Skip clearing the canvas before painting the contents of opaque layers

Skip clearing the canvas before painting the contents of opaque layers

The SkCanvas doesn't need to be cleared before painting the layer contents
if the layer is known to be opaque (e.g. the root layer). Bypassing the clear
gives a sizeable perf boost.

This patch also switches to calling clear() instead of drawRect() for clearing the canvas pixels as clear has been shown to be significantly faster. 

BUG=279612
Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=219894

[Vangelis Kokkevis, 2013-08-27 02:00:46]

Ready for review.

[enne (OOO), 2013-08-27 17:13:04]

https://codereview.chromium.org/23484005/diff/9001/cc/resources/content_layer...
File cc/resources/content_layer_updater.cc (right):

https://codereview.chromium.org/23484005/diff/9001/cc/resources/content_layer...
cc/resources/content_layer_updater.cc:71: canvas->drawRect(layer_sk_rect,
paint);
Could you just have the first branch of this if/else and remove the second? I
think it makes the code more confusing because it looks as though there's some
contract that pixels outside the content rect should be preserved.

The two ContentLayerUpdater-derived classes both size their canvases to
content_rect.size(), so you could just add a comment to PaintContents that
passing a larger canvas to PaintContents may write to pixels outside the content
rect if you think this is dangerous.

Alternatively, you could just pass a gfx::Vector2d offset instead of a gfx::Rect
to this function for the translation and then create the content_rect internally
based on that offset and canvas size.

[Vangelis Kokkevis, 2013-08-27 19:59:59]

Thanks, Enne. Modified the code as per your suggestion. PTAL.

[enne (OOO), 2013-08-27 20:19:11]

lgtm

[commit-bot: I haz the power, 2013-08-27 20:19:19]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/vangelis@chromium.org/23484005/19001

[commit-bot: I haz the power, 2013-08-28 01:10:02]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/vangelis@chromium.org/23484005/19001

[commit-bot: I haz the power, 2013-08-28 03:30:58]

Change committed as 219894