ACES Output Transforms VWG
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Meeting #120, September 27th, 1pm PT
Jeffrey D Mathias
- Kevin Wheatley: Pekka about his latest version using techniques from the gamut mapper in the chroma compression.
- Pekka Riikonen: The chroma compression wasn't exact in where it reaches to, and Alex suggested using the "reach mode". I've done that in the version called . The chroma compression uses a space I came up with, and I calculated the maximum M value at the display white J. M is normalized by this value multiplied by a gain value in the UI, currently set to 1. This means it won't compress or expand beyond that compression space. What I'm currently using is close to AP1. I was able to use a wider compression gamut than before, because it is now more accurate where it goes to. This gives us what Alex was looking for. It greatly improved the inverse compared to v44. It changes the rendering a little. Magenta is lighter. Saturation changes a little, which we can tweak later if we want. I think it looks good, even with the default gamut mapper, not the reach gamut mapper.
- Kevin Wheatley: So we can't combine this with the gamut mapper, because it uses different primaries. One concern I had what is the combined effect of two compressions could introduce kinks, perhaps outside that space.
- Pekka Riikonen: I've noted before that my compression curve does not have a continuous derivative. I haven't found one that is and also has a closed form inverse. But any kinks should be outside the gamut, at least for P3 and Rec.709.
- Kevin Wheatley: We need to check the effect of the composite compression around the hues. There could be weirdness from applying the same techniques with two unaligned gamuts.
- Pekka Riikonen: That could happen with the current one as well. If we used the same primaries for chroma compression and gamut compression the look would be different for each target.
- Kevin Wheatley: Have you looked at SDR / HDR matching?
- Pekka Riikonen: I have. With the old method the limit was scaled with peak luminance. Now it's constant. I need to find a different approach.
- Kevin Wheatley: Won't the use of max M for peak J mean you have different scaling for HDR? You compress less for HDR because you start bigger, so you perceive it as more colorful.
- Pekka Riikonen: The distance from max M to the locus increases with peak luminance. But it's a normalization factor, not a boundary. It's normalized to the cusp of the compression space. But it might work if we didn't do that.
- Kevin Wheatley: Improving the inverse makes it promising.
[Pekka showed the new version vs v44 with images]
- Pekka Riikonen: Unfortunately reds do desaturate faster with v45. The compression goes higher as J increases, so reds compress more. You can also see it with magenta with this flower.
- Kevin Wheatley: Does it affect the ability to reach the corners.
- Pekka Riikonen: It still has the issue with the red corner, but the others seem fine.
[Pekka showed a 3D plot with a small slice missing at the red cusp, and pointed to an odd distortion around magenta / blue / cyan]
- Kevin Wheatley: That may be what Jed is to.
- Pekka Riikonen: I think the red corner is reachable if we reduce compression. AP1 reach mode gets there, but everything else is pulled in.
- Kevin Wheatley: AP1 is a little arbitrary, so maybe there's a better option.
- Pekka Riikonen: Maybe the chroma compression space, which is a little bigger than AP1. For rendering AP0 actually looks quite good, but even more doesn't get reached. I think the chroma compression works well, but for gamut mapping, how far do we want to reach? It's still quite arbitrary.
- Christopher Jerome: What was your objective when adjusting the primaries?
- Pekka Riikonen: One thing was the inverse, and the other factor was that the space could be bigger because it is more accurate. Also there'e no eccentricity factor any more as it's no longer necessary for a good inverse.
- Christopher Jerome: Is there a place for the stock Hellwig or HK eccentricity?
- Pekka Riikonen: Luke said the way we use M it would make no difference. It has a small impact on the look of the rendering.
- Christopher Jerome: On the plots the inverse look very good.
- Pekka Riikonen: Anything that goes outside on the inverse is the gamut mapper. The chroma compression only affects the interior. The chroma compression works the same as before. I'm just changing the limit where it stops compressing.
- Kevin Wheatley: In the plot green and blue invert to clear points, but the green looks "chopped off".
- Pekka Riikonen: Yes, and I don't understand why it doesn't go to the corner of the compression space. The old one was similar, but did have a point.