If different illumination is not convincing that there is an infinitude of variations for a single portrait, consider that you could also vary the thermostat of the room in which it is stored. Each incremental temperature affects each atom of the painting differently. You could freeze half of it, put the other half in a sauna. Still not enough? You could display it on a planet with high gravity or with low gravity. Each atom in a different energy state yet again. Any painting in the Milky Way galaxy is receding at nearly light speed from galaxies at cosmological distances. Another infinitude of energies of which the matter that makes up a painting is a small, almost insignificant part made up mostly of empty space. You get the idea? Energy and mass are equivalent, but energy always has an infinitude of directions, energies, etc. Matter mimics some of these, but does not perfectly quantize energy sufficiently even to justify thinking of it as a collection of discrete particles and / or relative positions. Don't even get me started on how the uncertainty principle figures in. Discrete mathematics as applied to this problem is only a figment of some mathematician's fevered imagination.