Recap of discussion:
Note - there was a lot of all over the place discussion this meeting, if you want the real context, it would be a good one to listen to the recording.
- @LarsB - exposure invariance only possible without limiting input/output - we need to make make clear we are talking about scene-referred in the Progress Report so display limits are not relevant
- List things that are out of scope in Progress Report
- Is exposure invariance truly necessary? Lars points out luminance might change - but for scene-referred, luminance can be negative and therefore not useful.
- @joachim.zell brought up chromatic aberration - nice to have reference images here, but not really an issue for a gamut mapping algorithm
- How do we deal with way out of gamut red but in gamut green? This is actually the most common problem. @carolalynn : this is where the safety gamut comes into play
- How do we identify what is ‘right’ or ‘good’? Discussion around camera manufacturers color pipeline vs aces for visual reference
- Lars - working on a 2020 to r709 mapping, cyan is the hardest. What are the hardest colors in ACES to map? Group responded with blues for sure.
- @Troy_James_Sobotka proposed that a macbeth chart would be a good place to start for colors we care about? At least as a ground truth - macbeth colors should not ever be affected by our algorithm as a base level of acceptability. 190 or 24 chart?
- What do we do with values outside of AP0? Do we clamp? Treat them as valid?
- Lars referenced the Reinhart tonemapping equation. Applied to luminance only. Thomas and Troy noted that it causes hue distortion. https://www.desmos.com/calculator/udgcerxlsb
- JZ - do we need a golden eye group? Present mapped images and see what people think - maybe eventually, after our own group has done visual/measurable testing
- @Thomas_Mansencal raised a grading operator downstream of gamut mapping to ‘steer’ hues back on track. Matthias’ Nuke script has this option, but it’s global – introduces new artefacts when qualified. This is problematic for the zone of trust.
- We need to remember that there will normally be grading as well. So if our gamut mapper ‘heals’ the image so it is easier for the colorist to get to where they want, that’s a big improvement on the current situation.