Would it help to get multiple images from the pieces on the belt, each illuminated from a different direction by some kind of strobe? Then the shadows could be incorporated in detecting some shape information. Might even help with translucent pieces.
Colored strobes may also help separating out different color pieces, although I expect that would be overkill.
Colored strobes may also help separating out different color pieces, although I expect that would be overkill.