`ModelExperimental` - Default material

According to the glTF spec,

If material is undefined, then a default material MUST be used.

The “default material” is described here: https://www.khronos.org/registry/glTF/specs/2.0/glTF-2.0.html#default-material

This should be easy to handle, there’s a couple things to check:

• Ensure that the MaterialPipelineStage doesn’t throw an error if primitive.material is undefined. A no-op is probably okay
• Ensure that the shader defaults match what is described in the spec
• find a model to test with

[ptrgags]

Oh @jjhembd here’s A modified Sandcastle example that sets up the default material. Basically I (ab)used a CustomShader’s REPLACE_MATERIAL mode so the material is set to the default rather than what was in the glTF.

The relevant code is this part

// Modified this shader to use the default material
const textureUniformShader = new Cesium.CustomShader({
  lightingModel: Cesium.LightingModel.PBR,
  mode: Cesium.CustomShaderMode.REPLACE_MATERIAL,
  fragmentShaderText: `
    void fragmentMain(FragmentInput fsInput, inout czm_modelMaterial material)
    {
    }
    `,
});

EDIT: Also note that the lighting looks weird for the default material since the normal is set to a constant +z direction. This was mainly because we haven’t implemented #6506

[ptrgags]

Hi @jjhembd,

Is there anywhere else I need to check to make sure we always get a default material?

At this point I think it’s really just ModelExperimentalFS.glsl that needs to be updated since that’s hard-coding defaults in the shader.

As far as the ModelComponents.Material() goes, I think we covered all the main cases in the above discussion, but I guess it doesn’t hurt to do a search through the code for ModelComponents.Material ModelComponents.MetallicRoughness and ModelComponents.SpecularGlossiness to double check nothing is hard-coding an unexpected default.

Also, in PntsLoader…

That’s a good question… .pnts not technically a glTF file in the first place (unlike .b3dm and .i3dm which wrap a .glb file). So I don’t think this particular case has a correct answer, it’s more about what we think looks good as a default. Might be good to compare Model vs ModelExperimental rendering the same .pnts model and try to make them match.