tree 8867f461ebb605fb8ada6b92811140d287284b62
parent 025b9144680456d871a9c41e36237ec0f4b35c0d
author Michael Ludwig <michaelludwig@google.com> 1620921743 -0400
committer Skia Commit-Bot <skia-commit-bot@chromium.org> 1620934673 +0000

Fix overdraw from unstable perspective math

There were two issues leading to the corruption seen in the linked
chromium issue.

1. The draw's bounds were calculated based on the quad being clipped
to w>= epsilon, which is what happens when the AA inset/outset is done.
But for non-aa quads, the fillrect and texture ops did no clipping,
assuming that the GPU would be sufficient. However, this can produce
non-aa draws that exceed the calculated bounds, misleading the clip
stack into incorrectly removing the scissor, etc.
2. Precision issues within CropToRect meant some perspective quads'
barycentric coordinates would become degenerate and compute to (0,0,1),
making it appear as if the render target/scissor were contained within
it. This meant we'd turn it into a rectangular clear.

These changes appear to address the corruption on Linux and Windows, but
there are still rendering artifacts from poor aa inset/outset
calculations. These artifacts are at least limited to the clip properly.
A better rendering method that does not rely on line intersections
will address these artifacts, but this CL is a reasonable temporary
mitigation.

Bug: chromium:1204347
Change-Id: I3c67d4efe70313ae7c98abc0a57b5b047c83890d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/407821
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>