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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]: 36a82314a0.
This commit is contained in:
Campbell Barton 2023-01-26 16:05:21 +11:00
parent c1beaea80f
commit 564673b8ea
Notes: blender-bot 2023-02-14 03:52:45 +01:00 
Referenced by issue #71137, off-by-one bug in spline interpolation produces wrong geometry
1 changed files with 15 additions and 1 deletions
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16
source/blender/blenkernel/intern/curve.cc
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@ -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 {