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Volumetric Fog + Post Process Fog

Hello UE4 Rendering wizards!

I have a conundrum that I wanted to share - it’s something I’ve been beating my head against for many months.

Consider this scenario:

I am making a (primarily) underwater game, and must therefore make use of fog effects. Exponential height fog is undesirable because it doesn’t mask correctly at the waterline, so I have opted to use a post process fog masked by custom depth. That all works fine.

Now here’s the tricky part.

I would very much like to add the ‘volumetric’ fog effects that were introduced in 4.16 on top of my custom fog solution, so that my water will appear to have rays of light passing through it. Unfortunately the post process fog and volumetric fog do not play nice together, resulting in some very ugly sorting issues (pictured here)

Can anyone think of a way to get volumetric fog and post process fog to sort correctly, or is it simply not possible?

Any help is much appreciated!

Cheers,
Alex