fast inverse square root

The selection of 0x5f3759df as a constant prompted much of the original speculation surrounding the fast inverse square root function.

For a practical example from Quake III, see Fast inverse square root.

The following code is the fast inverse square root implementation from Quake III Arena, stripped of C preprocessor directives, but including the exact original comment text:

The advantages in speed offered by the fast inverse square root kludge came from treating the longword containing the floating point number as an integer then subtracting it from a specific constant, 0x5f3759df.

In order to assist calculation of these shaders, id Tech 3 implements a specific fast inverse square root function, which attracted a significant amount of attention in the game development community for its clever use of integer operations.

Quake III Arena used the fast inverse square root algorithm to speed graphics processing unit computation, but the algorithm has since been implemented in some dedicated hardware vertex shaders using field-programmable gate arrays (FPGA).