WebGLNodeBuilder: Move sizeNode under positionNode

[wmcmurray]

I faced a "race condition" bug while using PointsNodeMaterial with positionNode + sizeNode.

How:
I was changing the position of each points with a couple nodes, then I re-used one of those nodes inside the node hierarchy of my sizeNode.

Result:
The position of each points was 0,0,0 because, in the resulting shader code, every variables generated by my nodes where appended where the size is defined (after gl_PointSize = size;), therefore the position definition above was referencing a variable that is not yet set (something like this: transformed = nodeVar7;), therefore the position was not set.

I wonder if there could be other race conditions like this ? Would it be a good idea to always put all variables at the top instead ?

cc @sunag

[sunag]

Would you have a small example just to confirm what am I thinking?

[wmcmurray]

Sure ! 😄
Here is the bug in action: https://codepen.io/wmcmurray/pen/dyVRMog?editors=0010
The problem arise on line 65, if you comment it, everything will work as intended !

[sunag]

@wmcmurray Thanks and great example.
Before merging your PR I would like to make sure this cannot be fixed in the flow process.

[wmcmurray]

@sunag Yes feel free to come up with a better solution ! 🙂 I realized (while making the example) that the problem is a bit more complex than I initially thought !

[sunag]

The WebGLNodeBuilder slot sequence must follow the same defined in ShaderLib/ShaderChunk of Three.js core. For this reason this change was necessary. The same thing would don't happen in WebGPU because WebGPUNodeBuilder is fully based in flow process. Thanks!