SceneGraph and Transforms #41184

New Issue

Inês Almeida · 2014-07-25T22:24:16+02:00

Inês Almeida commented

2014-07-25 22:24:16 +02:00

The scene graph currently handles transforms by storing independent components: location, orientation and scale.

Every frame all objects' world transforms are calculated relative to their parents' and their own local components. Doing these computations component wise causes several problems when a parent is scaled and rotated, and the child rotated too (because in a 4x4 matrix, scale and matrix overlap, and stored separately they have no notion about each other. the child should have the parent scale applied along the parent's axis, not it's own). There is no way to store shear, and also problems with negative scale.

At the end, a 4x4 matrix is computed from the individual components for use in OpenGL.

I propose changing the use of independent components to a 4x4 matrix. The advantages would be correct behaviour with hierarchical transformations and (imho) simpler code. The disadvantages that obtaining the individual components back from the matrix - decomposition - is a complex operation that has problems at least in case of reflexive matrices. (and speed problems for real time?) Still checking who would need this..
I also propose taking the chance to replace the MoTo library.

What is the problem with MoTo?
The Motion Toolkit version we are using is from 2002! with several patches on top. It is limited, not optimized and uses doubles instead of floats contrasting with the rest of blender and what should be used in a GPU.
MoTo doesn't provide anything that we can not use from BLI_math (in C), Eigen3 or bullet2, so why maintain an extra library?
Moto can (should if kept) be updated, it has a new repo with activity from a couple of months. The updated version is quite different, it is template based (not forcing doubles), optimized and has support for several new concepts: diagonals, planes, dual quats, bounding boxes.

Developer documentation for the scene graph is here .

The scene graph currently handles transforms by storing independent components: location, orientation and scale. Every frame all objects' world transforms are calculated relative to their parents' and their own local components. Doing these computations component wise causes several problems when a parent is scaled and rotated, and the child rotated too (because in a 4x4 matrix, scale and matrix overlap, and stored separately they have no notion about each other. the child should have the parent scale applied along the parent's axis, not it's own). There is no way to store shear, and also problems with negative scale. At the end, a 4x4 matrix is computed from the individual components for use in OpenGL. I propose changing the use of independent components to a 4x4 matrix. The advantages would be correct behaviour with hierarchical transformations and (imho) simpler code. The disadvantages that obtaining the individual components back from the matrix - decomposition - is a complex operation that has problems at least in case of reflexive matrices. (and speed problems for real time?) Still checking who would need this.. I also propose taking the chance to replace the MoTo library. What is the problem with MoTo? The Motion Toolkit version we are using is from 2002! with several patches on top. It is limited, not optimized and uses doubles instead of floats contrasting with the rest of blender and what should be used in a GPU. MoTo doesn't provide anything that we can not use from BLI_math (in C), Eigen3 or bullet2, so why maintain an extra library? Moto can (should if kept) be updated, it has a [new repo ](http://code.google.com/p/motion-toolkit/) with activity from a couple of months. The updated version is quite different, it is template based (not forcing doubles), optimized and has support for several new concepts: diagonals, planes, dual quats, bounding boxes. Developer documentation for the scene graph is [here ](http://wiki.blender.org/index.php?title=User:Brita/GSoC_BGE_cleanup_and_support/doc/sg).

Inês Almeida commented

2014-07-25 22:24:16 +02:00

Changed status to: 'Open'