There were several issues involved into triangle ribbons hair:
- Even for the viewport rendering the blender scene camera was
used for orientation. This made hair triangles oriented to
the scene camera, not to the viewport camera.
- Triangle orientation was actually supposing the camera is
perspective. Triangles weren't oriented properly for the
orthographic camera resulting in different hair width across
it's length.
This issues are solved now, but there are some related TODOs:
- Rotating viewport doesn't re-orient the triangles, so after
viewport navigation hair might not look correct. However,
with this fix toggling viewport render (to force hair sync)
makes viewport render correct.
This isn't so much trivial fix, would require making BVH
aware of the dynamic triangle orientation, so they get
properly oriented without full hair re-sync.
- Panorama camera behavior didn't change but looks like it
should, however not really sure atm what's the right thing
to do here.
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