Fix T95649: Corrective Smooth can cause vertices to jump

Calculate a tangent per loop, apply the transformation in multiple
spaces and accumulate the result weighting by the loop-angle.

Note that previously only the tangents Z axis (normal)
was properly accumulated, the X and Y axes only contained the values
from a single loop (the last one written to), meaning only two edges
contributed to the result. Now the transformation from all loops are
taken into account.

In practice the difference between both methods is minimal,
only for more extreme bending it can be noticed which is often when the
previous method didn't work as well.

1 files changed, 6 insertions, 2 deletions

diff --git a/source/blender/makesdna/DNA_modifier_types.h b/source/blender/makesdna/DNA_modifier_types.h
index 787f52f9891..7625f04fefa 100644
--- a/source/blender/makesdna/DNA_modifier_types.h
+++ b/source/blender/makesdna/DNA_modifier_types.h
@@ -1856,9 +1856,13 @@ enum {
 };
 
 typedef struct CorrectiveSmoothDeltaCache {
-  /* delta's between the original positions and the smoothed positions */
+  /**
+   * Delta's between the original positions and the smoothed positions,
+   * calculated loop-tangent and which is accumulated into the vertex it uses.
+   * (run-time only).
+   */
   float (*deltas)[3];
-  unsigned int totverts;
+  unsigned int deltas_num;
 
   /* Value of settings when creating the cache.
    * These are used to check if the cache should be recomputed. */