Unsharp Mask Sharpening

The "unsharp" in the name refers to the blurred copy that gets subtracted — the technique comes from the film era, where a negative was sandwiched with a defocused positive. It is a purely optical geometric process, not invented detail: it increases the contrast of edges that are already there.

The threshold skips flat areas. Without it, noise in skies and skin gets amplified exactly as much as genuine edge detail, which is the usual reason over-sharpened photos look gritty rather than crisp.