[Bf-blender-cvs] [baad410a5c5] soc-2018-hair-shader: Fix h value not being computed correctly.

[Brecht Van Lommel]

Commit: baad410a5c5b15158b06d9b9a9d9c86522261cea
Author: Brecht Van Lommel
Date:   Sun May 27 01:03:49 2018 +0200
Branches: soc-2018-hair-shader
https://developer.blender.org/rBbaad410a5c5b15158b06d9b9a9d9c86522261cea

Fix h value not being computed correctly.

The new code only works for thick curves, not ribbons, but should hopefully
work correctly for that case. If we already know the normal we don't need to
look at the curve geometry, which simplifies things.

===================================================================

M	intern/cycles/kernel/closure/bsdf_hair_principled.h

===================================================================

diff --git a/intern/cycles/kernel/closure/bsdf_hair_principled.h b/intern/cycles/kernel/closure/bsdf_hair_principled.h
index fe32e0c1ddb..3a7950ad191 100644
--- a/intern/cycles/kernel/closure/bsdf_hair_principled.h
+++ b/intern/cycles/kernel/closure/bsdf_hair_principled.h
@@ -158,15 +158,35 @@ ccl_device int bsdf_principled_hair_setup(KernelGlobals *kg, ShaderData *sd, Pri
 	bsdf->v = sqr(0.726f*bsdf->v + 0.812f*sqr(bsdf->v) + 3.700f*pow20(bsdf->v));
 	bsdf->s =    (0.265f*bsdf->s + 1.194f*sqr(bsdf->s) + 5.372f*pow22(bsdf->s))*M_SQRT_PI_8_F;
 
+	/* Compute local frame, aligned to curve tangent and ray direction. */
+	float3 X = normalize(sd->dPdu);
+	float3 Y = safe_normalize(cross(X, sd->I));
+	float3 Z = safe_normalize(cross(X, Y));
+
+#if 0
+	/* TODO: this seems to give wrong results, and h should be in the -1..1 range? */
 	float curve_r;
 	float3 curve_P = curve_center(kg, sd, &curve_r);
-	float3 X = normalize(sd->dPdu);
-	float3 Y = normalize(cross(X, sd->I));
 	float h = safe_divide(dot(Y, sd->P - curve_P), curve_r);
+	kernel_assert(fabsf(h) <= 2.0f);
+#else
+	/* TODO: this only works for thick curves where sd->Ng is the normal
+	 * pointing from the center of the curve to the shading point. For
+	 * ribbons we need to find another solution. */
+
+	/* h -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 sine of the angle
+	 * between sd->Ng and Z, as seen from the tangent X. */
+
+	/* TODO: we convert this value to a cosine later and discard the sign, so
+	 * we could probably save some operations. */
+	float h = dot(cross(sd->Ng, X), Z);
+
+	kernel_assert(fabsf(h) < 1.0f + 1e-4f);
+#endif
 
 	kernel_assert(isfinite3_safe(Y));
 	kernel_assert(isfinite_safe(h));
-	kernel_assert(fabsf(h) <= 2.0f);
 
 	bsdf->geom = make_float4(Y.x, Y.y, Y.z, h);