for a camera ray, compute the actual range of the hair width that the
current pixel covers, and only integrate that subset, to prevent a
ribbon-like appearance in close-up looks.
When the hair covers less than one pixel on the screen or when the ray
is not camera ray, the model works the same as before.
The valid interval from incoming direction was shared between
`...eval_r()` and `...eval_residual()`, so compute that in `...eval()` instead.
The valid interval from outgoing direction was computed for
`...eval_r()` to further reduce the integration interval. This part is
removed because the check `dot(wo, wm) > 0` is relatively cheap inside the loop.
The geometry normal of the curve might not be precisely orthogonal to
the tangent due to interpolation. Previously, the tangent was adjusted
to be orthogonal to the normal. However, the normal is linearly
interpolated, which is less accurate than the tangent computed using Catmull-Rom.
This commit keeps the tangent and adjust the normal instead, besides
better accuracy, it ensures a smooth transition when the cross-section
shifts between circular and elliptical.
Weizhen Huang