In what units is fog density expressed?

I imagine these are the same as the old fixed function OpenGL pipeline, which I believe are as follows:

Linear: f = inverse_lerp(fog_start_distance, fog_end_distance, distance) Exp: f = exp(-fog_density * distance) Exp2: f = exp(-(fog_density * distance) ^ 2)

f = clamp(f, 0.0, 1.0)

C = f Cr + (1.0 - f) Cf

where: f is fog factor C is resulting fragment color Cr is original fragment color Cf is fog color