Cycles: reduce shadow terminator artifacts

Offset rays from the flat surface to match where they would be for a smooth
surface as specified by the normals. In the shading panel there is now a
Shading Offset (existing option) and Geometry Offset (new).

The Geometry Offset works as follows:
* 0: disabled
* 0.001: only terminated triangles (normal points to the light, geometry
  doesn't) are affected
* 0.1 (default): triangles at grazing angles are affected, and the effect
  fades out
* 1: all triangles are affected

Limitations:
* The artifact is still visible in some cases, it could be that some quads
  require to be treated specifically as quads.
* Inconsistent normals cause artifacts.
* If small objects cast shadows to a big low poly surface, the shadows can
  appear to be in a wrong place - because the surface moved slightly above
  the geometry. This can be noticed only at grazing angles to light.
* Approximated surfaces of two non-intersecting low-poly objects can overlap
  that causes off-the-wall shadows.

Generally, using one or a few levels of subdivision can get rid of artifacts
faster than before.

Differential Revision: https://developer.blender.org/D11065

1 files changed, 3 insertions, 6 deletions

diff --git a/intern/cycles/kernel/closure/bsdf.h b/intern/cycles/kernel/closure/bsdf.h
index 6070fd983f5..6f2f2ebb202 100644
--- a/intern/cycles/kernel/closure/bsdf.h
+++ b/intern/cycles/kernel/closure/bsdf.h
@@ -462,7 +462,7 @@ ccl_device_inline int bsdf_sample(KernelGlobals *kg,
   else {
     /* Shadow terminator offset. */
     const float frequency_multiplier =
-        kernel_tex_fetch(__objects, sd->object).shadow_terminator_offset;
+        kernel_tex_fetch(__objects, sd->object).shadow_terminator_shading_offset;
     if (frequency_multiplier > 1.0f) {
       *eval *= shift_cos_in(dot(*omega_in, sc->N), frequency_multiplier);
     }
@@ -488,12 +488,9 @@ ccl_device_inline
               const float3 omega_in,
               float *pdf)
 {
-  /* For curves use the smooth normal, particularly for ribbons the geometric
-   * normal gives too much darkening otherwise. */
-  const float3 Ng = (sd->type & PRIMITIVE_ALL_CURVE) ? sd->N : sd->Ng;
   float3 eval;
 
-  if (dot(Ng, omega_in) >= 0.0f) {
+  if (dot(sd->N, omega_in) >= 0.0f) {
     switch (sc->type) {
       case CLOSURE_BSDF_DIFFUSE_ID:
       case CLOSURE_BSDF_BSSRDF_ID:
@@ -589,7 +586,7 @@ ccl_device_inline
     }
     /* Shadow terminator offset. */
     const float frequency_multiplier =
-        kernel_tex_fetch(__objects, sd->object).shadow_terminator_offset;
+        kernel_tex_fetch(__objects, sd->object).shadow_terminator_shading_offset;
     if (frequency_multiplier > 1.0f) {
       eval *= shift_cos_in(dot(omega_in, sc->N), frequency_multiplier);
     }