After Rec 709, the next term that is used often in the world of film is the LUT table. Let those who have never used them be the first to use them… Exactly! LUTs are used by both beginner creators and large post-production studios. So what are they and why do they have a slightly bad reputation? More about this in this entry.
WHAT IS A LUT?
LUT, or Look-Up Table, is nothing more than a conversion table that changes the color values (RGB) in a given image. In other words, it corrects contrast, color, or both, allowing you to quickly make these changes during post-production. LUT files are most often in the .cube format, but there are exceptions, such as the .alf4c format for Arri Alexa 35 cameras or .vlt for Panasonic cameras. You can open them with a regular notepad and then it will turn out that the mentioned “table” looks something like this (of course, this is only a fragment):
0.041033 0.041033 0.041033
0.062173 0.038275 0.040764
0.081106 0.034035 0.040525
0.099236 0.027164 0.040322
0.116776 0.011288 0.040156
0.132918 0.000000 0.040014
In the table, we see rows of numbers that indicate the output value of the RGB channels, and each row indicates one sample. This particular appearance of this table results from several facts:
- This is a 3d LUT, which, unlike older 1d LUTs, allows to make changes to all RGB values at once, taking into account their interdependence.
- LUTs are based on the idea of sampling, which means that instead of storing output values for every possible input value, they take and modify samples at regular intervals, using a mathematical formula to estimate the values between them (interpolation). The input values are therefore “predefined” and only the output values are in the file.
- The number of input/output values depends on the file type; we distinguish 17x17x17, 33x33x33 and 65x65x65 LUTs, which means that on each axis there are 17, 33 or 65 regularly spaced samples, respectively.
MEANING OF GAMUT AND GAMMA
The idea of LUTs is quite simple, but in order for them to be well implemented, you need to be aware of a certain basic principle: the LUT is prepared only for a certain specific gamut and gamma of the input and output data. This information should be included in the title and/or header of the code itself, because it is crucial for the proper operation of the file! Here is an example: “# Creative transform designed for Davinci Wide Gamut Intermediate input and output.” Therefore, using this LUT, e.g. in the Rec 709 space, will give different or completely different effects from the intended ones, which is often the reason for misunderstandings and bad reputation.
TYPES OF LUTS
As previously mentioned, LUTs can be divided according to the complexity of the interaction (1d, 3d) or the density of the sample grid (x17, x33, x65), but now let’s look at their practical application. There are three types here:
- Technical LUTs: are used to convert between different gamuts / gammas, most often to normalize files in the so-called log to Rec 709. Such LUTs are often available free of charge from camera manufacturers (e.g. Sony). After using them, the image goes from faded and “flat” to much more contrast and saturated.
- Creative LUTs: this type is used to set a specific mood or visual style without changing the gamut/gamma. Creative LUTs can imitate the look of classic film stock, such as Kodak or Fuji, or give the image specific color tones that fit the aesthetics of a given project.
- Hybrid LUTs: As the name suggests, these are tables that combine features of both of the above.
A specific subtype of hybrid LUTs are the so-called Show LUTs. These are LUTs created by colorists specifically for cameras and used on film sets. Show LUTs help the director and cinematographer see a pre-colored image on the preview monitor that more closely matches the final effect. While they are mainly used to control appearance on set, they can also be a starting point for further post-production.
WHERE LUTS ARE ACTUALLY USED?
LUTs are used virtually everywhere we talk about digital image reproduction. Of course, we are talking about post-production programs, cameras and monitors. Monitors are probably an example of the rarest application, because only the highest-end ones are able to accept and use LUTs. They are professionally calibrated, e.g. to Rec 709 and gamma 2.4, and in combination with special devices or cards, such as Blackmagic Design Decklink, they create a reference set for accurate image work.
ADVANTAGES (AND DISADVANTAGES?) OF LUTS
The advantages of LUT tables certainly include:
- speed of work,
- repeatability of effects,
- ease of use.
And these three features are enough to make LUTs so extremely popular and widely used. Nevertheless, please remember that:
- in fact, they are not flexible at all, apart from the possibility of reducing their impact, nothing else can be corrected,
- they are designed for specific gamuts/gammas and if used contrary to their intended purpose, they may cause definitely undesirable effects,
- may be poorly designed or overly aggressive, especially when applied to 8-bit material, which may result in artifacts and/or banding.
SUMMARY
LUTs from reliable sources and used consciously are a powerful tool in the arsenal of a filmmaker, editor or colorist. They allow you to quickly and effectively adjust the color of your video, giving it a specific mood or restoring natural colors after recording in logarithmic color profiles. Such LUTs are not only used by professionals, but also created by them. On the other hand, we have countless sales offers of dubious origins of LUTs, which magically and with a few clicks are supposed to make the material almost straight from the camera become “cinematic”, which, combined with improper implementation in a given project, can result in truly tragic effects. LUTs are indispensable for use in both cameras and monitors. So what about post-production? Well, even the best LUTs, by their nature, have obvious limitations in matching almost any given image. For this reason, although in appropriate cases and with conscious use they will be a great solution, often it will be necessary to build an individual look based on tools that allow for any modification.
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