3D Tiles: Make KHR_Materials_Common work with batch tables and CESIUM_RTC extensions

[JDepooter]

Addresses the problem described in #4669

I've updated the code which generates shaders for the KHR_Materials_Common extension. The update addresses two related issues:

• When a batch table is used, a batch id attribute and parameter are needed in the generated vertex shader.
• When the CESIUM_RTC extension is used, the generated shader should use the CESIUM_RTC_MODELVIEW semantic for the model view matrix. This change also fixes the shader generation for glTF models loaded via the Model::fromGltf() function.

One thing I'm not certain is correct is the type of the BATCHID shader parameter. I've set it to WebGLConstants.UNSIGNED_SHORT, based on this link. Could anybody comment whether this needs to be adjusted based on the actual data type of the batch id values in the glTF buffer?

[lilleyse]

These changes look great! I'll throw together a sample tileset soon for testing.

[lilleyse]

Format as:
if (options.addBatchIdToGeneratedShaders) {

[lilleyse]

Could anybody comment whether this needs to be adjusted based on the actual data type of the batch id values in the glTF buffer?

Yes this will need to be adjusted based on the actual type in the buffer. The spec may need to be clearer in saying that the batch id can be any type.

[lilleyse]

Quick question @lexaknyazev @pjcozzi

Does a parameter's type equal the data type stored in the buffer or the type declared in the shader? If batchId is stored as unsigned shorts, should type be 5123 or 5126.

"technique": {
    "attributes": {
        "a_batchId": "batchId"
    },
    "parameters": {
        "batchId": {
            "semantic": "BATCHID",
            "type": 5123
        }
    }
}

Shader

attribute float a_batchId;

[lilleyse]

Thanks @lexaknyazev for the update.

@JDepooter the type can be WebGLConstants.FLOAT. I also updated the 3D Tiles spec: CesiumGS/3d-tiles#154

[JDepooter]

@lilleyse I've updated the formatting as requested.

With respect to the type of the BATCHID parameter, if the parameter type must match the data type stored in the buffer, there could be situations where multiple shaders would need to be generated per material. If two primitives use the same material, but have different batch id types, they would need different shaders.

[lilleyse]

Yeah, I'm hoping the answer is that the type should actually just be 5126 for float, but am waiting on a response to that.

[lilleyse]

Not quite final, but the following tileset works on this branch:
BatchedWithKHRMaterialsCommon.zip

[JDepooter]

@lilleyse I've updated the type to WebGLConstants.FLOAT

[pjcozzi]

Thanks again for the contribution, @JDepooter!

@lilleyse we have a CLA so please merge when ready.

[lilleyse]

I added a simple test with the sample tileset I linked.

Thanks @JDepooter.