Cohen–Sutherland
Line clipping with four-bit outcodes that accepts or rejects most lines without a single division.
The plane around a rectangular window is split into nine regions, and each endpoint gets a four-bit code: above, below, right, left. If both codes are zero, the line is entirely inside. If a bitwise AND of the two is non-zero, both ends sit on the same outer side and the line can be thrown away. Only the leftover cases need an actual intersection: move the offending endpoint to the boundary and try again.
Danny Cohen and Ivan Sutherland came up with it in 1967 for a flight simulator, where most of the world was always off-screen. Liang–Barsky (1984) is faster for lines that genuinely need clipping, but the outcode trick has outlived it — the same bitmasks are used to cull objects cheaply in frustum culling.