When all the DI (Digital Intermediate) stuff started about ten years ago, one of the most difficult things to achieve technically was to match the colour of the interactive display with the output of the lab. At that time, we started to hear about LUTs: Lookup Tables. Since then, LUTs have been implemented pretty much everywhere but they still remain a mystery to a lot of people.
As a digital cinema consultant I do receive quite often calls and mails of people asking me for LUTs for this camera, for this lab, for this monitor. But to answer correctly, it’s often a little more complicated than that.
What is a LUT?
A LUT is a table, so technically it’s just a bunch of numbers stacked into a text file or a spreadsheet.
Here is an example:
4095 0 4095
4095 0 4095
0 1535 0
0 1535 255
0 1535 511
0 1535 767
0 1535 1023
0 1535 1279
0 1535 1535
0 1535 1791
0 1535 2047
…
The concept is simple; to every value at the input, there’s a corresponding value at the output.
For example, input is:
R,G,B=232,121,211
the output is:
R,G,B=243,92,176
it gives me a different colour.
Lookup tables have been used in maths for a lot of things that don’t have anything to do with colours: anything that is too complex to calculate by hand (for a long time scientists used tables for logarithmic functions) or is just based on a physical phenomenon that can be measured but not modelized by a mathematical function; film colour transformation is one of those phenomenon.
insert image: calibration
2 approaches to DI
There are 2 main workflows for grading to film out:
The first one is to monitor the signal with a standard colourspace like Rec709 and have the lab to print it to film with the same colours.
Pros: easy to setup, you don’t have the responsibility of the output, it’s the lab to get it right.
Cons: some colours from Rec709 are not possible to reproduce on film. There are several ways to remap those impossible colours. So sometimes it’s just impossible to have
The second one is to have the output of the colour grading system emulating the output of the film. This is more complex.
insert image: calib
Pros: It’s possible to reach a match so good you can play the digital against the print together in butterfly screening (half of the screen is digital, other half is print). The best quality, used by the high-end labs and postproductions.
Cons: More complex to setup. You have the responsability of the colour consistency with the film out and a lot of parameters are not in your hands. Almost impossible to do black and white.
There is also a way to create “zebra” LUTs that tell you while grading in video mode which colours won’t be possible to reproduce on film.
Other LUTs
At the beginning of DI there were mostly 2 types of signal: video and film scan. Most video monitors were CRTs so it was quite easy to achieve Rec709 for HD colourspace. Today we need more LUTs for different reasons:
- Input LUT for digital cameras: to put them back in something your grading system can understand, like Cineon log or video signal.
- Onset/Offline LUTs: if the camera records a flat signal (like the Panavision Genesis’ Panalog) you want to monitor and do the offline editing with a LUT that brings it back to something nicer, or emulating a film output.
- LUTs for displays: today’s displays have a lot of different technologies and having them reach Rec709 is sometimes impossible without the use of LUTs. For example the JVC-DILA projector have a very wide gamut that can’t be limited using the projector’s controls.
3D LUTs?
We have a number of 3D things at the moment, but 3D LUTs are just one kind of arrangement for LUTs.
- A 1D LUT will apply the same correction to all channels. Technically it only affects the gamma.
- On 2D LUTs (which are technically 3x1D LUT) there is no interdependencies between RB channels, they are processed independently, which means you can’t really deal with forbidden or impossible colours.
- In a 3D LUT there is an output for every RGB triplet, which means you can remap impossible colours to whatever film does with it and truely emulate the film output
