Fix Principled BSDF specular color for black base color

Specular color is set to black instead of white inside the Principled BSDF
when the base color is set to fully black. This is contradictory to the sample
code of the Disney BRDF in BRDF Explorer. This patch aligns both
implementations.

Differential Revision: https://developer.blender.org/D10448

intern/cycles/kernel/shaders/node_principled_bsdf.osl

@@ -50,7 +50,7 @@ shader node_principled_bsdf(string distribution = "Multiscatter GGX",
 
   float m_cdlum = luminance(BaseColor);
   color m_ctint = m_cdlum > 0.0 ? BaseColor / m_cdlum :
-                                  color(0.0, 0.0, 0.0);  // normalize lum. to isolate hue+sat
+                                  color(1.0, 1.0, 1.0);  // normalize lum. to isolate hue+sat
 
   /* rotate tangent */
   if (AnisotropicRotation != 0.0)

intern/cycles/kernel/svm/svm_closure.h

@@ -308,7 +308,7 @@ ccl_device void svm_node_closure_bsdf(KernelGlobals *kg,
             float3 m_ctint = m_cdlum > 0.0f ?
                                  base_color / m_cdlum :
                                  make_float3(
-                                     0.0f, 0.0f, 0.0f);  // normalize lum. to isolate hue+sat
+                                     1.0f, 1.0f, 1.0f);  // normalize lum. to isolate hue+sat
             float3 tmp_col = make_float3(1.0f, 1.0f, 1.0f) * (1.0f - specular_tint) +
                              m_ctint * specular_tint;
 

source/blender/gpu/shaders/material/gpu_shader_material_principled.glsl

@@ -2,7 +2,7 @@
 vec3 tint_from_color(vec3 color)
 {
   float lum = dot(color, vec3(0.3, 0.6, 0.1));  /* luminance approx. */
-  return (lum > 0.0) ? color / lum : vec3(0.0); /* normalize lum. to isolate hue+sat */
+  return (lum > 0.0) ? color / lum : vec3(1.0); /* normalize lum. to isolate hue+sat */
 }
 
 float principled_sheen(float NV)