[Bf-blender-cvs] [564673b8ea2] master: Fix T71137: curve minimum twist producing wrong geometry

[Campbell Barton]

Commit: 564673b8ea2860cc0d0b3a0f673a315693d3f5eb
Author: Campbell Barton
Date:   Thu Jan 26 16:05:21 2023 +1100
Branches: master
https://developer.blender.org/rB564673b8ea2860cc0d0b3a0f673a315693d3f5eb

Fix T71137: curve minimum twist producing wrong geometry

Only one point should be used to create a reference rotation for other
points to follow. Using two caused the resulting twist to be
asymmetric, especially noticeable on symmetrical, cyclic curves.

An update to [0] which broke curve_to_mesh & deform_modifiers tests,
now this change only applies to cyclic curves as the final result was
much greater for non-cyclic curves because of a difference between how
end-point directions are calculated (see code-comments for details).

Alternate fix to D11886 which caused T101843.

[0]: 36a82314a0f5de65b54f6d8343a2899ed4e37010.

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

M	source/blender/blenkernel/intern/curve.cc

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

diff --git a/source/blender/blenkernel/intern/curve.cc b/source/blender/blenkernel/intern/curve.cc
index 744ef0e8009..61dc8a22077 100644
--- a/source/blender/blenkernel/intern/curve.cc
+++ b/source/blender/blenkernel/intern/curve.cc
@@ -2250,6 +2250,19 @@ static void make_bevel_list_3D_minimum_twist(BevList *bl)
   BevPoint *bevp2, *bevp1, *bevp0; /* Standard for all make_bevel_list_3D_* functions. */
   int nr;
   float q[4];
+  const bool is_cyclic = bl->poly != -1;
+  /* NOTE(@campbellbarton): For non-cyclic curves only initialize the first direction
+   * (via `vec_to_quat`), necessary for symmetry, see T71137.
+   * Otherwise initialize the first and second points before propagating rotation forward.
+   * This is historical as changing this can cause significantly different output.
+   * Specifically: `deform_modifiers` test: (`CurveMeshDeform`).
+   *
+   * While it would seem correct to only use the first point for non-cyclic curves as well
+   * the end-points direction is initialized from the input handles (instead of the directions
+   * between points), there is often a bigger difference in the first and second directions
+   * than you'd otherwise expect. So using only the first direction breaks compatibility
+   * enough it's best to leave it as-is. */
+  const int nr_init = bl->nr - (is_cyclic ? 1 : 2);
 
   bevel_list_calc_bisect(bl);
 
@@ -2260,7 +2273,8 @@ static void make_bevel_list_3D_minimum_twist(BevList *bl)
   nr = bl->nr;
   while (nr--) {
 
-    if (nr + 3 > bl->nr) { /* first time and second time, otherwise first point adjusts last */
+    if (nr >= nr_init) {
+      /* Initialize the rotation, otherwise propagate the previous rotation forward. */
       vec_to_quat(bevp1->quat, bevp1->dir, 5, 1);
     }
     else {