Technically if your pixel operation is commutative and reversible (as could be with alpha blending), you could store a buffer of pixels at the current position,

Technically if your pixel operation is commutative and reversible (as could be with alpha blending), you could store a buffer of pixels at the current position, then undo the previous pixel by current window at that buffer and then re-apply the operation with the new pixel value, and then directly send this to the display? Am I missing something apart from the fact that alpha blending is not actually commutative and/or reversible; and the fact that nobody implemented this yet?