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Something that didn't occur to me the last time I saw this: if each point is writing to the same RT, is your play space limited by how much worldspace it covers? Or is it just local to the player since anything further away is too far to matter?

This is neat. It doesn't actually have anything to do with the light itself, so it's just a general purpose "clear line-of-sight fog within radius." It could work just as well for, say, an explosion cutting through smoke or helicopter rotor wash, right?

something that could be even cooler: instead of actually masking out the smoke in the shader, sample this mask in the smoke particle system itself and stop spawning new smoke particles when you're occluded - this way the smoke would dissipate nicely when the light first hits it