Blender Game Engine does not fully implement Parent-Child relationships. #33512

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Tously Sid · 2012-12-13T09:55:17+01:00

Tously Sid commented

2012-12-13 09:55:17 +01:00

Category: Animation

%%%The .blend file includes a sphere parented to a cube, the cube has been scaled, and the sphere has been rotated. The 3D view shows the expected behavior, the sphere rotates within the scaled cube and is completely confined to the cube's bounds. Rotate the sphere to see how the sphere manages to rotate within the cube. Activate the game engine by pressing 'P'. The sphere correctly implements its parent's scaling and rotation properties, however it is no longer confined to it's parent's area of operation, rotating itself outside of its bounds.

Expected behavior: The sphere should only be able to rotate within the confines of its parent's boundaries as it does in the 3D editor/when rendering. This behavior is immensely useful in cases where the end user needs to rotate an object while keeping certain scaling properties, such as a ball squashing and stretching as it bounces, or a wheel slightly depressing due to its load while remaining easy to rotate.

Side note: Lattices are not a valid workaround as it seems the game engine does not support it.%%%

**Category**: Animation %%%The .blend file includes a sphere parented to a cube, the cube has been scaled, and the sphere has been rotated. The 3D view shows the expected behavior, the sphere rotates within the scaled cube and is completely confined to the cube's bounds. Rotate the sphere to see how the sphere manages to rotate within the cube. Activate the game engine by pressing 'P'. The sphere correctly implements its parent's scaling and rotation properties, however it is no longer confined to it's parent's area of operation, rotating itself outside of its bounds. Expected behavior: The sphere should only be able to rotate within the confines of its parent's boundaries as it does in the 3D editor/when rendering. This behavior is immensely useful in cases where the end user needs to rotate an object while keeping certain scaling properties, such as a ball squashing and stretching as it bounces, or a wheel slightly depressing due to its load while remaining easy to rotate. Side note: Lattices are not a valid workaround as it seems the game engine does not support it.%%%

Tously Sid commented

2012-12-13 09:55:17 +01:00

Changed status to: 'Open'

Tously Sid commented

2012-12-13 11:09:52 +01:00

%%%Added a second .blend, this one uses a Cube instead of an Empty, same result unfortunately.%%%

Tously Sid commented

2012-12-13 11:12:48 +01:00

%%%Or rather I attempted to submit a second .blend file but the server sent a "missing parameter" error. Download here: http://blenderartists.org/forum/attachment.php?attachmentid=206630&d=1355393266%%%

Dalai Felinto commented

2012-12-13 12:41:29 +01:00

%%%add a message (comment) when you attach the file and send. otherwise it fails.%%%

Tously Sid commented

2012-12-13 20:47:19 +01:00

%%%Thanks, here's the file: %%%

Tously Sid commented

2012-12-14 08:46:57 +01:00

%%%Please read this helpful post by MarcoIT which implements a solution through python: http://blenderartists.org/forum/showthread.php?275058-Not-Solvable-By-End-User-Game-Engine-state-is-not-identical-to-the-editor&p=2260075&viewfull=1#post2260075

I've upload his .blend file here as well, it's titled "bugScale.blend"%%%

%%%Please read this helpful post by MarcoIT which implements a solution through python: http://blenderartists.org/forum/showthread.php?275058-Not-Solvable-By-End-User-Game-Engine-state-is-not-identical-to-the-editor&p=2260075&viewfull=1#post2260075 I've upload his .blend file here as well, it's titled "bugScale.blend"%%%

Arnaud Loonstra commented

2013-03-20 11:45:22 +01:00

%%%I can confirm this bug. Important remark in the forum thread by MarcoIT:

"i think to have found the bug,
it is in localTransform(4x4) that , make a transformation of position and orientation , but not the scale with the parent obj.

in the blend below , that make the sphere with the code should be done automatically with parent + applyRotation .afaik

(to note that worldTransform is almost new , (from 2.62) and the scale is not used pretty much in game) "

I've been diving in the BGE scenegraph lately so I'm going to take a look at this to get myself more aquainted with the code. This shouldn't be too hard to fix, right...

right...%%%

%%%I can confirm this bug. Important remark in the forum thread by MarcoIT: "i think to have found the bug, it is in localTransform(4x4) that , make a transformation of position and orientation , but not the scale with the parent obj. in the blend below , that make the sphere with the code should be done automatically with parent + applyRotation .afaik (to note that worldTransform is almost new , (from 2.62) and the scale is not used pretty much in game) " I've been diving in the BGE scenegraph lately so I'm going to take a look at this to get myself more aquainted with the code. This shouldn't be too hard to fix, right... right...%%%

Tously Sid commented

2013-03-20 11:51:49 +01:00

%%%Thanks for checking this out Arnaud. Good luck.%%%

Arnaud Loonstra commented

2013-03-20 11:54:28 +01:00

%%%This might be related to #19216%%%

Arnaud Loonstra commented

2013-05-02 15:16:13 +02:00

%%%Also related: #28908%%%

Jorge Bernal commented

2014-05-18 20:25:49 +02:00

Added subscriber: @JorgeBernalMartinez

Inês Almeida self-assigned this 2014-06-17 12:10:16 +02:00

Francesco Siddi commented

2014-07-24 00:57:43 +02:00

Added subscriber: @fsiddi

Francesco Siddi commented

2014-07-24 00:57:43 +02:00

Adding a simplified version of the enginegameparentchildrelationships.blend file.

parent_child_simplified.blend

Adding a simplified version of the enginegameparentchildrelationships.blend file. [parent_child_simplified.blend](https://archive.blender.org/developer/F99296/parent_child_simplified.blend)

Inês Almeida commented

2014-07-25 13:38:48 +02:00

One more file, changing the suzanne rotation is what makes the bug noticable
parent_child_simplified_suzane.blend

One more file, changing the suzanne rotation is what makes the bug noticable [parent_child_simplified_suzane.blend](https://archive.blender.org/developer/F99499/parent_child_simplified_suzane.blend)

Inês Almeida commented

2014-07-25 18:50:55 +02:00

What is happening here is due to the GE using component wise hierarchical transforms, keeping loc, rot (as a 3x3 mat) and scale separated. This has noticeable problems when a child inherits a scale in different axis than it's own, because there is simply nowhere to store that information.

I do think this is a serious problem, solvable only by doing all the hierarchical transforms math in a different way. I think this would be a good chance change everything more in depth, so I am opening a design thread for discussion leaving my input there as I spent quite some time with this bug. It does not have a quick solution.

The right file this time:
parent_child_simplified_suzane.blend

What is happening here is due to the GE using component wise hierarchical transforms, keeping loc, rot (as a 3x3 mat) and scale separated. This has noticeable problems when a child inherits a scale in different axis than it's own, because there is simply nowhere to store that information. I do think this is a serious problem, solvable only by doing all the hierarchical transforms math in a different way. I think this would be a good chance change everything more in depth, so I am opening a design thread for discussion leaving my input there as I spent quite some time with this bug. It does not have a quick solution. The right file this time: [parent_child_simplified_suzane.blend](https://archive.blender.org/developer/F99530/parent_child_simplified_suzane.blend)

Inês Almeida removed their assignment 2014-08-23 20:14:43 +02:00

Inês Almeida commented

2014-08-23 20:14:43 +02:00

Added subscriber: @brita