Calculating skin joint matrix - use mesh node transform? about gltf-tutorials

[dakom]

Along with the language of the tutorial, if the above is correct then I believe it would also mean changing this pseudocode:

jointMatrix(j) =
  globalTransformOfNodeThatTheMeshIsAttachedTo^-1 *
  globalTransformOfJointNode(j) *
  inverseBindMatrixForJoint(j);

to rather be:

jointMatrix(j) =
  globalTransformOfSkeletonRootNode^-1 *
  globalTransformOfJointNode(j) *
  inverseBindMatrixForJoint(j);

I'm not quite clear on what to do in the case where there is no skeleton root node... e.g. whether it should be:

jointMatrix(j) =
  globalTransformOfJointNode(j) *
  inverseBindMatrixForJoint(j);

or in this particular case, the skeletonRootNode is the mesh node...
?

[shrinktofit]

I have the same question.

I don't know the meaning of skin.skeleton and how does it effect the calculation of the joint matrix.

In my case, I simply ignore it and do follow the tutorial gltfTutorial_020_Skins and everything seems OK.

[emackey]

@donmccurdy Do you know the answer, or the right person to ask?

[emackey]

/cc KhronosGroup/glTF#1270

[donmccurdy]

Just replied on KhronosGroup/glTF#1270. I believe it is correct to ignore it.

[emackey]

Having learned a bit more about skinning this year, I believe the tutorial's pseudocode is correct.

The vertex shader that's drawing a skinned mesh will attempt to draw it wherever in the world the node that owns the skinned mesh is located. But the mesh isn't supposed to be there, instead the mesh is supposed to be wrapped around the bones, which might be completely elsewhere. So the transform needs the inverse of the skinned mesh node's position, to recover the glTF "world" coordinates, and from there must travel to the joint's world coordinates.

The skeleton node is safe for implementations to ignore, and safe to omit from glTF. I believe it serves as a marker between full-fledged heavyweight scene nodes, versus lighter-weight no-mesh-allowed joint nodes. I believe it's purely a performance optimization, or a crutch for systems that demand to draw a sharp distinction between nodes and joints.

[javagl]

I think that, after a longer discussion and various fixes, this should also be resolved as of #64

If it is not, feel free to reopen.