A - camera
B - translucent plane
C - solid wall
A <==> B - postprocess for fog/mud water
B <==> C - clean air
Is there any way to get A-B distance instead of A-C?
you might be able to get it if you write the translucent plane/planes to a custom depth buffer. here is an example: Custom Depth in Unreal Engine 4 - Tom Looman
hope that helps 