Fix T38407: Bone roll calculation flips local axes at wrong rotation angle.
Basic idea is now to have the transformes bones keep "facing" the armature's Z axis, see comments in code for details. That might not be ideal, but at least we now have humanly predictable and consistent results.
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		@@ -1182,6 +1182,7 @@ static void createTransArmatureVerts(TransInfo *t)
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				}
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			}
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			else {
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				const float Z[3] = {0.0f, 0.0f, 1.0f};
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				if (ebo->flag & BONE_TIPSEL) {
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					copy_v3_v3(td->iloc, ebo->tail);
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					copy_v3_v3(td->center, (t->around == V3D_LOCAL) ? ebo->head : td->iloc);
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@@ -1196,8 +1197,9 @@ static void createTransArmatureVerts(TransInfo *t)
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					ED_armature_ebone_to_mat3(ebo, td->axismtx);
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					if ((ebo->flag & BONE_ROOTSEL) == 0) {
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						/* To fix roll, see comments in transform_generic.c::recalcData_objects() */
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						td->extra = ebo;
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						td->ival = ebo->roll;
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						td->ival = ebo->roll - ED_rollBoneToVector(ebo, Z, false);
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					}
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					td->ext = NULL;
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@@ -1219,8 +1221,9 @@ static void createTransArmatureVerts(TransInfo *t)
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					ED_armature_ebone_to_mat3(ebo, td->axismtx);
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					td->extra = ebo; /* to fix roll */
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					td->ival = ebo->roll;
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					/* To fix roll, see comments in transform_generic.c::recalcData_objects() */
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					td->extra = ebo;
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					td->ival = ebo->roll - ED_rollBoneToVector(ebo, Z, false);
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					td->ext = NULL;
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					td->val = NULL;
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@@ -798,35 +798,25 @@ static void recalcData_objects(TransInfo *t)
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			if (!ELEM3(t->mode, TFM_BONE_ROLL, TFM_BONE_ENVELOPE, TFM_BONESIZE)) {
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				/* fix roll */
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				/* Previous method basically tried to get a rotation transform from org ebo Y axis to final ebo Y axis,
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				 * apply this same rotation to org ebo Z axis to get an "up_axis", and compute a new roll value
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				 * so that final ebo's Z axis would be "aligned" with that up_axis.
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				 * There are two issues with that method:
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				 *   - There are many cases where the computed up_axis does not gives a result people would expect.
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				 *   - Applying a same transform in a single step or in several smaller ones would not give the same
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				 *     result! See e.g. T38407.
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				 * Now, instead of trying to be smart with complex axis/angle handling, just store diff roll
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				 * (diff between real init roll and virtual init roll where bone's Z axis would be "aligned" with
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				 * armature's Z axis), and do the reverse to get final roll.
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				 * This method at least gives predictable, consistent results (the bone basically keeps "facing"
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				 * the armature's Z axis).
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				 */
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				for (i = 0; i < t->total; i++, td++) {
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					if (td->extra) {
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						float vec[3], up_axis[3];
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						float qrot[4];
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						float roll;
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						ebo = td->extra;
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						const float Z[3] = {0.0f, 0.0f, 1.0f};
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						if (t->state == TRANS_CANCEL) {
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							/* restore roll */
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							ebo->roll = td->ival;
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						}
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						else {
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							copy_v3_v3(up_axis, td->axismtx[2]);
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							if (t->mode != TFM_ROTATION) {
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								sub_v3_v3v3(vec, ebo->tail, ebo->head);
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								normalize_v3(vec);
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								rotation_between_vecs_to_quat(qrot, td->axismtx[1], vec);
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								mul_qt_v3(qrot, up_axis);
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							}
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							else {
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								mul_m3_v3(t->mat, up_axis);
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							}
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							/* roll has a tendency to flip in certain orientations - [#34283], [#33974] */
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							roll = ED_rollBoneToVector(ebo, up_axis, false);
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							ebo->roll = angle_compat_rad(roll, td->ival);
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						}
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						ebo = td->extra;
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						ebo->roll = td->ival + ED_rollBoneToVector(ebo, Z, false);
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					}
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				}
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			}
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