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Cycles: adjust normal in Principled Hair Huang to be orthogonal to the tangent 

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.
This commit is contained in:
Weizhen Huang 2023-12-12 22:23:11 +13:00 committed by Weizhen Huang
parent 17842d4d40
commit c592fa14f1
1 changed files with 5 additions and 5 deletions
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10
intern/cycles/kernel/closure/bsdf_principled_hair_huang.h
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@ -188,8 +188,8 @@ ccl_device int bsdf_hair_huang_setup(ccl_private ShaderData *sd,
/* Compute local frame. The Y axis is aligned with the curve tangent; the X axis is perpendicular
to the ray direction for circular cross-sections, or aligned with the major axis for elliptical
cross-sections. */
const float3 Y = safe_normalize(sd->dPdu);
const float3 X = safe_normalize(cross(Y, sd->wi));
bsdf->extra->Y = safe_normalize(sd->dPdu);
const float3 X = safe_normalize(cross(sd->dPdu, sd->wi));
/* h from -1..0..1 means the rays goes from grazing the hair, to hitting it at the center, to
* grazing the other edge. This is the cosine of the angle between `sd->N` and `X`. */
@ -199,6 +199,8 @@ ccl_device int bsdf_hair_huang_setup(ccl_private ShaderData *sd,
kernel_assert(isfinite_safe(bsdf->h));
if (bsdf->aspect_ratio != 1.0f && (sd->type & PRIMITIVE_CURVE)) {
/* Adjust `bsdf->N` to be orthogonal to `sd->dPdu`. */
bsdf->N = safe_normalize(cross(sd->dPdu, safe_normalize(cross(bsdf->N, sd->dPdu))));
/* Align local frame with the curve normal. */
if (bsdf->aspect_ratio > 1.0f) {
/* Switch major and minor axis. */
@ -214,14 +216,12 @@ ccl_device int bsdf_hair_huang_setup(ccl_private ShaderData *sd,
/* Fill extra closure. */
if (is_zero(bsdf->N) || !isfinite_safe(bsdf->N)) {
bsdf->extra->Y = Y;
/* Construct arbitrary local coordinate system. The implementation should ensure smooth
* transition along the hair shaft. */
make_orthonormals(Y, &bsdf->extra->Z, &bsdf->N);
make_orthonormals(bsdf->extra->Y, &bsdf->extra->Z, &bsdf->N);
}
else {
bsdf->extra->Z = safe_normalize(cross(bsdf->N, sd->dPdu));
bsdf->extra->Y = safe_normalize(cross(bsdf->extra->Z, bsdf->N));
}
const float3 I = make_float3(