Custom (matrix_model / matrix_normal) values for meshInstance

[AlexAPPi]

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Sometimes, for mesh instances, it is useful to have a dedicated proxy function that overrides the original position. For example, in the case of prefabs where world coordinates are defined relative to an internal root node, this helps avoid sending redundant data through the (model_matrix) | (matrix_normal) uniform channel.

It is worth considering adding such a feature.

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engine/src/scene/renderer/renderer.js

Line 832 in 4429ce7

setMeshInstanceMatrices(meshInstance, setNormalMatrix = false) {

Prototype:

    setMeshInstanceMatrices(meshInstance, setNormalMatrix = false) {
        const modelMatrix = meshInstance.customModelMatrix ?? meshInstance.node.worldTransform;
        this.modelMatrixId.setValue(modelMatrix.data);
        if (setNormalMatrix) {
            this.normalMatrixId.setValue((meshInstance.customNormalMatrix ?? meshInstance.node.normalMatrix).data);
        }
    }

[mvaligursky]

I think it would need more justification to consider this.

[AlexAPPi]

I think it would need more justification to consider this.

The engine currently handles matrices for instancing well: full matrices can be placed in the instance buffer if they are updated infrequently. For dynamic instancing, storing full matrices in the buffer is too expensive, so I use an index in the instance buffer while the actual matrices are stored in a texture — when I instantiate a prefab, the prefab’s global transform is read from the texture, while the prefab’s other parts use local coordinates relative to that prefab. The same applies to dynamic batches, where model coordinates are relative to the batch root. Switching to a workaround that avoids matrix textures would require major rework of engine logic and would greatly increase bandwidth usage and reduce performance.