Alright, this is an idea I had a long time ago but I just haven't had time to work on it so I thought I'd throw it out into the ether and see what ideas people have.
The goal is to increase the dynamic range. Yeah yeah I know it is already very large and better than digital etc. Set that aside, this has nothing to do with digital. This idea is more about making GND filters obsolete.
The idea is to put a mask onto the film which becomes opaque with exposure. Initially, I was thinking of a photochromic material such as one finds on the "transitions" lenses. But now I wonder if there is something less expensive....
Also, for colour film, this exposure mask would need to be as neutral as possible.