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What Is Inside A Cube File

Guide

What is inside a .cube file

A LUT sounds like a black box. It is a plain text file you can open in any editor: a few header lines, then a list of colours.

The header

This is the start of the 17-point file this site writes for Warm Portrait:

# Made with PictureFilters on 2026-10-24.
# Input: sRGB, display-referred, gamma-encoded. Output: the same.
# Colour only: grain, vignette, glow, halation and letterbox are not in this file.
# No photograph and no file name are stored in this file.
TITLE "Warm Portrait — PictureFilters"
LUT_3D_SIZE 17
DOMAIN_MIN 0.0 0.0 0.0
DOMAIN_MAX 1.0 1.0 1.0

Lines starting with # are comments. TITLE names the table. LUT_3D_SIZE says how many steps there are along each of red, green and blue — here 17, so 17 × 17 × 17 = 4,913 colours follow. DOMAIN_MIN and DOMAIN_MAX say what input range the steps cover; for an ordinary photo that is 0 to 1 on every channel.

The rows, red first

Then come the colours, one per line, three numbers each:

0.008800 0.008800 0.008800
0.074828 0.008660 0.007832
0.139477 0.008519 0.006896
0.204809 0.008373 0.005961
0.270884 0.008222 0.005026
…
0.978044 0.952390 0.901460

The first line is what pure black becomes; the last is what pure white becomes. In between, red changes fastest, then green, then blue — so line 2 is the first step of red with green and blue still at zero. Getting that order backwards is the classic way a home-made LUT swaps a picture’s red and blue. A colour that falls between grid points is found by blending the eight surrounding ones (trilinear interpolation), which is what the graphics chip does here when it applies any table.

The writer evaluates every grid point from the recipe itself at full precision and prints six decimals, so the file is the recipe, not a copy of an 8-bit texture. Choose 17, 33 or 65 points: 33 is the usual size, 65 the most faithful and about eight times larger.

What a .cube can and cannot carry

A table maps one colour to another, so it can carry everything a look and the colour controls do: strength, tone, colour, toning, colour filters, HSL and curves. It cannot carry anything that depends on where a pixel is or on its neighbours — grain, a vignette, glow, halation or letterbox bars — so those are left out of every .cube saved here, and the save button says so. Save one from the filter editor or any look on the library; try one from elsewhere on apply a LUT online.

  • The file assumes ordinary display-referred sRGB in and out. A LUT built for a camera's log footage expects a flat log picture and will look wrong on a phone photo.
  • Other applications may interpolate differently (tetrahedral rather than trilinear); differences are usually well under one level per channel at 33 points.

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