Rework canvas passes in prep for subpasses.

engine/src/flutter/impeller/aiks/aiks_unittests.cc

@@ -103,6 +103,30 @@ TEST_F(AiksTest, CanRenderClips) {
   canvas.ClipPath(
       PathBuilder{}.AddRect(Rect::MakeXYWH(0, 0, 500, 500)).CreatePath());
   canvas.DrawPath(PathBuilder{}.AddCircle({500, 500}, 250).CreatePath(), paint);
+  // ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
+}
+
+TEST_F(AiksTest, CanRenderGroupOpacity) {
+  Canvas canvas;
+
+  Paint red;
+  red.color = Color::Red();  //.WithAlpha(0.5);
+  Paint green;
+  green.color = Color::Green();  //.WithAlpha(0.5);
+  Paint blue;
+  blue.color = Color::Blue();  //.WithAlpha(0.5);
+
+  Paint alpha;
+  alpha.color = Color::Red().WithAlpha(0.5);
+
+  canvas.SaveLayer(alpha);
+
+  canvas.DrawRect({100, 100, 100, 100}, red);
+  canvas.DrawRect({120, 120, 100, 100}, green);
+  canvas.DrawRect({140, 140, 100, 100}, blue);
+
+  canvas.Restore();
+
   ASSERT_TRUE(OpenPlaygroundHere(canvas.EndRecordingAsPicture()));
 }
 

engine/src/flutter/impeller/aiks/canvas.cc

@@ -12,15 +12,13 @@
 namespace impeller {
 
 Canvas::Canvas() {
-  xformation_stack_.push({});
-  passes_.emplace_back(CanvasPass{});
+  Save(true);
 }
 
 Canvas::~Canvas() = default;
 
 void Canvas::Save() {
-  FML_DCHECK(xformation_stack_.size() > 0);
-  xformation_stack_.push(xformation_stack_.top());
+  Save(false);
 }
 
 bool Canvas::Restore() {
@@ -28,17 +26,16 @@ bool Canvas::Restore() {
   if (xformation_stack_.size() == 1) {
     return false;
   }
-  xformation_stack_.pop();
+  xformation_stack_.pop_back();
   return true;
 }
 
 void Canvas::Concat(const Matrix& xformation) {
-  const auto current_xformation = xformation_stack_.top().xformation;
-  xformation_stack_.top().xformation = xformation * current_xformation;
+  xformation_stack_.back().xformation = xformation * GetCurrentTransformation();
 }
 
 const Matrix& Canvas::GetCurrentTransformation() const {
-  return xformation_stack_.top().xformation;
+  return xformation_stack_.back().xformation;
 }
 
 void Canvas::Translate(const Vector3& offset) {
@@ -71,6 +68,7 @@ void Canvas::DrawPath(Path path, Paint paint) {
   entity.SetPath(std::move(path));
   entity.SetStencilDepth(GetStencilDepth());
   entity.SetContents(paint.CreateContentsForEntity());
+
   GetCurrentPass().PushEntity(std::move(entity));
 }
 
@@ -86,21 +84,23 @@ void Canvas::ClipPath(Path path) {
   entity.SetPath(std::move(path));
   entity.SetContents(std::make_shared<ClipContents>());
   entity.SetStencilDepth(GetStencilDepth());
+
   GetCurrentPass().PushEntity(std::move(entity));
 }
 
 void Canvas::DrawShadow(Path path, Color color, Scalar elevation) {}
 
 void Canvas::DrawPicture(const Picture& picture) {
-  for (const auto& pass : picture.passes) {
-    CanvasPass new_pass;
-    for (const auto& entity : pass.GetPassEntities()) {
-      auto new_entity = entity;
-      new_entity.SetTransformation(GetCurrentTransformation() *
-                                   entity.GetTransformation());
-      new_pass.PushEntity(std::move(new_entity));
+  for (const auto& stack_entry : picture.entries) {
+    auto new_stack_entry = stack_entry;
+    if (auto pass = new_stack_entry.pass) {
+      for (auto entity : pass->GetPassEntities()) {
+        entity.IncrementStencilDepth(GetStencilDepth());
+        entity.SetTransformation(GetCurrentTransformation() *
+                                 entity.GetTransformation());
+      }
     }
-    passes_.emplace_back(std::move(new_pass));
+    xformation_stack_.emplace_back(std::move(new_stack_entry));
   }
 }
 
@@ -145,21 +145,49 @@ void Canvas::DrawImageRect(std::shared_ptr<Image> image,
 
 Picture Canvas::EndRecordingAsPicture() {
   Picture picture;
-  picture.passes = std::move(passes_);
+  picture.entries = std::move(xformation_stack_);
   return picture;
 }
 
 CanvasPass& Canvas::GetCurrentPass() {
-  FML_DCHECK(!passes_.empty());
-  return passes_.back();
+  for (auto i = xformation_stack_.rbegin(), end = xformation_stack_.rend();
+       i < end; i++) {
+    if (i->pass.has_value()) {
+      return i->pass.value();
+    }
+  }
+  FML_UNREACHABLE();
 }
 
 void Canvas::IncrementStencilDepth() {
-  ++xformation_stack_.top().stencil_depth;
+  ++xformation_stack_.back().stencil_depth;
 }
 
 size_t Canvas::GetStencilDepth() const {
-  return xformation_stack_.top().stencil_depth;
+  return xformation_stack_.back().stencil_depth;
+}
+
+void Canvas::DrawRect(Rect rect, Paint paint) {
+  DrawPath(PathBuilder{}.AddRect(rect).CreatePath(), std::move(paint));
+}
+
+void Canvas::Save(bool create_subpass) {
+  // Check if called from the ctor.
+  if (xformation_stack_.empty()) {
+    FML_DCHECK(create_subpass) << "Base entries must have a pass.";
+    CanvasStackEntry entry;
+    entry.pass = CanvasPass{};
+    xformation_stack_.emplace_back(std::move(entry));
+  }
+
+  auto entry = CanvasStackEntry{};
+
+  entry.xformation = xformation_stack_.back().xformation;
+  entry.stencil_depth = xformation_stack_.back().stencil_depth;
+  if (create_subpass) {
+    entry.pass = CanvasPass{};
+  }
+  xformation_stack_.emplace_back(std::move(entry));
 }
 
 }  // namespace impeller

engine/src/flutter/impeller/aiks/canvas.h

@@ -4,9 +4,9 @@
 
 #pragma once
 
+#include <deque>
 #include <memory>
 #include <optional>
-#include <stack>
 #include <vector>
 
 #include "flutter/fml/macros.h"
@@ -23,11 +23,6 @@ namespace impeller {
 
 class Entity;
 
-struct CanvasStackEntry {
-  Matrix xformation;
-  size_t stencil_depth = 0u;
-};
-
 class Canvas {
  public:
   Canvas();
@@ -56,6 +51,8 @@ class Canvas {
 
   void DrawPath(Path path, Paint paint);
 
+  void DrawRect(Rect rect, Paint paint);
+
   void DrawImage(std::shared_ptr<Image> image, Point offset, Paint paint);
 
   void DrawImageRect(std::shared_ptr<Image> image,
@@ -72,8 +69,7 @@ class Canvas {
   Picture EndRecordingAsPicture();
 
  private:
-  std::stack<CanvasStackEntry> xformation_stack_;
-  std::vector<CanvasPass> passes_;
+  std::deque<CanvasStackEntry> xformation_stack_;
 
   CanvasPass& GetCurrentPass();
 
@@ -81,6 +77,8 @@ class Canvas {
 
   size_t GetStencilDepth() const;
 
+  void Save(bool create_subpass);
+
   FML_DISALLOW_COPY_AND_ASSIGN(Canvas);
 };
 

engine/src/flutter/impeller/aiks/canvas_pass.cc

@@ -26,4 +26,27 @@ const Entity& CanvasPass::GetPostProcessingEntity() const {
   return post_processing_entity_;
 }
 
+Rect CanvasPass::GetCoverageRect() const {
+  std::optional<Point> min, max;
+  for (const auto& entity : ops_) {
+    auto coverage = entity.GetPath().GetMinMaxCoveragePoints();
+    if (!coverage.has_value()) {
+      continue;
+    }
+    if (!min.has_value()) {
+      min = coverage->first;
+    }
+    if (!max.has_value()) {
+      max = coverage->second;
+    }
+    min = min->Min(coverage->first);
+    max = max->Max(coverage->second);
+  }
+  if (!min.has_value() || !max.has_value()) {
+    return {};
+  }
+  const auto diff = *max - *min;
+  return {min->x, min->y, diff.x, diff.y};
+}
+
 }  // namespace impeller

engine/src/flutter/impeller/aiks/canvas_pass.h

@@ -5,6 +5,7 @@
 #pragma once
 
 #include <memory>
+#include <optional>
 
 #include "flutter/fml/macros.h"
 #include "impeller/entity/contents.h"
@@ -20,6 +21,8 @@ class CanvasPass {
 
   void PushEntity(Entity entity);
 
+  Rect GetCoverageRect() const;
+
   const std::vector<Entity>& GetPassEntities() const;
 
   void SetPostProcessingEntity(Entity entity);
@@ -31,4 +34,10 @@ class CanvasPass {
   Entity post_processing_entity_;
 };
 
+struct CanvasStackEntry {
+  Matrix xformation;
+  size_t stencil_depth = 0u;
+  std::optional<CanvasPass> pass;
+};
+
 }  // namespace impeller

engine/src/flutter/impeller/aiks/picture.h

@@ -4,8 +4,8 @@
 
 #pragma once
 
+#include <deque>
 #include <memory>
-#include <vector>
 
 #include "flutter/fml/macros.h"
 #include "impeller/aiks/canvas_pass.h"
@@ -14,7 +14,7 @@
 namespace impeller {
 
 struct Picture {
-  std::vector<CanvasPass> passes;
+  std::deque<CanvasStackEntry> entries;
 };
 
 }  // namespace impeller

engine/src/flutter/impeller/aiks/picture_renderer.cc

@@ -29,10 +29,12 @@ bool PictureRenderer::Render(const Surface& surface,
     return false;
   }
 
-  for (const auto& pass : picture.passes) {
-    if (!entity_renderer_.RenderEntities(surface, parent_pass,
-                                         pass.GetPassEntities())) {
-      return false;
+  for (const auto& entry : picture.entries) {
+    if (auto pass = entry.pass) {
+      if (!entity_renderer_.RenderEntities(surface, parent_pass,
+                                           pass->GetPassEntities())) {
+        return false;
+      }
     }
   }
 

engine/src/flutter/impeller/entity/entity.cc

@@ -42,4 +42,8 @@ uint32_t Entity::GetStencilDepth() const {
   return stencil_depth_;
 }
 
+void Entity::IncrementStencilDepth(uint32_t increment) {
+  stencil_depth_ += increment;
+}
+
 }  // namespace impeller

engine/src/flutter/impeller/entity/entity.h

@@ -33,6 +33,8 @@ class Entity {
 
   void SetStencilDepth(uint32_t stencil_depth);
 
+  void IncrementStencilDepth(uint32_t increment);
+
   uint32_t GetStencilDepth() const;
 
  private:

engine/src/flutter/impeller/geometry/color.h

@@ -62,6 +62,10 @@ struct Color {
 
   static constexpr Color Blue() { return {0.0, 0.0, 1.0, 1.0}; }
 
+  constexpr Color WithAlpha(Scalar new_alpha) const {
+    return {red, green, blue, new_alpha};
+  }
+
   static constexpr Color AliceBlue() {
     return {240.0 / 255.0, 248.0 / 255.0, 255.0 / 255.0, 1.0};
   }

engine/src/flutter/impeller/geometry/path.cc

@@ -174,9 +174,20 @@ std::vector<Point> Path::CreatePolyline(
 }
 
 Rect Path::GetBoundingBox() const {
-  if (linears_.empty() && quads_.empty() && cubics_.empty()) {
+  auto min_max = GetMinMaxCoveragePoints();
+  if (!min_max.has_value()) {
     return {};
   }
+  auto min = min_max->first;
+  auto max = min_max->second;
+  const auto difference = max - min;
+  return {min.x, min.y, difference.x, difference.y};
+}
+
+std::optional<std::pair<Point, Point>> Path::GetMinMaxCoveragePoints() const {
+  if (linears_.empty() && quads_.empty() && cubics_.empty()) {
+    return std::nullopt;
+  }
 
   std::optional<Point> min, max;
 
@@ -209,9 +220,7 @@ Rect Path::GetBoundingBox() const {
     clamp(cubic.Extrema());
   }
 
-  const auto difference = *max - *min;
-
-  return {min->x, min->y, difference.x, difference.y};
+  return std::make_pair(min.value(), max.value());
 }
 
 }  // namespace impeller

engine/src/flutter/impeller/geometry/path.h

@@ -5,6 +5,7 @@
 #pragma once
 
 #include <functional>
+#include <optional>
 #include <vector>
 
 #include "impeller/geometry/path_component.h"
@@ -68,6 +69,8 @@ class Path {
 
   Rect GetBoundingBox() const;
 
+  std::optional<std::pair<Point, Point>> GetMinMaxCoveragePoints() const;
+
  private:
   struct ComponentIndexPair {
     ComponentType type = ComponentType::kLinear;

engine/src/flutter/impeller/geometry/point.h

@@ -4,6 +4,7 @@
 
 #pragma once
 
+#include <algorithm>
 #include <cmath>
 #include <string>
 
@@ -83,6 +84,14 @@ struct TPoint {
     return dx * dx + dy * dy;
   }
 
+  constexpr TPoint Min(const TPoint& p) const {
+    return {std::min<Type>(x, p.x), std::min<Type>(y, p.y)};
+  }
+
+  constexpr TPoint Max(const TPoint& p) const {
+    return {std::max<Type>(x, p.x), std::max<Type>(y, p.y)};
+  }
+
   constexpr Type GetDistance(const TPoint& p) const {
     return sqrt(GetDistanceSquared(p));
   }