[css-color-4] Evaluate static gamut mapping algorithms for oklab/oklch spaces about csswg-drafts

[svgeesus]

Thanks for starting this.

• Projecting in Lab to the Rec2020 gamut boundary along constant L

Why Lab and not Oklab, given the hue curvature of Lab (especially in blues, but also in orange to a lesser extent).

I agree that mapping to a smoothed gamut boundary may well be better.

• Projecting in Rec2020 YUV along constant Y

That is video luma, right? Where Y is computed without bothering to undo gamma-encoding first?

[ccameron-chromium]

Thanks for starting this.

• Projecting in Lab to the Rec2020 gamut boundary along constant L

Why Lab and not Oklab, given the hue curvature of Lab (especially in blues, but also in orange to a lesser extent).

Sorry, that was a typo. I fixed up the original comment.

I agree that mapping to a smoothed gamut boundary may well be better.

• Projecting in Rec2020 YUV along constant Y

That is video luma, right? Where Y is computed without bothering to undo gamma-encoding first?

In the experiments that I did (which was not very exhaustive), I was doing this in linear space, but that was just because it was easier to plug into the pipeline at that stage.

Another option was to find the nearest point in the L2 sense in YUV space (either linear or gamma encoded). I haven't investigated that thoroughly.

[svgeesus]

Sorry, that was a typo. I fixed up the original comment.

Ah okay, makes sense.

I was doing this in linear space

That makes the calculated lightness accurate, so good. It doesn't make the UV plane perceptually uniform, of course.