[Bf-blender-cvs] [7c85f11c427] master: Cycles: Change bake jittering to avoid issues with skinny triangles

[Lukas Stockner]

Commit: 7c85f11c427d0d07444d99c34acae5ece56d987e
Author: Lukas Stockner
Date:   Fri Dec 9 02:27:40 2022 +0100
Branches: master
https://developer.blender.org/rB7c85f11c427d0d07444d99c34acae5ece56d987e

Cycles: Change bake jittering to avoid issues with skinny triangles

Partially addresses T72011.

The problem here is that the previous barycentric clamping did not deal well
with skinny triangles and would end up generating "sub-pixel jittering"
locations that were actually >20 pixels away.

Differential Revision: https://developer.blender.org/D16727

===================================================================

M	intern/cycles/kernel/integrator/init_from_bake.h

===================================================================

diff --git a/intern/cycles/kernel/integrator/init_from_bake.h b/intern/cycles/kernel/integrator/init_from_bake.h
index cc3fbe3fe39..2a5bda50fb3 100644
--- a/intern/cycles/kernel/integrator/init_from_bake.h
+++ b/intern/cycles/kernel/integrator/init_from_bake.h
@@ -16,18 +16,40 @@
 
 CCL_NAMESPACE_BEGIN
 
-/* This helps with AA but it's not the real solution as it does not AA the geometry
- * but it's better than nothing, thus committed. */
-ccl_device_inline float bake_clamp_mirror_repeat(float u, float max)
+/* In order to perform anti-aliasing during baking, we jitter the input barycentric coordinates
+ * (which are for the center of the texel) within the texel.
+ * However, the baking code corrently doesn't support going to neighboring triangle, so if the
+ * jittered location falls outside of the input triangle, we need to bring it back in somehow.
+ * Clamping is a bad choice here since it can produce noticeable artifacts at triangle edges,
+ * but properly uniformly sampling the intersection of triangle and texel would be very
+ * performance-heavy, so cheat by just trying different jittering until we end up inside the
+ * triangle.
+ * For triangles that are smaller than a texel, this might take too many attempts, so eventually
+ * we just give up and don't jitter in that case.
+ * This is not a particularly elegant solution, but it's probably the best we can do. */
+ccl_device_inline void bake_jitter_barycentric(float &u,
+                                               float &v,
+                                               float2 rand_filter,
+                                               const float dudx,
+                                               const float dudy,
+                                               const float dvdx,
+                                               const float dvdy)
 {
-  /* use mirror repeat (like opengl texture) so that if the barycentric
-   * coordinate goes past the end of the triangle it is not always clamped
-   * to the same value, gives ugly patterns */
-  u /= max;
-  float fu = floorf(u);
-  u = u - fu;
-
-  return ((((int)fu) & 1) ? 1.0f - u : u) * max;
+  for (int i = 0; i < 10; i++) {
+    /* Offset UV according to differentials. */
+    float jitterU = u + (rand_filter.x - 0.5f) * dudx + (rand_filter.y - 0.5f) * dudy;
+    float jitterV = v + (rand_filter.x - 0.5f) * dvdx + (rand_filter.y - 0.5f) * dvdy;
+    /* If this location is inside the triangle, return. */
+    if (jitterU > 0.0f && jitterV > 0.0f && jitterU + jitterV < 1.0f) {
+      u = jitterU;
+      v = jitterV;
+      return;
+    }
+    /* Retry with new jitter value. */
+    rand_filter = hash_float2_to_float2(rand_filter);
+  }
+  /* Retries exceeded, give up and just use center value. */
+  return;
 }
 
 /* Offset towards center of triangle to avoid ray-tracing precision issues. */
@@ -144,12 +166,7 @@ ccl_device bool integrator_init_from_bake(KernelGlobals kg,
   }
 
   /* Sub-pixel offset. */
-  if (sample > 0) {
-    u = bake_clamp_mirror_repeat(u + dudx * (rand_filter.x - 0.5f) + dudy * (rand_filter.y - 0.5f),
-                                 1.0f);
-    v = bake_clamp_mirror_repeat(v + dvdx * (rand_filter.x - 0.5f) + dvdy * (rand_filter.y - 0.5f),
-                                 1.0f - u);
-  }
+  bake_jitter_barycentric(u, v, rand_filter, dudx, dudy, dvdx, dvdy);
 
   /* Convert from Blender to Cycles/Embree/OptiX barycentric convention. */
   const float tmp = u;