Object: Set Parent (Keep Transform Without Inverse)

**Relevant to Artists:** This patch adds an option to the Parenting
menu, `Object (Keep Transform Without Inverse)`, and Apply menu, `Parent
Inverse`. The operators preserve the child's world transform without
using the parent inverse matrix. Effectively, we set the child's origin
to the parent. When the child has an identity local transform, then the
child is world-space aligned with its parent (scale excluded).

**Technical:** In both cases, the hidden parent inverse matrix is
generally set to identity (cleared or "not used") as long as the parent
has no shear. If the parent has shear, then this matrix will not be
entirely cleared. It will contain shear to counter the parent's shear.
This is required, otherwise the object's local matrix cannot be properly
decomposed into location, rotation and scale, and thus cannot preserve
the world transform.

If the child's world transform has shear, then its world transform is
not preserved. This is currently not supported for consistency in the
handling of shear during the other parenting ops: Parent (Keep
Transform), Clear [Parent] and Keep Transform. If it should work, then
another patch should add the support for all of them.

Reviewed By: sybren, RiggingDojo
Differential Revision: https://developer.blender.org/D14581

2 files changed, 60 insertions, 0 deletions

diff --git a/source/blender/blenkernel/BKE_object.h b/source/blender/blenkernel/BKE_object.h
index eaeb6e6a2e4..faf878dfc2a 100644
--- a/source/blender/blenkernel/BKE_object.h
+++ b/source/blender/blenkernel/BKE_object.h
@@ -270,6 +270,17 @@ void BKE_object_apply_mat4(struct Object *ob,
                            const float mat[4][4],
                            bool use_compat,
                            bool use_parent);
+
+/**
+ * Use parent's world location and rotation as the child's origin. The parent inverse will
+ * become identity when the parent has no shearing. Otherwise, it is non-identity and contains the
+ * object's local matrix data that cannot be decomposed into location, rotation and scale.
+ *
+ * Assumes the object's world matrix has no shear.
+ * Assumes parent exists.
+ */
+void BKE_object_apply_parent_inverse(struct Object *ob);
+
 void BKE_object_matrix_local_get(struct Object *ob, float r_mat[4][4]);
 
 bool BKE_object_pose_context_check(const struct Object *ob);
diff --git a/source/blender/blenkernel/intern/object.cc b/source/blender/blenkernel/intern/object.cc
index 92c350c5208..5ff1f6b950f 100644
--- a/source/blender/blenkernel/intern/object.cc
+++ b/source/blender/blenkernel/intern/object.cc
@@ -3558,6 +3558,55 @@ void BKE_object_apply_mat4(Object *ob,
   BKE_object_apply_mat4_ex(ob, mat, use_parent ? ob->parent : nullptr, ob->parentinv, use_compat);
 }
 
+void BKE_object_apply_parent_inverse(struct Object *ob)
+{
+  /*
+   * Use parent's world transform as the child's origin.
+   *
+   * Let:
+   *    local = identity
+   *    world = orthonormalized(parent)
+   *
+   * Then:
+   *    world = parent @ parentinv @ local
+   *    inv(parent) @ world = parentinv
+   *    parentinv = inv(parent) @ world
+   *
+   * NOTE: If ob->obmat has shear, then this `parentinv` is insufficient because
+   *    parent @ parentinv => shearless result
+   *
+   *    Thus, local will have shear which cannot be decomposed into TRS:
+   *    local = inv(parent @ parentinv) @ world
+   *
+   *    This is currently not supported for consistency in the handling of shear during the other
+   *    parenting ops: Parent (Keep Transform), Clear [Parent] and Keep Transform.
+   */
+  float par_locrot[4][4], par_imat[4][4];
+  BKE_object_get_parent_matrix(ob, ob->parent, par_locrot);
+  invert_m4_m4(par_imat, par_locrot);
+
+  orthogonalize_m4_stable(par_locrot, 0, true);
+
+  mul_m4_m4m4(ob->parentinv, par_imat, par_locrot);
+
+  /* Now, preserve `world` given the new `parentinv`.
+   *
+   * world = parent @ parentinv @ local
+   * inv(parent) @ world = parentinv @ local
+   * inv(parentinv) @ inv(parent) @ world = local
+   *
+   * local = inv(parentinv) @ inv(parent) @ world
+   */
+  float ob_local[4][4];
+  copy_m4_m4(ob_local, ob->parentinv);
+  invert_m4(ob_local);
+  mul_m4_m4_post(ob_local, par_imat);
+  mul_m4_m4_post(ob_local, ob->obmat);
+
+  /* Send use_compat=False so the rotation is predictable. */
+  BKE_object_apply_mat4(ob, ob_local, false, false);
+}
+
 /** \} */
 
 /* -------------------------------------------------------------------- */