Match CPU/GPU logic for start tangents #705
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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.
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The current state of this is "works for me," and I'm feeling a bit uncertain whether it's fully portable. If we do get hit by fastmath issues, which is entirely plausible given the weak guarantees of the language, then I'm inclined to take this in a somewhat different direction: revert the encoding changes so that the final segment writes the unmodified point values, but change the GPU-side tangent calculation so that it's based on the encoded values rather than the cubic points after degree raising. So I'm uploading this hoping to get more feedback. If it generally works, I think we can go forward. |
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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.