A stored colour transform: a table that gives an output colour value for each input value, used in film and video to convert between encodings or to apply a look.
A one-dimensional LUT remaps each channel separately and can change tone but not mix channels. A three-dimensional LUT samples a grid of red, green and blue combinations and interpolates between the grid points, so it can change hue and saturation as well. A technical LUT converts between two defined encodings; a creative or show LUT carries an agreed look.
A LUT is not a filter that suits any footage. It expects one specific input encoding, and the same table applied to a different camera's log curve gives a different and usually wrong result.
A production may fix its look as a show LUT before the shoot and view every log recording through it on set.
Where a LUT sits matters more than what it contains: corrections belong before it, and a table left in the chain can clip the range an HDR master needs.
A LUT is described by its dimension and its size. A one-dimensional table lists an output for each sampled input level of a channel. A three-dimensional table samples a cube of red, green and blue combinations: a 33-point cube holds 35,937 entries and a 65-point cube 274,625, and values between the grid points are interpolated. The Common LUT Format, version 3.0, defines both kinds of node and discourages three-dimensional grids larger than 128 points per axis. A table also has an expected input and output encoding, which the numbers themselves do not reveal and which has to travel with the file.
Corrections are applied after a LUT that converts log footage to a display standard.
Highlights and shadows the table has already compressed or clipped cannot be recovered downstream. Exposure and balance corrections belong before the table, where the full recorded range is still present.
An HDR master is made from a project built around an SDR show LUT.
The table limits the image to the range it was built for. Dolby's grading guide instructs colourists to remove or modify any lookup tables that may be limiting dynamic range and colour in the source images.