Lookup table (LUT)

A lookup table is a precomputed mapping from input intensity to output value, stored as a table indexed by input value. For an 8-bit input, the table has 256 entries — one output value per possible input. Applying a LUT to an image is one of the simplest computations possible: for each pixel, read its value, look up the corresponding output, write the output to the result. The output can be the same type as the input (LUT remaps grayscale to grayscale, e.g., gamma correction), or a different type (LUT remaps grayscale to RGB, the standard "false color" rendering). LUTs are how display systems color-render scientific images, how tone-correction is implemented at the GPU level, and how many simple intensity transformations are computed in practice — a LUT is faster than recomputing the underlying function per pixel because the function is precomputed once and indexed thereafter.