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Watertightness regression in longpathdash #704

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raphlinus opened this issue Sep 26, 2024 · 0 comments · May be fixed by #705
Open

Watertightness regression in longpathdash #704

raphlinus opened this issue Sep 26, 2024 · 0 comments · May be fixed by #705
Milestone
Vello 0.3 release

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@raphlinus
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As of #695 we have a regression in longpathdash, generating artifacts consistent with loss of watertightness. The artifacts are present for both butt and round caps.

longpathdash_butt

longpathdash_round

The most likely explanation is a discrepancy between the way tangents are computed on the CPU when encoding the cap and on the GPU. My guess is that the above patch made these match for cubic segments but not for lines.

Thanks to @DJMcNab for noticing; there's a Zulip thread with a bit of discussion.
@raphlinus raphlinus added this to the Vello 0.3 release milestone Sep 26, 2024
raphlinus added a commit that referenced this issue Sep 26, 2024
@raphlinus
Match CPU/GPU logic for start tangents
a05c730 
When encoding a start tangent for an end cap or closed segment, use
logic designed to match the GPU computed tangent for the first segment.

Note: this changes the GPU calculation to write out the lerp calculation
explicitly rather than use the mix intrinsic, so we can rely on the
rounding behavior. In the presence of fastmath, the rounding behavior is
not guaranteed, but it is verified to work on M1.

Fixes #704, unless we get bitten by fastmath.
@raphlinus raphlinus linked a pull request Sep 26, 2024 that will close this issue
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Match CPU/GPU logic for start tangents linebender/vello
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@raphlinus