Wrong normals for instanced models with node scaling in glTF

[javagl]

The attached archive contains a tileset with three contents:

• Box.glb is just the "Box" glTF sample model
• boxScaled.i3dm is an I3DM that contains a version of the box that scaled by a factor of 100 in x-direction, but defines a SCALE_NON_UNIFORM of [0.01, 1.0, 1.0], resulting in a unit box to be rendered
• boxScaled.glb is a glTF asset that contains a version of the box that scaled by a factor of 100 in x-direction, and uses EXT_mesh_gpu_instancing with a SCALE of [1.0, 1.0, 1.0] (!), attached to a node with a scaling of [0.01, 1.0, 1.0] - also resulting in a unit box to be rendered

The non-uniform scaling of the node that refers to the EXT_mesh_gpu_instancing causes some error with the computation of the normals:

Bottom: Box.glb Center: boxScaled.i3dm Top: boxScaled.glb

The effect is highly dependent on the view direction...

This may be a hint that there's likely some place where the transformation of the normals with the transpose of the inverse of the modelview matrix does not properly take into account the matrix of the node that the instancing extension is attached to...

The archive with the tileset, tiles, and a Sandcastle for quick tests:

CesiumJS_instancing_normal_issue.zip

[ggetz]

Thanks for the report @javagl!

Next think I'd suggest is to check whether this is a regression that was introduced in 1.97, or if the issue was pre-existing.