You can definitely do that, however in practice, separating out by files makes the code very easy to test, since you can validate functionality per pixel type,

You can definitely do that, however in practice, separating out by files makes the code very easy to test, since you can validate functionality per pixel type, and then validate your outer loop by passing fake objects with correct functions. You can also have internal workings with static functions/variables that you can use as helpers, but invisible to outside code and not declared in header files.