Faking Simple Volumetric Fog

Here's an old school way of faking volumetric fog. I'm not sure who invented the idea, but we used to do it this way back in the Eighties.

Set up a scene.

Use a camera-based grad to fake a Z-depth render. Let gray represent distance from camera - the further, the brighter. Render in 16 bits.

Slam in a ground plane. This will represent the height of the top of your ground mist.

Render out another 16-bit Z-depth image with your elevated ground plane in place.

Subtract one 16-bit Z-depth image from the other. This will give you an image of fog ray traversal lengths - exactly as if you ray-marched through the volumteric fog.

You can tweak the contrast if you wish.

Use this volumetric ray-marched mask as a key mix between your rendered scene and an image of pure fog color. (I chose pure red for my full fog image)

Here are the results again. You can adjust fog density to taste by adjusting the density of the Z-depth difference mask.

This technique is pretty old-school. For it to work you have to stay out of the fog.