fix vertex alpha calculation in GrTesselator

fix vertex alpha calculation in GrTesselator

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2405813003

Committed: https://skia.googlesource.com/skia/+/8c8fceff4d4e8ec09f29748d77ed5510697a2995

[lsalzman1, 2016-10-10 22:08:09]

Description was changed from

==========
fix vertex alpha calculation in GrTesselator

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

to

==========
fix vertex alpha calculation in GrTesselator

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2405813003
==========

[lsalzman1, 2016-10-10 22:11:16]

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

[lsalzman1, 2016-10-10 22:11:17]

GrTesselator incorrectly calculates: d->fColor = SkAlphaMulQ(aaParams->fColor,
v->fAlpha)

Note that if fColor's alpha is 255, and v->fAlpha is 255, then (255 * 255)>>8
ends up as 254, so our opaque vertex color is no longer opaque.

The simple fix here is to correctly adjust the alpha to become a scale with
SkAlpha255To256.

This bug tripped up some Firefox unit tests that read back alpha from canvas.

[bsalomon, 2016-10-10 22:14:23]

Description was changed from

==========
fix vertex alpha calculation in GrTesselator

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2405813003
==========

to

==========
fix vertex alpha calculation in GrTesselator

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2405813003
==========

[bsalomon, 2016-10-10 22:14:23]

bsalomon@google.com changed reviewers:
+ senorblanco@google.com

[bsalomon, 2016-10-10 22:14:32]

+Stephen

[Stephen White, 2016-10-11 17:45:19]

LGTM

Note that there are known issues (artifacts) with the screenspace AA
tessellating path renderer, which is why it's still disabled in Chrome (via
SK_DISABLE_SCREENSPACE_TESS_AA_PATH_RENDERER). You may want to do the same for
now.

[lsalzman1, 2016-10-11 17:58:59]

The CQ bit was checked by lsalzman@mozilla.com

[commit-bot: I haz the power, 2016-10-11 17:59:11]

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 19:20:18]

Description was changed from

==========
fix vertex alpha calculation in GrTesselator

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2405813003
==========

to

==========
fix vertex alpha calculation in GrTesselator

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2405813003

Committed:
https://skia.googlesource.com/skia/+/8c8fceff4d4e8ec09f29748d77ed5510697a2995
==========

[commit-bot: I haz the power, 2016-10-11 19:20:19]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/8c8fceff4d4e8ec09f29748d77ed5510697a2995

[lsalzman1, 2016-10-11 19:44:30]

On 2016/10/11 17:45:19, Stephen White wrote:
> LGTM
> 
> Note that there are known issues (artifacts) with the screenspace AA
> tessellating path renderer, which is why it's still disabled in Chrome (via
> SK_DISABLE_SCREENSPACE_TESS_AA_PATH_RENDERER). You may want to do the same for
> now.

Noted. I will disable this in Firefox configs as well for now. We can revisit
whether to turn it back on in later updates, I guess. But hey, at least I found
a bug. :)

[Stephen White, 2016-10-11 19:45:04]

On 2016/10/11 19:44:30, lsalzman1 wrote:
> On 2016/10/11 17:45:19, Stephen White wrote:
> > LGTM
> > 
> > Note that there are known issues (artifacts) with the screenspace AA
> > tessellating path renderer, which is why it's still disabled in Chrome (via
> > SK_DISABLE_SCREENSPACE_TESS_AA_PATH_RENDERER). You may want to do the same
for
> > now.
> 
> Noted. I will disable this in Firefox configs as well for now. We can revisit
> whether to turn it back on in later updates, I guess. But hey, at least I
found
> a bug. :)

Indeed! Thanks for the fix.