07 Dec 2012

Colour Look Up Tables demystified

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As digital video cameras get better, and their users become more ambitious, LUTs are used to preserve or manipulate the images. Understanding LUTs is now key to setting up a production workflow

Anyone involved in anything other than the most basic camerawork or postproduction at the moment will quickly encounter the concept of a lookup table, or LUT. They’re a simple enough concept, but there are at least two main types with significantly different capabilities, and they’re used for both technical and creative purposes.

The purpose of a lookup table is to make precise and repeatable changes to an image’s luminance and colorimetry, and as the name suggests, the most basic type of lookup table comprises a simple list of numbers. A ten-bit lookup table comprises 1024 numbers for each RGB channel, each representing the output value for a given input value. If the 512th value of the red LUT happened to be 501, every red value of 512 that went in would come out as 501. That’s a one-dimensional lookup table, and while it can make changes to overall luminance and colour balance by altering the values in each individual RGB channel, it cannot, for instance, make a purely red object into a blue one, or affect overall saturation or hue.

A billion possible values

To do that, we need a three-dimensional lookup table, so called because every possible combination of three RGB input values is associated with a unique set of RGB output values – or at least, in theory. Since there are over a billion possible discrete colour values in a ten-bit RGB signal, usually a somewhat smaller set of output values is stored and the results for a particular input value are calculated from an average of the most nearby stored output values. Because it deals with sets of three numbers, a 3D LUT can be depicted as a cube, and some systems call this sort of LUT a cube. If a LUT allows hue modification – that is, changing one colour into another – it must be a 3D LUT.

Creative reasons for doing any of this are found in many video cameras, particularly those which allow the user to make changes to saturation and individual colours. Options called things like “matrix”, which were once upon a time implemented as analogue processing amplifiers, are now more likely to be done in a single step using digital signal processing based on a LUT. Outside the guts of a camera, it’s common to apply a LUT to monitoring on set to modify the output of a digital cinematography camera to approximate the intended final grade, while the recording of the scene is made without the LUT for later adjustment. Monitors from vendors like Cinetal may include the ability to load LUTs, perhaps from a flash card, while Blackmagic have their HDlink range, which can convert an SDI camera feed into a DVI signal suitable for an off-the-shelf monitor while applying a LUT (uploadable via USB) in the process. In these cases, the making of the LUT is a creative process, involving subjective viewing of the image by a colorist and adjustment of the values using a grading application.

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Phil Rhodes

Phil Rhodes is a Cinematographer, Technologist, Writer and above all Communicator. Never afraid to speak his mind, and always worth listening to, he's a frequent contributor to RedShark.

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