[fuchsia] Avoid creating separate system layers for PhysicalShapeLayers.

Change PhysicalShapeLayers to be composited by
Flutter by default. In this case, shadows will be
rendered too.

If there is embedded Fuchsia content in the scene
(a ChildSceneLayer),  PhysicalShapeLayers that 
appear above the embedded content will be turned
into their own Scenic layers.
In this case, there will be no shadows rendered, as
it is in the current implementation. We will change
this later after clients adjust their usage of PSL
once this change lands.

Author: mikejurka

flow/layers/child_scene_layer.cc

@@ -20,6 +20,7 @@ ChildSceneLayer::ChildSceneLayer(zx_koid_t layer_id,
 void ChildSceneLayer::Preroll(PrerollContext* context, const SkMatrix& matrix) {
   TRACE_EVENT0("flutter", "ChildSceneLayer::Preroll");
   set_needs_system_composite(true);
+  context->child_scene_layer_exists_below = true;
 
   // An alpha "hole punch" is required if the frame behind us is not opaque.
   if (!context->is_opaque) {
@@ -49,7 +50,9 @@ void ChildSceneLayer::UpdateScene(SceneUpdateContext& context) {
   auto* view_holder = ViewHolder::FromId(layer_id_);
   FML_DCHECK(view_holder);
 
-  view_holder->UpdateScene(context, offset_, size_, hit_testable_);
+  view_holder->UpdateScene(context, offset_, size_,
+                           sk_float_round2int(context.alphaf() * 255),
+                           hit_testable_);
 }
 
 }  // namespace flutter

flow/layers/layer.h

@@ -66,6 +66,11 @@ struct PrerollContext {
   float total_elevation = 0.0f;
   bool has_platform_view = false;
   bool is_opaque = true;
+#if defined(OS_FUCHSIA)
+  // True if, during the traversal so far, we have seen a child_scene_layer.
+  // Informs whether a layer needs to be system composited.
+  bool child_scene_layer_exists_below = false;
+#endif  // defined(OS_FUCHSIA)
 };
 
 // Represents a single composited layer. Created on the UI thread but then

flow/layers/layer_tree.cc

@@ -83,7 +83,7 @@ void LayerTree::UpdateScene(SceneUpdateContext& context,
       context,
       SkRRect::MakeRect(
           SkRect::MakeWH(frame_size_.width(), frame_size_.height())),
-      SK_ColorTRANSPARENT, SK_AlphaOPAQUE);
+      SK_ColorTRANSPARENT, SK_AlphaOPAQUE, "flutter::LayerTree");
   if (root_layer_->needs_system_composite()) {
     root_layer_->UpdateScene(context);
   }

flow/layers/opacity_layer.cc

@@ -9,11 +9,6 @@
 
 namespace flutter {
 
-// The OpacityLayer has no real "elevation", but we want to avoid Z-fighting
-// when using the system compositor.  Choose a small but non-zero value for
-// this.
-constexpr float kOpacityElevationWhenUsingSystemCompositor = 0.01f;
-
 OpacityLayer::OpacityLayer(SkAlpha alpha, const SkPoint& offset)
     : alpha_(alpha), offset_(offset) {
   // Ensure OpacityLayer has only one direct child.
@@ -40,8 +35,6 @@ void OpacityLayer::Preroll(PrerollContext* context, const SkMatrix& matrix) {
   SkMatrix child_matrix = matrix;
   child_matrix.postTranslate(offset_.fX, offset_.fY);
 
-  total_elevation_ = context->total_elevation;
-  context->total_elevation += kOpacityElevationWhenUsingSystemCompositor;
   context->is_opaque = parent_is_opaque && (alpha_ == SK_AlphaOPAQUE);
   context->mutators_stack.PushTransform(
       SkMatrix::MakeTrans(offset_.fX, offset_.fY));
@@ -52,17 +45,7 @@ void OpacityLayer::Preroll(PrerollContext* context, const SkMatrix& matrix) {
   context->mutators_stack.Pop();
   context->mutators_stack.Pop();
   context->is_opaque = parent_is_opaque;
-  context->total_elevation = total_elevation_;
 
-#if defined(OS_FUCHSIA)
-  if (needs_system_composite()) {
-    // When using the system compositor, do not include the offset since we
-    // are rendering as a separate piece of geometry and the offset will be
-    // baked into that geometry's transform.
-    frameRRect_ = SkRRect::MakeRect(paint_bounds());
-    set_paint_bounds(SkRect::MakeEmpty());
-  } else
-#endif
   {
     set_paint_bounds(paint_bounds().makeOffset(offset_.fX, offset_.fY));
     if (!context->has_platform_view && context->raster_cache &&
@@ -123,36 +106,10 @@ void OpacityLayer::Paint(PaintContext& context) const {
 #if defined(OS_FUCHSIA)
 
 void OpacityLayer::UpdateScene(SceneUpdateContext& context) {
-  FML_DCHECK(needs_system_composite());
-  TRACE_EVENT0("flutter", "OpacityLayer::UpdateScene");
-
-  ContainerLayer* container = GetChildContainer();
-  FML_DCHECK(!container->layers().empty());  // OpacityLayer can't be a leaf.
-
-  SceneUpdateContext::Transform transform(
-      context, SkMatrix::MakeTrans(offset_.fX, offset_.fY));
-
-  // Retained rendering: speedup by reusing a retained entity node if possible.
-  // When an entity node is reused, no paint layer is added to the frame so we
-  // won't call PhysicalShapeLayer::Paint.
-  LayerRasterCacheKey key(unique_id(), context.Matrix());
-  if (context.HasRetainedNode(key)) {
-    TRACE_EVENT_INSTANT0("flutter", "retained layer cache hit");
-    const scenic::EntityNode& retained_node = context.GetRetainedNode(key);
-    FML_DCHECK(context.top_entity());
-    FML_DCHECK(retained_node.session() == context.session());
-    context.top_entity()->embedder_node().AddChild(retained_node);
-    return;
-  }
-
-  TRACE_EVENT_INSTANT0("flutter", "cache miss, creating");
-  // If we can't find an existing retained surface, create one.
-  SceneUpdateContext::Frame frame(
-      context, frameRRect_, SK_ColorTRANSPARENT, alpha_,
-      kOpacityElevationWhenUsingSystemCompositor, total_elevation_, this);
-  frame.AddPaintLayer(container);
-
-  UpdateSceneChildren(context);
+  float saved_alpha = context.alphaf();
+  context.set_alphaf(context.alphaf() * (alpha_ / 255.f));
+  ContainerLayer::UpdateScene(context);
+  context.set_alphaf(saved_alpha);
 }
 
 #endif  // defined(OS_FUCHSIA)

flow/layers/opacity_layer.h

@@ -43,7 +43,6 @@ class OpacityLayer : public ContainerLayer {
   SkAlpha alpha_;
   SkPoint offset_;
   SkRRect frameRRect_;
-  float total_elevation_ = 0.0f;
 
   FML_DISALLOW_COPY_AND_ASSIGN(OpacityLayer);
 };

flow/layers/physical_shape_layer.cc

@@ -52,23 +52,32 @@ void PhysicalShapeLayer::Preroll(PrerollContext* context,
 
   context->total_elevation += elevation_;
   total_elevation_ = context->total_elevation;
+#if defined(OS_FUCHSIA)
+  child_layer_exists_below_ = context->child_scene_layer_exists_below;
+  context->child_scene_layer_exists_below = false;
+ #endif
+
   SkRect child_paint_bounds;
   PrerollChildren(context, matrix, &child_paint_bounds);
+
+#if defined(OS_FUCHSIA)
+  children_need_system_compositing_ = needs_system_composite();
+  if (child_layer_exists_below_) {
+    set_needs_system_composite(true);
+  }
+  context->child_scene_layer_exists_below =
+      context->child_scene_layer_exists_below || child_layer_exists_below_;
+#endif
   context->total_elevation -= elevation_;
 
   if (elevation_ == 0) {
     set_paint_bounds(path_.getBounds());
   } else {
-#if defined(OS_FUCHSIA)
-    // Let the system compositor draw all shadows for us.
-    set_needs_system_composite(true);
-#else
     // We will draw the shadow in Paint(), so add some margin to the paint
     // bounds to leave space for the shadow. We fill this whole region and clip
     // children to it so we don't need to join the child paint bounds.
     set_paint_bounds(ComputeShadowBounds(path_.getBounds(), elevation_,
                                          context->frame_device_pixel_ratio));
-#endif  // defined(OS_FUCHSIA)
   }
 }
 
@@ -78,30 +87,64 @@ void PhysicalShapeLayer::UpdateScene(SceneUpdateContext& context) {
   FML_DCHECK(needs_system_composite());
   TRACE_EVENT0("flutter", "PhysicalShapeLayer::UpdateScene");
 
-  // Retained rendering: speedup by reusing a retained entity node if possible.
-  // When an entity node is reused, no paint layer is added to the frame so we
-  // won't call PhysicalShapeLayer::Paint.
-  LayerRasterCacheKey key(unique_id(), context.Matrix());
-  if (context.HasRetainedNode(key)) {
-    TRACE_EVENT_INSTANT0("flutter", "retained layer cache hit");
-    const scenic::EntityNode& retained_node = context.GetRetainedNode(key);
-    FML_DCHECK(context.top_entity());
-    FML_DCHECK(retained_node.session() == context.session());
-    context.top_entity()->entity_node().AddChild(retained_node);
-    return;
-  }
+  // If there is embedded Fuchsia content in the scene (a ChildSceneLayer),
+  // PhysicalShapeLayers that appear above the embedded content will be turned
+  // into their own Scenic layers.
+  if (child_layer_exists_below_) {
+    // Reset paint bounds, because shadows are not rendered in this case
+    // (see comment in Paint()).
+    set_paint_bounds(path_.getBounds());
+
+    float global_scenic_elevation = context.GetGlobalElevationForNextScenicLayer();
+    float local_scenic_elevation_ =
+        global_scenic_elevation - context.scenic_elevation();
+    float z_translation = -local_scenic_elevation_;
+
+    // Retained rendering: speedup by reusing a retained entity node if
+    // possible. When an entity node is reused, no paint layer is added to the
+    // frame so we won't call PhysicalShapeLayer::Paint.
+    LayerRasterCacheKey key(unique_id(), context.Matrix());
+    if (context.HasRetainedNode(key)) {
+      TRACE_EVENT_INSTANT0("flutter", "retained layer cache hit");
+      scenic::EntityNode* retained_node = context.GetRetainedNode(key);
+      FML_DCHECK(context.top_entity());
+      FML_DCHECK(retained_node->session() == context.session());
 
-  TRACE_EVENT_INSTANT0("flutter", "cache miss, creating");
-  // If we can't find an existing retained surface, create one.
-  SceneUpdateContext::Frame frame(context, frameRRect_, color_, SK_AlphaOPAQUE,
-                                  elevation_, total_elevation_, this);
-  for (auto& layer : layers()) {
-    if (layer->needs_painting()) {
-      frame.AddPaintLayer(layer.get());
+      // Re-adjust the elevation.
+      retained_node->SetTranslation(0.f, 0.f, z_translation);
+
+      context.top_entity()->entity_node().AddChild(*retained_node);
+      return;
+    }
+
+    TRACE_EVENT_INSTANT0("flutter", "cache miss, creating");
+    // If we can't find an existing retained surface, create one.
+    SceneUpdateContext::Frame frame(context, frameRRect_, SK_ColorTRANSPARENT,
+                                    sk_float_round2int(context.alphaf() * 255),
+                                    "flutter::PhysicalShapeLayer",
+                                    z_translation, this);
+
+    frame.AddPaintLayer(this);
+
+    // Node: UpdateSceneChildren needs to be called here so that |frame| is still
+    // in scope (and therefore alive) while UpdateSceneChildren is being called.
+    if (children_need_system_compositing_) {
+      UpdateSceneChildren(context);
+    }
+  } else {
+    if (children_need_system_compositing_) {
+      UpdateSceneChildren(context);
     }
   }
+}
+
+void PhysicalShapeLayer::UpdateSceneChildren(SceneUpdateContext& context) {
+  float scenic_elevation = context.scenic_elevation();
+  context.set_scenic_elevation(scenic_elevation + local_scenic_elevation_);
 
-  UpdateSceneChildren(context);
+  ContainerLayer::UpdateSceneChildren(context);
+
+  context.set_scenic_elevation(scenic_elevation);
 }
 
 #endif  // defined(OS_FUCHSIA)
@@ -110,7 +153,20 @@ void PhysicalShapeLayer::Paint(PaintContext& context) const {
   TRACE_EVENT0("flutter", "PhysicalShapeLayer::Paint");
   FML_DCHECK(needs_painting());
 
+
+#if defined(OS_FUCHSIA)
+  // TODO: Re-enable shadow drawing here.
+  // Shadows are not rendered for PhysicalShapeLayers that exist as separate
+  // system services; this is to maintain compatibility with the previous
+  // implementation and has the added benefit of requiring smaller textures, since
+  // extra space is not needed for the shadows.
+  // This behavior might change  after clients adjust their usage of PhysicalShaperLayer
+  // to make elevation correlate to desired shadow size.
+  //if (false && !child_layer_exists_below_ && elevation_ != 0) {
+  if (elevation_ != 0) {
+#else
   if (elevation_ != 0) {
+#endif
     DrawShadow(context.leaf_nodes_canvas, path_, shadow_color_, elevation_,
                SkColorGetA(color_) != 0xff, context.frame_device_pixel_ratio);
   }

flow/layers/physical_shape_layer.h

@@ -37,11 +37,18 @@ class PhysicalShapeLayer : public ContainerLayer {
 
 #if defined(OS_FUCHSIA)
   void UpdateScene(SceneUpdateContext& context) override;
+
+  void UpdateSceneChildren(SceneUpdateContext& context);
 #endif  // defined(OS_FUCHSIA)
 
   float total_elevation() const { return total_elevation_; }
 
  private:
+#if defined(OS_FUCHSIA)
+  float local_scenic_elevation_ = 0.0f;
+  bool child_layer_exists_below_ = false;
+  bool children_need_system_compositing_ = false;
+#endif
   SkColor color_;
   SkColor shadow_color_;
   float elevation_ = 0.0f;

flow/layers/transform_layer.cc

@@ -56,8 +56,12 @@ void TransformLayer::UpdateScene(SceneUpdateContext& context) {
   TRACE_EVENT0("flutter", "TransformLayer::UpdateScene");
   FML_DCHECK(needs_system_composite());
 
-  SceneUpdateContext::Transform transform(context, transform_);
-  UpdateSceneChildren(context);
+  if (!transform_.isIdentity()) {
+    SceneUpdateContext::Transform transform(context, transform_);
+    UpdateSceneChildren(context);
+  } else {
+    UpdateSceneChildren(context);
+  }
 }
 
 #endif  // defined(OS_FUCHSIA)