cc: Add predictive pre-painting.

When scrolling quickly and with painting just barely able to keep up,
it's possible get into a bad state where we always do visible
painting and never get a chance to paint ahead of the fold. This
shows up as flickering at the bottom of the page.

This patch has two parts. First, it uses a scroll prediction when
choosing what to paint (both for 'visible' and off-screen painting).
Second, it adds a very simple scroll predictor based on the change
in the visibleContentRect. The second part should almost certainly
be improved (use impl-thread scroll/animation directions/velocities)
but this provides an initial estimate that helps in the most important
case (basic scrolling of big layers).

This doesn't actually change the priorities of textures in the scroll
direction (but changing the visible rect does this partially).
This is not an issue currently since the what-to-paint logic doesn't
strictly adhere to priority order.

BUG=121957


Review URL: https://chromiumcodereview.appspot.com/11193010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@163665 0039d316-1c4b-4281-b951-d872f2087c98

1 files changed, 7 insertions, 0 deletions

diff --git a/cc/tiled_layer.h b/cc/tiled_layer.h
index 0712c5f..d3ec856 100644
--- a/cc/tiled_layer.h
+++ b/cc/tiled_layer.h
@@ -83,6 +83,7 @@ private:
     bool haveTexturesForTiles(int left, int top, int right, int bottom, bool ignoreOcclusions);
     IntRect markTilesForUpdate(int left, int top, int right, int bottom, bool ignoreOcclusions);
     void updateTileTextures(const IntRect& paintRect, int left, int top, int right, int bottom, TextureUpdateQueue&, const OcclusionTracker*, RenderingStats&);
+    void updateScrollPrediction();
 
     UpdatableTile* tileAt(int, int) const;
     UpdatableTile* createTile(int, int);
@@ -91,6 +92,12 @@ private:
     bool m_skipsDraw;
     bool m_failedUpdate;
 
+    // Used for predictive painting.
+    IntSize m_predictedScroll;
+    IntRect m_predictedVisibleRect;
+    IntRect m_previousVisibleRect;
+    IntSize m_previousContentBounds;
+
     TilingOption m_tilingOption;
     scoped_ptr<LayerTilingData> m_tiler;
 };