Post-processing treats the rendered frame as footage to be finished: bloom, grain, vignettes, color grading, distortion, each a full-screen shader pass before display. The scene provides content; post provides cinematography.
The economics are excellent: one stack of passes restyles everything beneath it, which is how a system offers many scenes with one coherent look. It is also the natural home for global audio response, since a post effect touching every pixel makes the whole image react at once.
The discipline is ordering and restraint: effects compound in sequence, and each pass spends milliseconds from the frame budget. Photism’s composer chain is exactly this stack, with the taste knobs exposed as its LOOK controls.