​​ACES Output Transforms - Background Information

​​Introduction

​​Key pre-release versions

​​[Clicking each version title will take you directly to the version tag on Github.]

​​DC22

​​Overview
  • ​​Derived from film work by M. Uchida and Fuji team
  • ​​Very filmic based - tone scale was more contrasty, especially in the toe
  • ​​Dependent on lots of LUTs
  • ​​No closed-form inverse

​​v0.1 (also known as UT33)

​​Overview
  • ​​Ratio-restoring tone scale in ODT only
  • ​​Problems:
  • ​​lots of clipping steps which means:
  • ​​gamut coverage in OCES is poor relative to the OCES encoding primaries
  • ​​gamut coverage through the ODT is poor relative to the P3 encoding primaries
  • ​​overly complex
  • ​​non-invertible
​​
​​Rendering Steps
​​RRT
  1. ​​Tone scale: applied to ACES RGB independently
  1. ​​Clip negatives
  1. ​​3x3 adjustment matrix
  1. ​​Clip negatives
  1. ​​Per-hue contrast adjustments: several 11-point 1D LUTs with blending between 6 hue regions
  1. ​​Desaturation matrix: with equal 1/3 channel weighting, sat=0.86
  1. ​​Clip negatives
​​ODT
  1. ​​Ratio-restoring tone scale: applied to OCES RGB; norm = (R2 + G2 + B2) / (R + G + B)
  1. ​​3x3 matrix: OCES to XYZ
  1. ​​3x3 matrix: XYZ to display primaries
  1. ​​Clip 0-1
  1. ​​Inverse EOTF
​​

​​v0.1.1

​​Overview
​​Mostly the same as v0.1, but with a few changes in the ODT only. Specifically, the norm used in the ODT’s ratio-restoring tone scale was changed to use the max value.
​​In addition, an “inverse saturation” function was added in order to “reduce the rate of desaturation” (i.e. keep bright saturated lights more saturated).
​​
​​Rendering Steps
​​RRT - unchanged from v0.1
​​ODT
  1. ​​Ratio-restoring tone scale: applied to OCES RGB; norm = max(R,G,B)
  • ​​Introduced an inverse saturation function to “reduce the rate of desaturation”
  • ​​Desaturation of low-chroma highlights toward a neutral with the same norm
  1. ​​3x3 matrix: OCES to XYZ
  1. ​​3x3 matrix: XYZ to display primaries
  1. ​​Clip 0-1