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Why does the chromaticity diagram look like that?

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276 points samwillis | 3 comments | 26 Jul 24 18:19 UTC | HN request time: 0s | source
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mncharity ◴[27 Jul 24 00:10 UTC] No.41083489[source]▶
Does anyone know of a nice "pedagogical" color space? That is, one optimized for teaching and learning, for correctness rather than for simple math? Where the space's highly-noticeable characteristics are actual features of human perception, rather than the usual mess of "nope, that too is a model artifact" (mostly from optimizing for computation). And full-gamut, well behaved out to spectral locus. And with at least somewhat linear hues and color combination. Sort of the Munsell niche, but full gamut, and this century.

I wasn't able to find anything even close, for a "maybe teach color better by emphasizing spectra?" side project, so I kludged. CAM16UCS as state-of-the-art for perceptual color, untwisted with Jzazbz for linear hues (it also sanity checked absolute luminosity), with a rather-unprincipled mashing down of CAM's IIUC-non-perceptual near-locus silly blue tail. Implemented as lookup tables. If there is any related work out there, I'd love to hear of it. Tnx.

replies(2): >>41083563 #>>41084960 #
suzumer ◴[27 Jul 24 00:23 UTC] No.41083563[source]▶
Cam16 (as opposed to cam16 ucs) is perception based. It calculates chroma, lightness, and hue, and is based on the munsell color system. Hellwig and Fairchild recently simplifed the model mathematically, improving it's chroma accuracy.( http://markfairchild.org/PDFs/PAP45.pdf) Another, simpler, model is CIELAB, which outputs paramters L, a, and b, where L is lightness, hypot(a,b) is chroma, and arctan2(b,a) is the hue.
replies(1): >>41084210 #
1. mncharity ◴[27 Jul 24 03:13 UTC] No.41084210[source]▶
Thanks! IIRC(fuzzily - it's been a while), I chose -UCS for a more euclidean color difference metric - I should review that. My even fuzzier recollection, is CIELAB's visible gamut shape is very artifacty[1], perhaps misleadingly representing the volume outside sRGB/P3 for instance.

The pedagogical objectives of playing well with full visible 3D gamut, and spectral locus, and of avoiding shape artifacts (concavities, excursions), are... non-traditional. Characteristics which could be happily traded away in traditional uses of color spaces, for characteristics like model math and simplicity which here have near-zero value (lookup tables satisficing). And were - most spaces have "oh my, that's a hard downselect" bizarre visual hulls, and topologies outside of P3 or even sRGB can get quite strange. Thus the need to untwist CAM16's curving hue lines - they're not bad within sRGB, but by the time they hit visible hull, yipes, I recall some as near parallel to hull.

Having a color space to play with as a realistic 3D whole, seems not the kind of thing we collectively incentivize. A lot of science education content difficulty seems like that.

[1] https://commons.wikimedia.org/wiki/File:Visible_gamut_within...

replies(1): >>41084346 #
2. suzumer ◴[27 Jul 24 03:57 UTC] No.41084346[source]▶
CAM16's hue lines are curved by design. Hue is not linear with regards to xy chromaticity, as evidenced by the Abney effect[1].

[1] https://en.wikipedia.org/wiki/Abney_effect

replies(1): >>41086098 #
3. mncharity ◴[27 Jul 24 12:19 UTC] No.41086098[source]▶
But maybe not this[1] non-linear? Fun if real. But perhaps fitting was done within a gamut folks care more about, and model math then induced artifacts at the margins of the full visible gamut? I'd really love to know if that blue tail represents real perception.

[1] https://www.researchgate.net/profile/Volodymyr-Pyliavskyi/pu... [png] from https://ojs.suitt.edu.ua/index.php/digitech/article/download... [PDF dl] (Curiously, bing image search has this figure, but google doesn't.)