I hate to bring this up... But fog isn't transparent.
Why not take sprite A sans alpha, then sprite B (the fog) with alpha and cover sprite A, to net you sprite C, which has no alpha, and give it sprite A's origional alpha figures...
so.. if you have a bottle with a top. The bottle itself might be 50% transparent, and the top is solid. You cover with fog, and the top becomes more grey, and the bottles colors become more grey, and you can then still see through the bottle by 50%.
I don't see why you can't do that.
In other words... Do whatever you want to make the fog look. But replace the ultimate alpha with the origional sprite's alpha.
Why not take sprite A sans alpha, then sprite B (the fog) with alpha and cover sprite A, to net you sprite C, which has no alpha, and give it sprite A's origional alpha figures...
so.. if you have a bottle with a top. The bottle itself might be 50% transparent, and the top is solid. You cover with fog, and the top becomes more grey, and the bottles colors become more grey, and you can then still see through the bottle by 50%.
I don't see why you can't do that.
In other words... Do whatever you want to make the fog look. But replace the ultimate alpha with the origional sprite's alpha.