I think SDFs is the data underneath, but baked into SDF voxel tiles at runtime and not always evaluating SDF shapes. The SDF tiles still get splatted.
At one point it used triangles for the large splats, and still compute shaders for the small ones due to quad overdraw. I'm not sure if the final version stuck with that or went all compute, but quad complexity is also why nanite does compute shaders for small triangles and traditional rasterization for the larger ones.
In my experience it isn't really about quad overdraw/helper lanes, but rather about primitive assembly. On my RTX 4090, vertex shaders max out at about 20M tris/ms, while mesh shaders, which bypass primitive assembly, can reach 40M tris/ms. Maybe quad draw might be the next bottleneck after primitive assembly.
But it's a bit difficult to find the exact reason since profilers won't really tell you a concrete bottleneck when drawing vast amounts of tiny tris.
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