No vertex skinning when default material is used

[javagl]

The spec is (or was) silent on this point. The naive implementation of the "default material" (which should be used when no material was given) consists of a fragment shader that just emits the gray color, and a vertex shader that does the usual GL transforms.

However, this "default vertex shader" does not support skinning, because it does not have joint- and weight attributes.

This is not a big issue for now, especially considering the transition to glTF 2.0, but mentioned here so that it does not get lost.

[javagl]

For glTF 2.0, there is now a (preliminary?) solution, consisting of a vertex shader that is compiled with a certain number of joints ( https://github.com/javagl/JglTF/blob/master/jgltf-model/src/main/resources/pbr.vert#L18 ).

The solution might be backported to glTF 1.0, but the shaders may have to be updated anyhow, to improve the PBR support.

[javagl]

Crossref: donmccurdy/three-gltf-viewer#50