Fix #133229: Remove camera size optimization from SSS shader in Cycles

In Cycles, the subsurface scattering shader will switch to a
diffuse shader under a few different conditions to improve performance
and reduce noise.

This commit removes the "switch back to diffuse if the scattering
radius is less than a quarter of a pixel" optimization because in some
scenes, this can result in noticable lines as the shader transitions
between subsurface scattering and diffuse.

Pull Request: https://projects.blender.org/blender/blender/pulls/133245

intern/cycles/kernel/closure/bssrdf.h

@@ -305,20 +305,20 @@ ccl_device int bssrdf_setup(ccl_private ShaderData *sd,
   int bssrdf_channels = SPECTRUM_CHANNELS;
   Spectrum diffuse_weight = zero_spectrum();
 
-  /* Fall back to diffuse if the radius is smaller than a quarter pixel. */
-  float min_radius = max(0.25f * sd->dP, BSSRDF_MIN_RADIUS);
   if (path_flag & PATH_RAY_DIFFUSE_ANCESTOR) {
-    /* Always fall back to diffuse after a diffuse ancestor. Can't see it well then and adds
+    /* Always fall back to diffuse after a diffuse ancestor. Can't see it that well and it adds
      * considerable noise due to probabilities of continuing the path getting lower and lower. */
-    min_radius = FLT_MAX;
+    bssrdf_channels = 0;
+    diffuse_weight = bssrdf->weight;
   }
-
-  FOREACH_SPECTRUM_CHANNEL (i) {
-    if (GET_SPECTRUM_CHANNEL(bssrdf->radius, i) < min_radius) {
-      GET_SPECTRUM_CHANNEL(diffuse_weight, i) = GET_SPECTRUM_CHANNEL(bssrdf->weight, i);
-      GET_SPECTRUM_CHANNEL(bssrdf->weight, i) = 0.0f;
-      GET_SPECTRUM_CHANNEL(bssrdf->radius, i) = 0.0f;
-      bssrdf_channels--;
+  else {
+    FOREACH_SPECTRUM_CHANNEL (i) {
+      if (GET_SPECTRUM_CHANNEL(bssrdf->radius, i) < BSSRDF_MIN_RADIUS) {
+        GET_SPECTRUM_CHANNEL(diffuse_weight, i) = GET_SPECTRUM_CHANNEL(bssrdf->weight, i);
+        GET_SPECTRUM_CHANNEL(bssrdf->weight, i) = 0.0f;
+        GET_SPECTRUM_CHANNEL(bssrdf->radius, i) = 0.0f;
+        bssrdf_channels--;
+      }
     }
   }