Look-Up Table

Overview

A Look-Up Table (LUT) is a file that serves as a conversion table for color and tonal values. It maps each input RGB value of an image to a predefined output RGB value. This allows the look of a shot to be altered reproducibly and non-destructively, without the calculation having to be performed anew for each pixel – the result is simply "looked up" in the table.

In film and TV production, LUTs are typically divided into three functional areas:

• Technical/Display LUTs (Conversion LUTs): convert from one color space or transfer curve to another, for example, from camera log (e.g., ARRI LogC, Sony S-Log3) to Rec. 709.
• Calibration LUTs: correct deviations of individual monitors so that all displays on set and in post-production show images as identically and as close to the standard as possible.
• Creative LUTs (Look LUTs): apply a stylistic color grading and serve as a starting point or preview for the later grading process.

1D and 3D LUTs

LUTs are fundamentally distinguished into two types:

• 1D LUT: treats the red, green, and blue channels separately. It is suitable for adjustments to contrast, brightness, and transfer curve (gamma), but cannot independently alter hue and saturation.
• 3D LUT: maps the entire color space as a three-dimensional cube. Since the interaction between channels is captured here, hue and saturation can also be shifted precisely. Therefore, 3D LUTs are predominantly used for professional grading and on-set monitoring.

Format and Resolution

The most common exchange format is the file extension .cube – a plain text file containing the control points of the color cube. Accuracy depends on the number of control points per axis; intermediate values are interpolated. A resolution of 33 points per axis is common.

On-Set Usage

On location, LUTs are primarily used for monitoring. Since cameras usually record images in a flat log gradation, the raw signal appears desaturated and low in contrast. Via a viewing or monitoring LUT – often a log-to-Rec.709 conversion – the image is displayed in a meaningful representation on set monitors, while the actual recording remains unchanged in log format.

The DIT (Digital Imaging Technician) manages these LUTs, ensures consistent monitors through calibration LUTs, and can provide project-specific looks – often in conjunction with ASC-CDL values – as a preview of the intended grade. This way, directors, camera departments, and the rest of the crew see a reliable approximation of the final image result already on set.