Feature Request: OnBeforeRender on Material

Is your feature request related to a problem? Please describe.

Let’s say I have a custom material that is shared by two meshes. This material uses the inverse transform of the mesh. In this case, I have no way to send the inverse transform because it’s dependent from the actual bound mesh.

When creating an app with Three.js, it’s not really an issue because we control the flow of data. However, when wrapping Three.js in another library, it makes it somewhat difficult to share a ShaderMaterial whose parameters aren’t independent from the camera / mesh drawn.

Describe the solution you’d like

On the top of my mind:

• Create a onBeforeRender callback on the material, that would be in charge of updating per-object uniforms. This is similar to the onBeforeCompile callback, but called each time something is rendered with this material.

Describe alternatives you’ve considered

• Use onBeforeRender on each meshes. Annoying because all meshes using this material must be updated and a user would need to attach the callback himself
• Clone the material for each mesh

Additional context

This is highly related to the issue #16922. I think the problem is similar.

[sunag]

@DavidPeicho Look like we have a WebGL version for that 😃 https://github.com/mrdoob/three.js/pull/22504

[Mugen87]

Okay, after investigating this issue a refactoring of how bindings are managed is required. Right now, the binding and the actual data are handled in a single object. In the below example, bufferGPU represents the UBO and array the actual data.

https://github.com/mrdoob/three.js/blob/cb99e7e08cc97cb7a22f17e32cc7a22082022d67/examples/jsm/renderers/webgpu/WebGPUUniformsGroup.js#L22-L23

If a material is shared among multiple 3D objects, there is only one instance of bindings objects and thus only one data storage. This is not a problem for textures (because textures are defined on material level) however model related data like the MVP matrix can’t be processed this way. They have to be managed per 3D object and the UBO needs to be updated per object with the correct data.

I need some time to think about a possible solution for this…