Slimming Paint phase 2 compositing algorithm plumbing & skeleton display list API.

Source/core/compositing/DisplayListCompositingBuilder.cpp

+// Copyright 2014 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 "config.h"
+#include "core/compositing/DisplayListCompositingBuilder.h"
+
+#include "platform/graphics/paint/DisplayItemTransformTreeBuilder.h"
+#include "platform/graphics/paint/DisplayList.h"
+
+namespace blink {
+
+void DisplayListCompositingBuilder::build()
+{
+    DisplayItemTransformTreeBuilder transformTreeBuilder;

[pdr., 2015/07/24 05:37:51]

Can you talk a bit about how fixed position diffs might work here? I'm thinking
through how the diff format would need to contain layer diffs which seems to
introduce a chicken-and-egg issue.

[chrishtr, 2015/07/24 14:53:45]

Fixed position display items would be emitted as they are already today (as part
of the DisplayItemList), and the diff format would have no special treatment of
{Begin,End}FixedPositionDisplayItems. If they are part of the diff (meaning they
might have changed) then the corresponding part of the transform tree and layer
structure would be rebuilt.

Jeremy's design doc has additional details:

https://docs.google.com/document/d/1VWjdq8hCJlNbak5ZyAsnLh-0--Hl_wht0xyuagODl...

+    transformTreeBuilder.build(m_displayItemList, m_displayListDiff);
+    m_transformTree = transformTreeBuilder.releaseTransformTree();
+
+    buildGraphicsLayerTree();
+
+    m_rootGraphicsLayer.setTransformTree(m_transformTree.release());
+}
+
+void DisplayListCompositingBuilder::buildGraphicsLayerTree()
+{
+    // TODO(chrishtr): implement by walking the display list and transform tree, plus additional hints, to decide layerization,
+    // including overlap testing and optimizations such as squashing.
+
+    // Also create a DisplayList for each graphics layer that is a sliced window into the DisplayItemList for the page.
+
+    // And add pixel refs.
+
+    // Trivial implementation with only one composited layer:
+    OwnPtr<DisplayList> displayList = adoptPtr(new DisplayList(m_displayItemList));
+    for (unsigned i = 0; i < m_displayItemList.displayItems().size(); i++) {
+        displayList->offsets.append(i);
+    }
+    m_rootGraphicsLayer.setDisplayList(displayList.release());
+}
+
+} // namespace blink