Yeah, that’s a problem I had for ages as well, to fix it you can use this trick:
- Instead of using the mist pass, use the depth pass. By default, it has the full distance at every pixel, so to get a useable version, pass it through a normalize node to get it in a 0-1 range.
- Then, because it also has a lot of fireflies (but a lot less than the mist pass), pass it through a few despeckle nodes.
- I usually add one, play with the values until it removes the most noise, then add another one and repeat until the image is clean.
Then hopefully you get a clean mist pass!
1: normalized depth, 2: with despeckling
Hope that helps!