Cycles: add bump map shadow terminator softening term for diffuse BSDFs

This avoids artifacts for bump mapping and diffuse BSDFs, where the bump
normal deviates far from the actual normal.

Differential Revision: https://developer.blender.org/D5399

1 files changed, 33 insertions, 0 deletions

diff --git a/intern/cycles/kernel/closure/bsdf.h b/intern/cycles/kernel/closure/bsdf.h
index 5e26f90a878..c83e97d94c2 100644
--- a/intern/cycles/kernel/closure/bsdf.h
+++ b/intern/cycles/kernel/closure/bsdf.h
@@ -73,6 +73,28 @@ ccl_device_inline float bsdf_get_roughness_squared(const ShaderClosure *sc)
   return bsdf_get_specular_roughness_squared(sc);
 }
 
+/* An additional term to smooth illumination on grazing angles when using bump mapping.
+ * Based on "Taming the Shadow Terminator" by Matt Jen-Yuan Chiang,
+ * Yining Karl Li and Brent Burley. */
+ccl_device_inline float bump_shadowing_term(float3 Ng, float3 N, float3 I)
+{
+  float g = safe_divide(dot(Ng, I), dot(N, I) * dot(Ng, N));
+
+  /* If the incoming light is on the unshadowed side, return full brightness. */
+  if (g >= 1.0f) {
+    return 1.0f;
+  }
+
+  /* If the incoming light points away from the surface, return black. */
+  if (g < 0.0f) {
+    return 0.0f;
+  }
+
+  /* Return smoothed value to avoid discontinuity at perpendicular angle. */
+  float g2 = sqr(g);
+  return -g2 * g + g2 + g;
+}
+
 ccl_device_inline int bsdf_sample(KernelGlobals *kg,
                                   ShaderData *sd,
                                   const ShaderClosure *sc,
@@ -424,6 +446,11 @@ ccl_device_inline int bsdf_sample(KernelGlobals *kg,
       }
     }
   }
+  else if (label & LABEL_DIFFUSE) {
+    if (sc->N != sd->N) {
+      *eval *= bump_shadowing_term((label & LABEL_TRANSMIT) ? -sd->N : sd->N, sc->N, *omega_in);
+    }
+  }
 
   return label;
 }
@@ -535,6 +562,9 @@ ccl_device_inline
         eval = make_float3(0.0f, 0.0f, 0.0f);
         break;
     }
+    if (CLOSURE_IS_BSDF_DIFFUSE(sc->type)) {
+      eval *= bump_shadowing_term(sd->N, sc->N, omega_in);
+    }
   }
   else {
     switch (sc->type) {
@@ -621,6 +651,9 @@ ccl_device_inline
         eval = make_float3(0.0f, 0.0f, 0.0f);
         break;
     }
+    if (CLOSURE_IS_BSDF_DIFFUSE(sc->type)) {
+      eval *= bump_shadowing_term(-sd->N, sc->N, omega_in);
+    }
   }
 
   return eval;