Fast Inverse Square Root

The mathematical sleight-of-hand that made real-time 3D lighting possible on consumer hardware. By reinterpreting a float's bit pattern as an integer, subtracting from a magic constant, and applying a single Newton-Raphson step, the routine approximates 1/√x about four times faster than the FPU equivalent. The code reached the public when id Software released the Quake III source in 2005 — universally credited to John Carmack. But it predates the game; a 2003 investigation by Rys Sommefeldt traced versions of the trick back through SGI to Ardent Computer in the 1980s, with the magic constant 0x5F3759DF likely from Greg Walsh or Gary Tarolli. The true author is not settled. The comment that accompanies the magic line — // what the fuck? — was the right reaction even from the engineers who passed it down.

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