What is gb_depth exactly?

[half-potato]

What exactly are these quantities? I'm trying to add this denoiser to a different method and I don't understand how to adapt my program accordingly. gb_depth appears to contain z0 and |z0-z1|, but I'm not sure what z0 and z1 are. I see they are derived quantities from interpolating triangles on the mesh, but what would these values be for a volumetric renderer using quadrature?

[jmunkberg]

See Appendix B in our paper: https://arxiv.org/pdf/2206.03380.pdf

The depth values are computed here, from the clip space coordinates
https://github.com/NVlabs/nvdiffrecmc/blob/main/render/render.py#L228

We adapt the SVGF denoiser, so please refer to that paper for details about the depth guide to the denoiser.
That denoiser is designed for surface geometry, and the guides (normals, depth) are assumed to be noise free.
It is not trivial to apply these guides to a volumetric renderer, which has a fuzzy depth. See e.g.: https://research.nvidia.com/publication/2021-03_interactive-path-tracing-and-reconstruction-sparse-volumes
for a discussion of volume denoising.