Fix T82156: Object with constraints translates when parented

Avoid the evaluation of constraints when computing the parent-inverse
matrix.

Constraints are meant to be evaluated last; object transforms are
computed this order:

1. `parent->obmat` (the parent object's world matrix)
2. `ob->parentinv` (the object's parent-inverse matrix)
3. Object's loc/rot/scale
4. Object's constraint evaluation

When the constraints are used to compute the parent-inverse matrix,
their effect is moved from step 4 to step 2 in this list, potentially
rotating or scaling the object's local transform. This causes unwanted
movement as reported in T82156.

Reviewed By: looch

Differential Revision: https://developer.blender.org/D9413

1 files changed, 5 insertions, 1 deletions

diff --git a/source/blender/blenkernel/intern/object.c b/source/blender/blenkernel/intern/object.c
index 430d28f67c2..242c0edd5a4 100644
--- a/source/blender/blenkernel/intern/object.c
+++ b/source/blender/blenkernel/intern/object.c
@@ -3473,7 +3473,11 @@ void BKE_object_workob_calc_parent(Depsgraph *depsgraph, Scene *scene, Object *o
   workob->par2 = ob->par2;
   workob->par3 = ob->par3;
 
-  workob->constraints = ob->constraints;
+  /* The effects of constraints should NOT be included in the parent-inverse matrix. Constraints
+   * are supposed to be applied after the object's local loc/rot/scale. If the (inverted) effect of
+   * constraints would be included in the parent inverse matrix, these would be applied before the
+   * object's local loc/rot/scale instead of after. For example, a "Copy Rotation" constraint would
+   * rotate the object's local translation as well. See T82156. */
 
   BLI_strncpy(workob->parsubstr, ob->parsubstr, sizeof(workob->parsubstr));