Model fails to load if some primitives are Draco encoded and others are not

[lilleyse]

I can't share the data publicly, but I have a glTF that contains two primitives, one with triangles and the other with lines, and only the triangles are Draco encoded. The code assumes that if the extension is present then all primitives are Draco encoded, which causes a failure when processing the lines.

[lilleyse]

This happened with another model that had both lines and triangles.

[mramato]

@lilleyse @ggetz Any guess on how easy this will be to fix? Seems like it's starting to pop up more often.

[ggetz]

We check each primitive for the extension before attempting to decode: https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Scene/DracoLoader.js#L180

Then throughout we check if there's decoded data associated with the primitive, and use that: https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Scene/Model.js#L2549

This would be a matter of making sure the resources for the primitives without the extension are loaded and used properly.

[lilleyse]

Also came up in #7495.

[OmarShehata]

There's a sample glTF in that issue above ^ so it should make it easier to confirm that this works when we have a fix.