vpool (WebGPU): ghosted / setOpacity views flicker on every redraw of the same pose #63

With @thatopen-platform/fragments-beta 3.5.14 on WebGPU, a streamed model with ghosted elements (setOpacity) flickers. Drawing the same camera pose twice gives a different image.

Measured

That Open's own school_arq.frag and school_str.frag (from thatopen.github.io/engine_fragment/resources/frags/), converted with writeStreamedFiles, both ghosted with setOpacity(0.12). Three WebGPURenderer on Deno 2.9.6's own WebGPU (wgpu, Metal), 960×540. Pixels that differ between two redraws of one pose (of 518,400; a pixel counts when the RGB difference sums to over 24):

We saw the same flicker in Chrome on our own models. WebGL vpool does not flicker.

Cause, as far as we can see

• The culling compute pass compacts each frame's surviving triangles with atomicAdd (wgsl.mjs), so the indirect draw list comes out in a different order every frame.
• Faded elements are drawn through one translucent mesh (vpool-fade, alpha blended, no depth write) in each element's own colour.
• Translucent faces of different colours blend to a different result in a different order. With one colour the blend is order-independent, which matches the second row.

Possible fixes

Build the draw list in a stable order (a prefix sum instead of atomicAdd), or sort the fade list. A depth pre-pass for the ghost does stop the flicker, but vpool's occlusion culling reprojects the previous frame's depth and then culls everything inside the ghost, so that isn't a fix.

Workaround we use

Give every vpool-fade face one colour: material.colorNode = vec4(vec3(0.85), colorNode.w). The image is stable, but every ghost looks the same.

Note: vpool only draws streamed models, so a plain .frag loaded with load() won't show this. Screenshots and the harness are available if useful.