ACES Gamut Compression User Guide
The purpose of this document is to elaborate on suggested user workflows for on set, dailies, visual effects, and finishing using the ACES 1.3 Reference Gamut Compression (RGC). We will not be going into detail on technical specification, for that please refer to the Technical Documentation:
Users of ACES are experiencing problems with clipping of colors and the resulting artifacts (loss of texture, intensification of color fringes). This clipping occurs at two stages in the pipeline.
- Conversion from camera raw RGB or from the manufacturer’s encoding space into ACES AP0
- Conversion from ACES AP0 into the working color space ACES AP1
These pixel values are particularly problematic when their negative components cause issues in compositing, and may also produce artifacts when viewed through an ACES Output Transform.
An LMT referred to as the was created as a temporary solution, but that affected all colors. A new solution was needed which preserved colors within a “zone of trust”, only altering the most saturated values.
Various options were investigated, and the working group finally settled on a simple RGB ratio based algorithm which compresses values based on their distance from an achromatic axis. This makes no attempt to ascertain the “correct” value for a pixel, since the nature of the problem means that these pixels may have no correct colour. Nor is it overly concerned with the aesthetic appearance of the result. It simply “heals” the problem pixels, to produce new RGB values which are less problematic when used in subsequent compositing or grading operations. Aesthetic modifications are left for the user to apply later.
(placeholder UI image)
what the group did and generally how it works
examples of it working
what it is not (color correction)
As can be seen in the example below, artifacts such as the magenta solarization seen on the nose of the Okja toy are greatly reduced by application of the algorithm.
As visualized in the flowchart above, a production should plan to utilize the gamut compression as a part of the viewing pipeline in every area of production - from on set all the way to finishing.
It is important to note that in all of these steps, the Reference Gamut Compression should be applied FIRST after the IDT, before all other work, grades, or output viewing transforms.
- On Set
- Live Grading: if your production is utilizing an on set grading software, such as Pomfort Livegrade, use it to apply the Reference Gamut Compression. This will create a 3D LUT for you, which will be passed to your LUT box for viewing on a monitor.
- In-Camera: the production can create a 3D LUT of the appropriate size (normally 33x33x33 max) with the Reference Gamut Compression added to the viewing pipeline to load into the camera.
- Use a dailies generation software, such as Colorfront or Resolve, to import the original camera footage, and apply the Reference Gamut Compression as a part of your viewing pipeline for export to desired media.
- Use media supplied from dailies, and back from VFX, to verify media as work progresses.
- General Workflow (flowchart)
- On Set to Finishing
- Apply everywhere, except VFX pulls (but don’t invert on the way back from VFX)
- first thing after the IDT
Reference Gamut Compression Parameters