Here’s a handy tip for flattening out bitmaps with some light variation across the surface as found on this website:
- Open image in Photoshop
- Duplicate Background layer [ Ctrl-J ]
- Open Filter > Blur > Average…
- Double click Background layer to change it into movable layer.
- Move it to the top of the stack, change it’s opacity to 50% [ 5 ] and blending mode to Linear Light
- Open Filter > Other > High Pass… and play with Radius parameter to control the effect
Obviously it’s not going to work on all bitmaps, but the few I’ve tried it on have worked out a treat.