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Fundamentals · 6 min read

How a LUT is built: what is inside a .cube file and what it cannot hold

A LUT feels like a filter, but it is closer to a phone book. For a list of input colours, it stores the output colour. That is all. Knowing this explains every strange thing a LUT has ever done to your footage.

What is in the file

Open a .cube file in a text editor. There is a short header, with a line such as LUT_3D_SIZE 33, and then rows of three numbers. Each row is one output colour: red, green, blue.

The size tells you how many steps there are on each axis. A 33-point LUT samples red, green and blue at 33 levels each: 33 × 33 × 33 = 35,937 colours. A 65-point LUT stores 274,625. Any colour that falls between those points is worked out from its nearest neighbours.

Input: an even grid of coloursOutput: where the LUT sends each one
Illustration of the idea, not a real product LUT. A 33-point LUT stores 35,937 of these points. Every colour in between is interpolated from its nearest neighbours.

From a grade to a .cube

  1. Pick the input. The grade is built on footage in one known profile, for example S-Log3 / S-Gamut3.Cine.
  2. Build the look in nodes, ending in Rec.709: the transform, the contrast curve, the colour response.
  3. Test it widely. Faces of different skin tones, daylight, tungsten, neon, underexposed and overexposed shots. A look that only works on the shot it was designed on is not a product.
  4. Export. Resolve sends that grid of colours through the node tree and writes down what comes out.
  5. Repeat per camera. The look stays the same and the input transform changes, giving one file per profile.

That last step is why CINELOCO LUT ships as five files: Sony S-Log3, Blackmagic Film Gen 5, Apple Log, Apple Log 2 and Rec.709.

What a LUT cannot hold

A LUT maps colour to colour. It has no idea where a pixel is in the frame or what is next to it. So these never survive the export:

  • Windows, masks and tracking
  • Glow, halation and blur
  • Film grain
  • Sharpening and noise reduction

If a product promises grain or halation "in the LUT", it is not in the LUT. Those live in a PowerGrade, where each one is a node. More on that in LUT vs PowerGrade.

Why a LUT breaks on badly exposed footage

The table has one answer per input colour. If your shot is two stops under, your skin tones arrive at the values where the table expected shadows, and it treats them as shadows. The LUT is not wrong. It is answering a different question. Fix exposure and white balance first, then apply it.

Strength

A LUT has no intensity control of its own. In Resolve, put it on its own node and lower Key Output Gain. In Premiere Pro, use the Intensity slider under Creative. In Final Cut Pro, use the Mix slider on the Custom LUT effect.

Common questions

Is a 65-point LUT better than a 33-point LUT?

It stores more points, which helps with very steep colour changes. For most looks a 33-point LUT is accurate and loads everywhere, including cameras and monitors.

Why does a LUT look different in two editors?

Usually because the input is different: one app is applying its own automatic conversion before the LUT, or the clip is being read at a different range. Turn off automatic colour management for that clip and apply the LUT to the untouched log image.

Can a LUT contain film grain?

No. A LUT only maps one colour to another. Grain, glow, halation and masks need a node tree.