Samples in the rome-gltf repository can not be displayed due to errors

[cx20]

I tried to display the following model in Cesium.js.
https://github.com/mrdoob/rome-gltf/blob/master/files/models/life_soup/quadruped_fox.gltf

However, it was not displayed due to an error.
http://jsdo.it/cx20/6z4e

Cesium.js:479 
[Cesium WebGL] Shader program link log: Too many vertex attributes.

Is this due to the limitation of Cesium.js?

[emackey]

First reported in #6416 (comment)

[bagnell]

I see the same error. That glTF has 22 vertex attributes, but WebGL only supports 16. This is a limitation of WebGL not Cesium. See webglreport.com for how many vertex attributes WebGL supports for you (see Max Vertex Attributes under Vertex Shader on the right).

According to webglstats.com, only a small percentage of hardware supports MAX_VERTEX_ATTRIBS > 16.

[emackey]

I would guess that ThreeJS is just limiting the number, so it doesn't overflow. Not sure if that produces the correct results or not.

[pjcozzi]

@emackey perhaps worth an implementation note in the glTF spec? Could you start a discussion in that repo?

I'm surprised and embarrassed that this hasn't come up before except in the more specific case of skins and morph targets.

[cx20]

I tried some other libraries that supports other animations. The link is a link to the sample.
Three.js, Hilo3d, PlayCanvas and the previous version of Babylon.js were able to display animations.

Library	rome-gltf animation
three.js	✅
Babylon.js v3.1	✅
Babylon.js v3.2	❌
Cesium.js v1.44	❌
Grimoire.js	❌
ClayGL	❌
Hilo3d	✅
PlayCanvas	✅

[PrincessGod]

@emackey @bagnell @pjcozzi @twpayne @cx20 Hi, all~

I found this problem can be optimized.

First decide how many morph targets can be applied according to MAX_VERTEX_ATTRIBS.

Then according to each target's weight to decide use which targets, which means sort by weight and use bigger targets.