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, 2 insertions, 1 deletions

diff --git a/intern/cycles/kernel/kernel_types.h b/intern/cycles/kernel/kernel_types.h
index 86b3b3b3365..85d75b36e5f 100644
--- a/intern/cycles/kernel/kernel_types.h
+++ b/intern/cycles/kernel/kernel_types.h
@@ -1479,7 +1479,8 @@ typedef struct KernelObject {
   float cryptomatte_object;
   float cryptomatte_asset;
 
-  float shadow_terminator_offset;
+  float shadow_terminator_shading_offset;
+  float shadow_terminator_geometry_offset;
   float pad1, pad2, pad3;
 } KernelObject;
 static_assert_align(KernelObject, 16);