By David Winter | | 7 minute read
Using the right LUTs can be a complex area of the editing workflow, especially for those without formal training. This guide will give you all the information you need to help create perfect, pro-level media content.
What is a LUT?
Most people understand that a LUT — which stands for Lookup Table — is a way of making a picture look different, mainly through having been encouraged to buy them on the promise that they'll make pictures look a specific way. They can sometimes even do that, but they have other uses, too.
At the most basic level, a lookup table is a list of every possible colour in a digital image, each associated with a new, modified output colour, so it can turn any input colour into any output colour. The output colours are selected so that the LUT has the overall effect of - for instance - making the whole image brighter, or darker, or bluer, or any other modification. Sometimes these effects can be extreme, turning red into green or blue into purple. It's more usual for them to be subtle, though, particularly when they are used to convert pictures from one technical standard to another or correct inaccuracies in a display.
Many modern workflows will use LUTs, although they often will be selected behind the scenes, as part of the setup in a piece of post-production software. Selecting a different standard on a monitor, such as Adobe RGB or sRGB, might also imply LUTs being used internally by the display itself. A monitor with calibration features might generate one internally, and a camera with different colour and brightness encoding options might be internally selecting different ones when users ask for different things.
The crucial understanding is that a LUT is akin to a filter on a camera's lens. It has no understanding of what the image is; it simply modifies colours. If, for instance, we colour-balance a camera to daylight and then add an orange filter to shoot under tungsten light, the image should look correct. If the camera was already balanced to shoot under tungsten light, though, and we added the orange filter, the image would look orangeish and incorrect. LUTs behave the same way, and where we have the option to manually select one we must understand what it's intended to do and how it's intended to be used.
What we've described here is a three-dimensional 3D LUT, which is theoretically capable of translating any colour into any other colour. 1D LUTs operate like the curves control in Photoshop and can't change any colour into any other colour, but can make colour balance, contrast and brightness controls.
You can learn more about both types further down the page.
LUT Applications
TECHNICAL
The most common application for LUTs is in the translation of one technical standard to another, such as when a camera crew shoots in a low-contrast log mode (see Brightness Encoding) but we want to monitor the resulting image on a conventional display.
Cameras might not use the same red, green and blue primary colours as the display (see Colour Encoding), and so we might also need to correct colour. This is done in almost every monitor and camera, often with a simple menu selection, though it's often possible to load custom LUTs into cameras, displays and workstations.
CALIBRATION
Displays may not be completely accurate, varying slightly in colour rendition and brightness depending on age as well as small variations in the manufacturing process. Calibration uses an optical device - a probe - to measure the light output of the monitor, creating a LUT designed to correct for any inaccuracy. Assuming the display was reasonably accurate to begin with, colour changes are often extremely subtle.
Some displays, particularly those intended for colour-critical work, can load calibration LUTs internally, or even generate their own when a calibration probe is plugged in. Naturally, it is only ever useful for that display and may need to be regularly updated as the display ages.
In theory, many displays can be calibrated so that any residual error is smaller than the human eye can see, at least within the absolute capability of the display. However, displays have limitations that can't be altered. Domestic televisions based on TFT-LCD technology may have a minimum black level that is visibly greyish in a darkened room, and a LUT can't cause the display to generate a black any darker than that.
Similarly, many domestic OLED displays become less able to display highly saturated colours at very high brightness levels, and again, that can't fully correct the problem.
CREATIVE
A creative LUT is intended to modify an image for artistic rather than technical reasons. Many are available for download, promising to simulate the look of almost any imaginable photographic process. Photochemical effects from the classic days of film are popular, with colour reversal, bleach bypass, or emulation of specific film stocks that may be based on scientific measurements of that stock's characteristics.
Software capable of colour grading is often capable of outputting a LUT representing the state of any grade created using it, so it's easy to generate creative LUTs then load them into cameras, displays, or other software. This is often done as part of the camera department's preparation for a film or TV production, and may be supplied as a rough guide to begin the grading process. This helps directors of photography who have not only a creative but also a political requirement to ensure everyone involved is familiar with the intended results. LUTs used on set are usually understood to be an approximation of the final result which may later change slightly in grading.
A creative LUT does not have any understanding of the images it's processing. One designed to work on material shot in a log mode (see Brightness Encoding) by a camera of one manufacturer is unlikely to work well on material shot in a different log mode on a different camera by a different manufacturer. Beyond simple technical mismatches, though, the look of any image is heavily dependent on lighting, production design and exposure choices, and it's a mistake to assume that LUTs can apply any look to any image. One designed for bright day exteriors, for instance, may not be usable on moody night interiors.
Productions interested in using this type of show LUT in on-set monitoring for a particular project should take care that it behaves as intended in a wide variety of circumstances, particularly if the LUT imposes a very noticeable change - an extreme look - on the image. To design a show LUT and use it for monitoring is to redesign the colour and brightness processing of the camera and that should not be done lightly. Prominent productions have created serious problems by using poorly-designed creative LUTs, and extensive testing is essential. Buying LUTs from a trusted supplier, which will hopefully have been extensively tested, can help here, although there will still be a need to make compatible lighting, production design and exposure choices.
LUT Technologies
1D LUTs
Often represented in a file ending .lut, a one-dimensional LUT is really three one-dimensional LUTs in a single file. Each LUT handles one RGB channel, so that any input red value can be associated with any output red value, and the same for green and blue. A 1D LUT approximates the capabilities of the curves filter in Photoshop. It can't be used to turn, say, red into green, but it can be used to normalise brightness and contrast and to adjust colour balance, and is much easier for devices and workstations to process.
3D LUTs
Because humans have vision based on three broad primary colours, colour in film and TV is represented in a three-dimensional space. In RGB imaging, that's often been represented as a cube, with one axis representing red, one green, and one blue. Often each of those values is represented with a 10-bit number, or a number between 0 and 1024, so that each edge of the cube is divided into 1024 sections. As a result of that 10-bit precision, the cube is capable of encoding over a billion colours.
Keeping a list of a billion input colours, each associated with a specific output colour, would represent an unworkable processing load. The file size itself would be over four gigabytes in size, a significant proportion of the memory available on even a high-end GPU. To avoid this problem, most 3D LUTs are stored sparsely - not every value is included, with the output values estimated based on the nearest input values. Often, the stored values represent a 33 entries on each side of the cube, for a total of 35,937 values, as opposed to the 1.07 billion that would be required for a full-resolution cube. That's enough precision for many applications, although LUTs with 17 or 65 entries on each side of the cube are also found, and more is better.
Low-resolution LUTs, or ones involving very bold, high contrast or high saturation changes to the input image, can reveal reduced resolution as a kind of banding, similar to quantisation noise (see Brightness Encoding). Sometimes this is exacerbated by LUTs based on an automatic assessment of images that lack sufficient colour range for proper analysis, or adding multiple ones together.
COLOUR TRANSFORMS
Not every modification to colour in modern film and TV work is based on a LUT. Sometimes, colour and brightness changes can be made mathematically, by running a calculation on the values of each pixel. This sort of mathematics may be called a colour transform, and generally has a lower processing load. Converting images between technical standards can sometimes be done as a mathematical transform, and may be implemented that way in cameras and displays.
Mathematical colour transforms are most often encountered by users (rather than camera designers) in the form of something like the American Society of Cinematographers' Color Decision List (often given in .cdl files), which defines a simple way to make relatively simple colour, brightness and saturation changes to images in a way that's widely understood. A CDL contains values for lift, gamma and gain of each RGB channel, roughly analogous to the three trackballs on some grading systems, and an overall saturation control.
A CDL can be accurately converted into a 3D LUT, but a 3D LUT can't (with trivial exceptions) be converted into a CDL. The 3D LUT can represent complex changes to hue and saturation - such as making all the greens bluer, or selective desaturation of highlights - which the CDL can't. Other mathematical colour transforms are more complex and can do more, depending on their purpose.
ON SET
A cautious cinematographer will have the digital imaging technician ensure that on-set monitors are calibrated. At the same time, a custom-designed show LUT, or one bought off the shelf, may be in use. Even if it isn't, though, it's unlikely that on-set monitors will properly display the low-contrast log output from the camera (see Brightness Encoding).
Even if we simply ask the camera to output what's often a conventional Rec. 709 image (see Brightness Encoding and Colour Encoding), we're still using a LUT, just one built into the camera.
IN THE EDIT
Sometimes, a digital imaging technician will make offline editing copies of the camera original media, probably using the show LUT, if any. If we're editing the camera original footage directly, we might use either the show or some other LUT to make the material viewable on the displays we have in the edit suite, which might just be a laptop display, designed to the sRGB standard (see Brightness Encoding and Colour Encoding).
We may need to choose a different LUT when exporting material for different viewing devices. Laptops, tablets and phones may use different colour and brightness standards to broadcast-oriented displays, requiring different LUTs to ensure the viewer's phone looks the same as the TV in the lounge.
IN GRADING
As in the edit, the process of grading is likely to involve several LUTs, to ensure that the timeline image, monitor image and rendered output are correct. Because post-production workflows are often very variable between different projects, giving specific guidance is difficult.
Particularly, though, modern workflows are increasingly likely to demand different deliverables, potentially targeting conventional TV broadcast, HDR broadcast, cinema, and online distribution, all of which might demand different monitoring - or at least different settings on the same monitor - and so different LUTs to suit.
Tutorial: How to make use of Lightworks' Quick LUTs
Our in-house editor shows how easy it is to control the look of your video using LUT conversion.
