Understanding Geometry Updates with WebGPURenderer

When working with the WebGPURenderer in Three.js, there are some specific behaviors to be aware of when updating geometry, especially when the number of vertices changes.

The Issue

The behavior you're experiencing where geometry updates don't take effect until the material is also updated is related to how WebGPURenderer handles geometry changes. This is particularly relevant when the vertex count changes, as in your example where you're going from 3 vertices to 4 vertices.

Why Material Updates Are Required

With WebGPURenderer, when you replace a geometry with a new one that has a different number of vertices, the renderer needs to recreate the internal GPU buffers and pipeline state. However, this recreation isn't automatically triggered just by changing the geometry reference.

When you update the material (or force a material update), it signals to the renderer that the rendering pipeline needs to be reevaluated, which includes checking the geometry's attributes and structure.

Solutions

Here are a few approaches to handle this situation:

1. Force material needsUpdate flag:

   JAVASCRIPTCopied
   mesh.geometry = newGeometry;
   mesh.material.needsUpdate = true;

2. Use the geometry's built-in update methods when possible instead of replacing the entire geometry:

   JAVASCRIPTCopied
   // If only updating existing attributes without changing vertex count
   mesh.geometry.attributes.position.array = newPositionArray;
   mesh.geometry.attributes.position.needsUpdate = true;

3. Dispose both geometry and material when making significant changes:

   JAVASCRIPTCopied
   mesh.geometry.dispose();
   const newGeometry = new BufferGeometry();
   // Set up new geometry...
   mesh.geometry = newGeometry;
   
   // Either create a new material or update the existing one
   mesh.material.needsUpdate = true;

4. For WebGPURenderer specifically, you might need to force a renderer context update:

   JAVASCRIPTCopied
   mesh.geometry = newGeometry;
   renderer.info.reset(); // This can help force internal cache updates

Best Practices

• Always dispose of old geometries to prevent memory leaks
• When changing vertex counts, be prepared to update materials or force renderer updates
• Consider using geometry update methods rather than complete replacements when possible
• For WebGPURenderer, stay updated with the latest Three.js versions as this renderer is still evolving

This behavior is specific to how WebGPURenderer manages GPU resources and may differ from other renderers like WebGLRenderer.