphPositions()) {
+      Vector2 position = glyph_position.position * Vector2();
+      Rect glyph_rect = Rect(position + glyph_bounds.origin, glyph_bounds.size);
+      result = result.has_value() ? result->Union(glyph_rect) : glyph_rect;
+    }
+  }
+
+  return result;
+}
+
 bool TextFrame::AddTextRun(TextRun run) {
   if (!run.IsValid()) {
     return false;

impeller/typographer/text_frame.h

@@ -21,6 +21,14 @@ class TextFrame {
 
   ~TextFrame();
 
+  //----------------------------------------------------------------------------
+  /// @brief      The conservative bounding box for this text frame.
+  ///
+  /// @return     The bounds rectangle. If there are no glyphs in this text
+  ///             frame, std::nullopt is returned.
+  ///
+  std::optional<Rect> GetBounds() const;
+
   //----------------------------------------------------------------------------
   /// @brief      The number of runs in this text frame.
   ///