Implement rational overflow clipping in SPv2

third_party/WebKit/Source/core/paint/PaintPropertyTreeBuilder.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/paint/PaintPropertyTreeBuilder.h"
 
 #include <memory>
 #include "core/dom/DOMNodeIds.h"
 #include "core/frame/FrameView.h"
 #include "core/frame/LocalFrame.h"
@@ skipping 565 matching lines @@
       ColorFilter mask_color_filter;
       bool has_mask = ComputeMaskParameters(
           mask_clip, mask_color_filter, object, context.current.paint_offset);
       if (has_mask) {
         force_subtree_update |= properties.UpdateMaskClip(
             context.current.clip, context.current.transform,
             FloatRoundedRect(mask_clip));
         output_clip = properties.MaskClip();
 
         // TODO(crbug.com/683425): PaintArtifactCompositor does not handle
-        // grouping (i.e. descendant-dependent compositing reason) properly
-        // yet. This forces masked subtree always create a layer for now.
+        // grouping (i.e. descendant-dependent compositing reason) properly yet.
+        // This forces masked subtree always create a layer for now.
         compositing_reasons |= kCompositingReasonIsolateCompositedDescendants;

[trchen, 2017/05/03 21:13:22]

(Unrelated) I think this no longer applies. I shall try remove this hack in a
separate CL.

       } else {
         if (auto* properties = object.GetMutableForPainting().PaintProperties())
           force_subtree_update |= properties->ClearMaskClip();
+
+        bool has_filter = object.IsBoxModelObject() &&
+                          object.StyleRef().HasFilter() &&
+                          ToLayoutBoxModelObject(object).Layer();
+        if (!has_filter) {
+          // [fx-containment, step 1]: push clip of any contained content below
+          // the clip of an effect that commutes with clips.
+          // context.fixed_position.clip is the "highest" in the clip tree that
+          // such a contained clip could be.
+          output_clip = context.fixed_position.clip;

[trchen, 2017/05/03 21:13:22]

fixed_position.clip is not necessarily shallower than in-flow clip, due to CSS
clip. Why is this needed though? The default from line #560 is as shallow as you
could get.

It may be beneficial to push it further down, e.g. allocating a smaller buffer
with canvas->saveLayer, but I think PaintChunksToCcLayer is in a better position
for it (for composited effects, cc already does it when computing render surface
bounds).

[chrishtr, 2017/05/03 21:20:59]

It's required to implement fx-containment, step 1 I think - no clips may escape
effects.

I don't get why fixed_position.clip might be deeper than in-flow clip. Can you
post an example?

+        }
       }
 
       SkBlendMode blend_mode =
           object.IsBlendingAllowed()
               ? WebCoreCompositeToSkiaComposite(kCompositeSourceOver,
                                                 style.BlendMode())
               : SkBlendMode::kSrcOver;
 
       CompositorElementId compositor_element_id =
           style.HasCurrentOpacityAnimation()
@@ skipping 108 matching lines @@
         force_subtree_update |= properties->ClearFilter();
     }
   }
 
   const auto* properties = object.PaintProperties();
   if (properties && properties->Filter()) {
     context.current_effect = properties->Filter();
     // TODO(trchen): Change input clip to expansion hint once implemented.
     const ClipPaintPropertyNode* input_clip =
         properties->Filter()->OutputClip();
+    // [fx-containment, step 1]: clips may not escape effects which
+    // move pixels. This only needs to happen if HasFilterThatMovesPixels(),
+    // but we do it for all filters to match SPv1 heuristics (see above).
     context.input_clip_of_current_effect = context.current.clip =
         context.absolute_position.clip = context.fixed_position.clip =
             input_clip;
   }
 }
 
 static bool NeedsCssClip(const LayoutObject& object) {
   return object.HasClip();
 }
 
@@ skipping 554 matching lines @@
                            context.force_subtree_update);
 
     context.force_subtree_update |= object.SubtreeNeedsPaintPropertyUpdate();
   }
 
   if (object.CanContainAbsolutePositionObjects())
     context.container_for_absolute_position = &object;
 }
 
 }  // namespace blink