implement A8 destination fast-path for SkPixelInfo::CopyPixels

implement A8 destination fast-path for SkPixelInfo::CopyPixels

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2407093002

Committed: https://skia.googlesource.com/skia/+/a2415accf1f184c4df6cffc2ccdd38f8d61031f6

[lsalzman1, 2016-10-11 04:28:58]

Description was changed from

==========
implement A8 destination fast-path for SkPixelInfo::CopyPixels

BUG=skia:
==========

to

==========
implement A8 destination fast-path for SkPixelInfo::CopyPixels

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2407093002
==========

[lsalzman1, 2016-10-11 04:33:02]

lsalzman@mozilla.com changed reviewers:
+ reed@google.com

[lsalzman1, 2016-10-11 04:33:02]

When trying to extract alpha values from an SkPixmap or a raster SkImage,
readPixels() can be slow due to punting to the canvas drawing fallback.

SkBitmap has a more efficient implementation in extractAlpha(), but it is
currently cumbersome to peek at the pixmap of the underlying SkImage, then
install that onto a bitmap, just to use that implementation.

This patch refactors things so SkPixelInfo::CopyPixels has the actual fast-path
code which then extractAlpha()/readPixels()/writePixels()/etc. are all using.

This way one can simply happily use readPixels() on an SkImage or SkPixmap and
not worry.

[reed1, 2016-10-11 16:19:24]

Is there already a test that exercises this readPixels -> gets alpha ?

[lsalzman1, 2016-10-11 17:57:39]

On 2016/10/11 16:19:24, reed1 wrote:
> Is there already a test that exercises this readPixels -> gets alpha ?

Not currently, but I can adjust Rtests/ReadPixelsTest.cpp to cover this pretty
easily, it looks like.

[lsalzman1, 2016-10-11 19:24:27]

There is now a shiny new subtest for kAlpha_8_SkColorType in ReadPixelsTest. It
all works. Passes before and after the change, so the behavior is compatible.

[reed1, 2016-10-11 20:39:57]

lgtm

[lsalzman1, 2016-10-11 21:08:52]

The CQ bit was checked by lsalzman@mozilla.com

[commit-bot: I haz the power, 2016-10-11 21:08:57]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-10-11 21:29:15]

Description was changed from

==========
implement A8 destination fast-path for SkPixelInfo::CopyPixels

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2407093002
==========

to

==========
implement A8 destination fast-path for SkPixelInfo::CopyPixels

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2407093002

Committed:
https://skia.googlesource.com/skia/+/a2415accf1f184c4df6cffc2ccdd38f8d61031f6
==========

[commit-bot: I haz the power, 2016-10-11 21:29:16]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/a2415accf1f184c4df6cffc2ccdd38f8d61031f6