LUTs - Look Up Tables

Moving images from one colour space or bit depth to another requires the use of a 'look up table', commonly referred to as a 'LUT', which accurately remaps the data. What a LUT allows within the Digital Intermediate is to display an image on a viewing device, such as a projector, without changing the original data or degrading the image in any shape or form.

Converting an image from one format to another will ultimately adjust it somehow, for example jpeg is an 8 bit linear image, where as Cineon is a 10 bit Log image. Converting a cineon frame to jpeg, reduces the bit depth therefore throwing valuable colour information away that cannot be regained. You can also add compression which further degrades the image, where as Cineon is uncompressed.

Of all the differences between how film reacts to light and how your monitor displays it, as I have mentioned on many occasions (far too many), film is logarithmic and a monitor (CRT or Projector) is linear. So what does that mean? On a monitor, there is a one-to-one correspondence between energy (think exposure) and brightness. Each time you increase the signal to the monitor by 1 volt, you get exactly the same incremental increase in brightness.

On film, however, the increase in brightness (emulsion density) is a result of the logarithm of the increase in exposure, in other words the correlation does not have a linear response.

The left graph is a representation of the linear display of your monitor. In the centre, the log sensitivity of film (exaggerated for clarity). To the right, the difference between the two. Note that on the Log graph between the inner black and white squares is a near linear section, a conversion technique that was an accepted standard for a number of years. This was to simply take the values between these two points and clip the data top and bottom to give an accepted (not necessarily acceptable) broadcast linear image from a logarithmic image

The point is we need to use and trust the LUT to accurately display a Logarithmic image correctly within the limited colour space and bit depth of the preferred choice of monitor or projector.

 

Illegal Colours

It is important to understand that due to the very nature of the way we can only capture and display a finite amount of colour, we are at a disadvantage in how to correctly display the full gamut represented by film. Obviously there is only a certain amount of colour we can visibly see as shown in the CIE chart on the left - the CIE colour coordinate system was developed in France in 1931, which displays all the wavelengths of light visible to humans.

Through these images we can ascertain that film captures a significant amount of the available colour space and, due to the limitations of phosphors in CRT technology, there is significant difference. I should point out these only are basic representations to illustrate the point, different film stocks and monitors will all have a different CIE graph to represent their individual characteristics.

The problem is that colours may be represented on film that need adjustment to look like they actually would on film. If we are not careful when we are correcting we can push the image and create images that are illegal or out of 'gamut' that simply will not look the same when printed back to film.

 

What is a LUT?

In the film industry, 3D LUTs (lookup tables) in essence are used to calculate preview colours for a monitor or digital projector of how an image will be reproduced on the final film print. A 3D LUT is defined as a 3D lattice where each axis is an entry point for one of the three colour components. Output values from the 3D LUT are calculated by interpolating between the 8 (or more) nearest points in the lattice.

Alternatively from the master timeline (2k/4K) a LUT, sometimes known as a cube - an HD or SD cube can be applied to the original data and output directly to tape, this adheres to the SMPTE standard for colour. This methodology stops the material having to undergo other correction/telecine grading sessions to transfer from film to tape, which can and has in the past, changed the look that was achieved throughout the DI process. This removes a large uncontrollable part of the older accepted workflow to achieve a complete workflow from film to cinema, to sell through (home DVD/Blu Ray). This also achieves a greater continuity within the film itself and deliverables needed.

Cubes may be of various sizes and bit depths. Often 17x17x17 cubes are used as 3D LUTs. The most common practice is to use RGB 10bit/component log images as the input to the 3D LUT. An interpolation engine is needed for calculating the output values from an input triple by looking for the eight nearest points defined within the 3D LUT lattice. Kodak, Arri and Filmlight all produce software or individual profiling for individual monitors or projectors.

 

Print Characterization

When you are grading a film in session, ideally you need to set your LUT/Cube to match the profile of the intended print stock. The high dynamic range and the high densities of print film can be measured precisely, producing a cube that can be precisely applied to a correctly calibrated monitor, LCD or projector. It is interesting to note that Filmlight and Arri do this as part of their service and create generic cubes for individual print stocks, whichever is your preference. Whereas Kodak only tend to work to their own stocks, which is something to bare in mind if you are working with Fuji.

 

Top 5 Reasons Not To Blindly Trust Your Monitor (even if it is calibrated ...)

  1. Film is pigment-based YCM, monitors are light-based RGB.
  2. Monitor is a linear color space, film is log.
  3. Phosphors have much less dynamic range than film.
  4. Displays 16 bit images as 8 bit.
  5. Shows significantly more shadow detail than the film will