Python: obj.parent = new_parent_obj, clears obj.matrix_parent_inverse for no reason. #62431

New Issue

Jose Conseco · 2019-03-10T22:14:33+01:00

Jose Conseco commented

2019-03-10 22:14:33 +01:00

I want to change object parent form Cube to Suzane, so that it won't move (matrix_basis, and matrix_world should be the same).
We search for new obj.matrix_parent_inverse2, in code below. It should work (proof on bottom), but last line clears new obj.matrix_parent_inverse

def set_new_parent_bug(obj, new_parent):
     new_m_p_inv = new_parent.matrix_world.inverted() * obj.parent.matrix_world * obj.matrix_parent_inverse
     obj.matrix_parent_inverse = new_m_p_inv
     obj.parent = new_parent # this clears m_parent_inv !! BUG?

In blend file I have script with broken function set_new_parent_bug(), and workaround function set_new_parent() that backups obj.matrix_parent_inverse, so that after changing parent and it being cleared to Identity Matrix, we can still use backup for calculation of new matrix_parent_inverse (for new parent)

Parenting_bug.blend
But affects both blender 2.8 and 2.79
In short using obj.parent = xxx, should not clear obj.matrix_parent_inverse

And below proof that the math is valid (we search for new matrix_parent_inverse so that objects stays in place).
mat_world should be same with old parent and new parent

old_m_w == new_m_w
old_parent_m_w @ obj.m_p_inv1 @ obj.m_basis  ==  new_parent_m_w @ **obj.m_p_inv2** @ obj.m_basis
new_parent_m_w.inv() @ old_parent_m_w @ obj.m_p_inv1 @ obj.m_basis  ==  new_parent_m_w.inv() @ new_parent_m_w @ **obj.m_p_inv2** @ obj.m_basis
new_parent_m_w.inv() @ old_parent_m_w @ obj.m_p_inv1   ==   **obj.m_p_inv2**

I want to change object parent form Cube to Suzane, so that it won't move (matrix_basis, and matrix_world should be the same). We search for new obj.matrix_parent_inverse2, in code below. It should work (proof on bottom), but last line clears new obj.matrix_parent_inverse ``` def set_new_parent_bug(obj, new_parent): new_m_p_inv = new_parent.matrix_world.inverted() * obj.parent.matrix_world * obj.matrix_parent_inverse obj.matrix_parent_inverse = new_m_p_inv obj.parent = new_parent # this clears m_parent_inv !! BUG? ``` In blend file I have script with broken function **set_new_parent_bug()**, and workaround function **set_new_parent()** that backups obj.matrix_parent_inverse, so that after changing parent and it being cleared to Identity Matrix, we can still use backup for calculation of new matrix_parent_inverse (for new parent) [Parenting_bug.blend](https://archive.blender.org/developer/F6796732/Parenting_bug.blend) But affects both blender 2.8 and 2.79 In short using obj.parent = xxx, should not clear obj.matrix_parent_inverse And below proof that the math is valid (we search for new matrix_parent_inverse so that objects stays in place). mat_world should be same with old parent and new parent ``` old_m_w == new_m_w old_parent_m_w @ obj.m_p_inv1 @ obj.m_basis == new_parent_m_w @ **obj.m_p_inv2** @ obj.m_basis new_parent_m_w.inv() @ old_parent_m_w @ obj.m_p_inv1 @ obj.m_basis == new_parent_m_w.inv() @ new_parent_m_w @ **obj.m_p_inv2** @ obj.m_basis new_parent_m_w.inv() @ old_parent_m_w @ obj.m_p_inv1 == **obj.m_p_inv2** ```

Jose Conseco commented

2019-03-10 22:14:33 +01:00

Added subscriber: @JoseConseco

Brecht Van Lommel commented

2019-03-11 14:40:52 +01:00

Added subscriber: @brecht

Brecht Van Lommel commented

2019-03-11 14:40:52 +01:00

Changed status from 'Open' to: 'Archived'

Brecht Van Lommel closed this issue

2019-03-11 14:40:52 +01:00

Brecht Van Lommel self-assigned this 2019-03-11 14:40:52 +01:00

Brecht Van Lommel commented

2019-03-11 14:40:52 +01:00

You can manually set matrix_parent_inverse to the old value if you need to, but I don't think preserving is the most commonly useful behavior.

Even if we did want to change it, it would break backward compatibility and I don't think it's worth it.

You can manually set `matrix_parent_inverse` to the old value if you need to, but I don't think preserving is the most commonly useful behavior. Even if we did want to change it, it would break backward compatibility and I don't think it's worth it.

Philipp Oeser commented

2019-03-11 15:29:58 +01:00

Added subscriber: @lichtwerk

Philipp Oeser commented

2019-03-11 15:29:58 +01:00

Might be misunderstanding, but if you only want to change parent without moving, why not just do something like:

obj.parent = new_parent
obj.matrix_parent_inverse = new_parent.matrix_world.inverted()

Might be misunderstanding, but if you only want to change parent without moving, why not just do something like: ``` obj.parent = new_parent obj.matrix_parent_inverse = new_parent.matrix_world.inverted() ```

Jose Conseco commented

2019-03-12 00:53:11 +01:00

@lichtwerk, this will work only if for old parent:
obj.matrix_parent_inverse would be same as obj.parent.matrix_world.inverted() (which is true only at time of parenting, as long as we don't move parent).
then obj.matrix_parent_inverse * obj.parent.matrix_world == Identity Matrix. And obj.m_w == basically obj.matrix_basis
But as soon as we move obj.parent, then above equation wont give you identity matrix, and obj.matrix_world will change (child will follow change of parent obj transform).
To test you can open blend file attached to task, and before running the script offset first parent object (cube). You will see, that with your method child object won't stay in one place.

@lichtwerk, this will work only if for old parent: obj.matrix_parent_inverse would be same as obj.parent.matrix_world.inverted() (which is true only at time of parenting, as long as we don't move parent). then obj.matrix_parent_inverse * obj.parent.matrix_world == Identity Matrix. And obj.m_w == basically obj.matrix_basis But as soon as we move obj.parent, then above equation wont give you identity matrix, and obj.matrix_world will change (child will follow change of parent obj transform). To test you can open blend file attached to task, and before running the script offset first parent object (cube). You will see, that with your method child object won't stay in one place.

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Python: obj.parent = new_parent_obj, clears obj.matrix_parent_inverse for no reason. #62431