Cycles: Fix excessive sampling weight of glossy Principled BSDF components

If there was any specularity in the Principled BSDF, it would get a sampling
weight of one regardless of its actual impact.

This commit makes Cycles estimate the contribution of the component and adjust
the weighting accordingly, which greatly improves the noise characteristics of
the Principled BSDF in many cases.

Note that this commit might slightly change the brightness of areas when using
MultiGGX and high roughnesses, but the new brightness is more accurate and
closer to the result of Branched Path Tracing. See T51836 for details.

Differential Revision: https://developer.blender.org/D2677

1 files changed, 15 insertions, 3 deletions

diff --git a/intern/cycles/kernel/closure/bsdf_microfacet_multi.h b/intern/cycles/kernel/closure/bsdf_microfacet_multi.h
index 165666d5368..22d0092093a 100644
--- a/intern/cycles/kernel/closure/bsdf_microfacet_multi.h
+++ b/intern/cycles/kernel/closure/bsdf_microfacet_multi.h
@@ -360,13 +360,17 @@ ccl_device int bsdf_microfacet_multi_ggx_aniso_setup(MicrofacetBsdf *bsdf)
 	return bsdf_microfacet_multi_ggx_common_setup(bsdf);
 }
 
-ccl_device int bsdf_microfacet_multi_ggx_aniso_fresnel_setup(MicrofacetBsdf *bsdf)
+ccl_device int bsdf_microfacet_multi_ggx_aniso_fresnel_setup(MicrofacetBsdf *bsdf, const ShaderData *sd)
 {
 	if(is_zero(bsdf->T))
 		bsdf->T = make_float3(1.0f, 0.0f, 0.0f);
 
 	bsdf->type = CLOSURE_BSDF_MICROFACET_MULTI_GGX_FRESNEL_ID;
 
+	float F0 = fresnel_dielectric_cos(1.0f, bsdf->ior);
+	float F = average(interpolate_fresnel_color(sd->I, bsdf->N, bsdf->ior, F0, bsdf->extra->cspec0));
+	bsdf->sample_weight *= F;
+
 	return bsdf_microfacet_multi_ggx_common_setup(bsdf);
 }
 
@@ -379,12 +383,16 @@ ccl_device int bsdf_microfacet_multi_ggx_setup(MicrofacetBsdf *bsdf)
 	return bsdf_microfacet_multi_ggx_common_setup(bsdf);
 }
 
-ccl_device int bsdf_microfacet_multi_ggx_fresnel_setup(MicrofacetBsdf *bsdf)
+ccl_device int bsdf_microfacet_multi_ggx_fresnel_setup(MicrofacetBsdf *bsdf, const ShaderData *sd)
 {
 	bsdf->alpha_y = bsdf->alpha_x;
 
 	bsdf->type = CLOSURE_BSDF_MICROFACET_MULTI_GGX_FRESNEL_ID;
 
+	float F0 = fresnel_dielectric_cos(1.0f, bsdf->ior);
+	float F = average(interpolate_fresnel_color(sd->I, bsdf->N, bsdf->ior, F0, bsdf->extra->cspec0));
+	bsdf->sample_weight *= F;
+
 	return bsdf_microfacet_multi_ggx_common_setup(bsdf);
 }
 
@@ -489,7 +497,7 @@ ccl_device int bsdf_microfacet_multi_ggx_glass_setup(MicrofacetBsdf *bsdf)
 	return SD_BSDF|SD_BSDF_HAS_EVAL|SD_BSDF_NEEDS_LCG;
 }
 
-ccl_device int bsdf_microfacet_multi_ggx_glass_fresnel_setup(MicrofacetBsdf *bsdf)
+ccl_device int bsdf_microfacet_multi_ggx_glass_fresnel_setup(MicrofacetBsdf *bsdf, const ShaderData *sd)
 {
 	bsdf->alpha_x = clamp(bsdf->alpha_x, 1e-4f, 1.0f);
 	bsdf->alpha_y = bsdf->alpha_x;
@@ -503,6 +511,10 @@ ccl_device int bsdf_microfacet_multi_ggx_glass_fresnel_setup(MicrofacetBsdf *bsd
 
 	bsdf->type = CLOSURE_BSDF_MICROFACET_MULTI_GGX_GLASS_FRESNEL_ID;
 
+	float F0 = fresnel_dielectric_cos(1.0f, bsdf->ior);
+	float F = average(interpolate_fresnel_color(sd->I, bsdf->N, bsdf->ior, F0, bsdf->extra->cspec0));
+	bsdf->sample_weight *= F;
+
 	return SD_BSDF|SD_BSDF_HAS_EVAL|SD_BSDF_NEEDS_LCG;
 }