You're speaking about film?
scan your negative with a scanner or take a picture of it with a light at the background.
A bit like in this video > https://www.youtube.com/watch?v=LXuwnt5ba8M
then once in GIMP go to Colors > Invert.
Then instead of opening the Curves like in the video above, in GIMP open the Levels (Colors > Levels...) and do like the guy with Photoshop > click on a the white pipette and find something that you think should be white on the photo (same for the gray, if white does not works well -> Reset between trials)
Or you can just try the "Auto Input Levels" option in first place, it can work well and it's simple to use
Screenshot below:
> Left the "auto" button, might works well,
> right the 2 button to pick white or gray on a picture
scan your negative with a scanner or take a picture of it with a light at the background.
A bit like in this video > https://www.youtube.com/watch?v=LXuwnt5ba8M
then once in GIMP go to Colors > Invert.
Then instead of opening the Curves like in the video above, in GIMP open the Levels (Colors > Levels...) and do like the guy with Photoshop > click on a the white pipette and find something that you think should be white on the photo (same for the gray, if white does not works well -> Reset between trials)
Or you can just try the "Auto Input Levels" option in first place, it can work well and it's simple to use
Screenshot below:
> Left the "auto" button, might works well,
> right the 2 button to pick white or gray on a picture
Patrice