WebGPU/TSL equivalent of `gl_DrawID`

Questions
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1

Hello, Im looking for the TSL equiv of WebGL’s gl_DrawID

For context I’m trying to sample a custom batching texture just like how the batching matrices data texture is sampled internally.

Internally drawIndex/instanceIndex is used for this purpose here, but when I use the same code, those index variables are always the same value (null) it looks like. https://github.com/mrdoob/three.js/blob/2eefcd6db11dba2d55dd49a51e7c529b6c20ef28/src/nodes/accessors/BatchNode.js#L90

Here is a minimal working WebGL example I’d like to recreate in WebGPU

https://codesandbox.io/p/devbox/thirsty-bush-f7z3c3?workspaceId=ws_5NvLnrKyk5Nd2YftNHw3Fy

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Here is my WebGPU attempt so far. I am aware of Multi draw not being supported in WebGPU so far but even when using forceWebGL flag on the renderer, this example does not work.
https://codesandbox.io/p/devbox/gracious-bassi-yhm3xg?workspaceId=ws_5NvLnrKyk5Nd2YftNHw3Fy

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Any help would be very much appriciated. Thanks!

2

Fixed. Stupid mistake :man_facepalming:Needed to move material definition before BatchedMesh initialization and change colorNode to the following.

  material.colorNode = TSL.Fn(() => {
    return TSL.color(TSL.float(GetIndirectIndexNode(TSL.drawIndex)).div(n));
  })();

Important to cast GetIndirectIndexNode result to float before division, else, the division result will be an int.

Fixed demo:

https://codesandbox.io/p/devbox/inspiring-brook-w6jjgw?workspaceId=ws_5NvLnrKyk5Nd2YftNHw3Fy

3

I’m glad you fixed it, I was actually looking into your code yesterday which lead to BatchNode: Optimize generated shader code. by Mugen87 · Pull Request #32203 · mrdoob/three.js · GitHub .

To clarify the title: The TSL object drawIndex only works with WebGL 2 right now and only if the draw call was triggered by a multidraw command. Otherwise it is always 0. In WebGPU, drawIndex is currently not supported and triggers an error. When at some point multi-draw-indirect is supported, we can revisit this issue.

4

That’s great. Yes sounds good on multi draw support. What is the fallback on WebGPU? Is it regular instanced drawing?

if so, will instanceIndex work as intended?

5

instanceIndex is used many times in the WebGPU examples and should work as expected.

6

Just tested. Works as intended. Thanks!