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I'm looking to create a node group which returns white for all upwards-facing regions that have a direct path to the sky - a simple example would be a 'snowfall' matte.

I have the z-normal working fine, but I'm running into the issue where regions which should be occluded (from above) still end up in the matte.

Is there any way to do this with nodes or OSL? The thing I'm thinking is impossible in cycles nodes is doing that extra ray-trace upwards to see if there's any geometry there.

This is the current result, as you can see, under the arch, and near Suzanne's mouth are showing snow - I want to create a mask of the regions which don't have a clear path upwards.

current shader output

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