So if I make a transmap for the clouds or in my case fog, that has values of grey in it, it will translate a different amounts of transparency depending on how dark the area is when applied to an image.

I.e. Black areas will be 100% transparent, White areas will be 0% non transparent and areas that are 50% on the grey scale will be more or less 50% transparent?

If that's right I'll get to work on it.

The reason that I asked is that I've only used transmaps to outline things that I applied to a shape i.e. a star shaped object on a clothpane to remove the unwanted area.  Or to make holes in things.

So they were just black and white.

I'm learning something new every day. LOL

If you shoot a mime, do you need a silencer?