Clean up multisampled color mask workaround for DrawingBuffer.

Clean up multisampled color mask workaround for DrawingBuffer.

On old AMD GPUs, glColorMask() doesn't work correctly for multisampled
renderbuffers. The old workaround used a GL_RGB renderbuffer, resolved to a
GL_RGBA renderbuffer, and then blitted to the destination texture (GL_RGBA).

The new workaround uses glColorMask() with a GL_RGBA renderbuffer. On GPUs where
glColorMask() doesn't work correctly, the alpha channel of the destination
texture is cleared to 1 after the resolve. On all other GPUs, no additional
clear is necessary.

BUG=595948
Committed: https://crrev.com/824f8b72a0ecde812abe61c0582f0f57bd230d6e
Cr-Commit-Position: refs/heads/master@{#391902}

[erikchen, 2016-05-05 00:46:49]

erikchen@chromium.org changed reviewers:
+ kbr@chromium.org

[erikchen, 2016-05-05 00:46:49]

kbr: Please review.

[Ken Russell (switch to Gerrit), 2016-05-05 18:19:23]

Thanks, looks great. Despite the fact that a second full-canvas pass (the clear)
is done, I think this is a lot simpler and will save VRAM. LGTM

https://codereview.chromium.org/1953623002/diff/40001/third_party/WebKit/Sour...
File third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp (right):

https://codereview.chromium.org/1953623002/diff/40001/third_party/WebKit/Sour...
third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp:779: // alpha
channel. Clear the alpha channel of |m_fbo|.
Please mention in the comment that there were bugs in the handling of
glColorMask with multisampled renderbuffers on some GPUs, in particular older
AMD GPUs on Mac OS X.

[erikchen, 2016-05-05 18:33:27]

https://codereview.chromium.org/1953623002/diff/40001/third_party/WebKit/Sour...
File third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp (right):

https://codereview.chromium.org/1953623002/diff/40001/third_party/WebKit/Sour...
third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp:779: // alpha
channel. Clear the alpha channel of |m_fbo|.
On 2016/05/05 18:19:23, Ken Russell wrote:
> Please mention in the comment that there were bugs in the handling of
> glColorMask with multisampled renderbuffers on some GPUs, in particular older
> AMD GPUs on Mac OS X.

Done.

[erikchen, 2016-05-05 18:33:30]

The CQ bit was checked by erikchen@chromium.org

[erikchen, 2016-05-05 18:33:31]

The patchset sent to the CQ was uploaded after l-g-t-m from kbr@chromium.org
Link to the patchset: https://codereview.chromium.org/1953623002/#ps60001
(title: "Comments from kbr and fix up a conditional.")

[commit-bot: I haz the power, 2016-05-05 18:34:30]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1953623002/60001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1953623002/60001

[commit-bot: I haz the power, 2016-05-05 21:31:59]

Committed patchset #4 (id:60001)

[commit-bot: I haz the power, 2016-05-05 21:33:21]

Description was changed from

==========
Clean up multisampled color mask workaround for DrawingBuffer.

On old AMD GPUs, glColorMask() doesn't work correctly for multisampled
renderbuffers. The old workaround used a GL_RGB renderbuffer, resolved to a
GL_RGBA renderbuffer, and then blitted to the destination texture (GL_RGBA).

The new workaround uses glColorMask() with a GL_RGBA renderbuffer. On GPUs where
glColorMask() doesn't work correctly, the alpha channel of the destination
texture is cleared to 1 after the resolve. On all other GPUs, no additional
clear is necessary.

BUG=595948
==========

to

==========
Clean up multisampled color mask workaround for DrawingBuffer.

On old AMD GPUs, glColorMask() doesn't work correctly for multisampled
renderbuffers. The old workaround used a GL_RGB renderbuffer, resolved to a
GL_RGBA renderbuffer, and then blitted to the destination texture (GL_RGBA).

The new workaround uses glColorMask() with a GL_RGBA renderbuffer. On GPUs where
glColorMask() doesn't work correctly, the alpha channel of the destination
texture is cleared to 1 after the resolve. On all other GPUs, no additional
clear is necessary.

BUG=595948

Committed: https://crrev.com/824f8b72a0ecde812abe61c0582f0f57bd230d6e
Cr-Commit-Position: refs/heads/master@{#391902}
==========

[commit-bot: I haz the power, 2016-05-05 21:33:22]

Patchset 4 (id:??) landed as
https://crrev.com/824f8b72a0ecde812abe61c0582f0f57bd230d6e
Cr-Commit-Position: refs/heads/master@{#391902}