Not All LUTs Are Created Equal

Today there are many stylized “look LUTs” available, designed to make your footage appear more cinematic. But what are these LUTs really doing under the hood?

Analysis

When analyzed against a stress-test image, many of these LUTs exhibit severe banding and color artifacts – negatively affecting the image quality.

The following image has a generic film display LUT applied. On the surface, everything may appear normal:

However, increasing the viewing gamma to 4.0 reveals some problematic areas hiding in the shadows:

In a different example of a LUT implementing a “film look”, isolating the individual red, green, and blue channels shows the unevenness in saturated areas:

While the LUT may appear to work on some shots, inevitably these kinds of LUTs will encounter an image that “breaks” them and sometimes no amount of creative grading can fix the problem. The solution lies in the overall LUT construction.

Quality LUTs

Some may be tempted to abandon LUTs altogether, however LUTs provide the ability to non-linearly adjust color. This is ideal for remapping out-of-gamut colors or when mimicking the characteristics of film.

The real issue comes down to quality, and as the title of this article states: not all LUTs are created equal.

A good LUT doesn’t introduce any artifacts of its own – it cleanly transforms the input color to the output color.

If the source footage contains banding, obviously the LUT can’t improve that (garbage in, garbage out). However, when applied to a high bit-depth image with the expected input parameters, a well-made LUT can work beautifully.

One example of a high-quality look-up table is our free REDcolor4 LUT, which was built to avoid the issues shown above:

When considering using a LUT for a project, it’s a good idea to thoroughly stress-test it beforehand, rather than find out later it’s causing unfixable artifacts.

Download

You can download the free LUT Stress Test Image here:
https://truecolor.us/downloads/lut-stress-test-image

To use the stress test image: first apply a LUT, then see how it behaves when increasing/decreasing the gamma (applied after the LUT). Also, try viewing the individual red, green, and blue color channels as well.