Cycles: Scale denoising albedo contribution of Principled BSDFs according to average fresnel

The Principled BSDF uses Microfacet closures that include a fresnel term,
which are a special case since their weight tends to be near white even
if their average contribution is fairly low.

The sample weight is scaled by the average fresnel weight to account for
this, but the denoising albedo still used the unscaled weight.

This was fine for the original denoiser, but apparently OIDN can't handle
the resulting albedo pass well. Therefore, this commit adds the described
scaling to the albedo pass contribution as well.

This problem was described in T69770.

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D6289

1 files changed, 3 insertions, 9 deletions

diff --git a/intern/cycles/kernel/closure/bsdf_microfacet_multi.h b/intern/cycles/kernel/closure/bsdf_microfacet_multi.h
index 9780dd87415..a5fe989bcd1 100644
--- a/intern/cycles/kernel/closure/bsdf_microfacet_multi.h
+++ b/intern/cycles/kernel/closure/bsdf_microfacet_multi.h
@@ -402,9 +402,7 @@ ccl_device int bsdf_microfacet_multi_ggx_aniso_fresnel_setup(MicrofacetBsdf *bsd
 
   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;
+  bsdf_microfacet_fresnel_color(sd, bsdf);
 
   return bsdf_microfacet_multi_ggx_common_setup(bsdf);
 }
@@ -424,9 +422,7 @@ ccl_device int bsdf_microfacet_multi_ggx_fresnel_setup(MicrofacetBsdf *bsdf, con
 
   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;
+  bsdf_microfacet_fresnel_color(sd, bsdf);
 
   return bsdf_microfacet_multi_ggx_common_setup(bsdf);
 }
@@ -582,9 +578,7 @@ 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;
+  bsdf_microfacet_fresnel_color(sd, bsdf);
 
   return SD_BSDF | SD_BSDF_HAS_EVAL | SD_BSDF_NEEDS_LCG;
 }