Why does the chromaticity diagram look like that?

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.

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.

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...

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

> Does anyone know of a nice "pedagogical" color space? Where the space's highly-noticeable characteristics are actual features of human perception

When talking with students about color, I find the HSL space the easiest to employ. From a color maths point of view I have been told that it is very messy, which is one reason why Adobe stopped using it in version 3 of Photoshop. But from the perceptual point if view, it is the artists favorite.

Perhaps a better option is the Muncell color space. Muncell chopped up the entirety of the color domain into thousands of small chunks. The distance between each chunk was a single units of 'barely perceptual difference' which he established through meticulous user testing. Hence the green domian was much larger than the yellow. The story behind his development of this space makes for facinatting reading. He was an artist (a painter) yet his work paved the way for more modern spaces.

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.)

> When talking with students about color, I find the HSL space the easiest to employ. [...]

Nod. I was exploring how/whether an emphasis on spectra might be used to more successfully explain and teach color. Motivated by observations of wide-spread profound failure, like first-tier graduate students saying "the Sun doesn't have a color; it's rainbow color", and color instruction content, err, exercising diverse artistic license. So I'd hoped for Munsell-like web interactives, but with principled coupling to spectra. Hopefully as easily understood as HSL, if less convenient as a more-abstracted artist UI. I like implicit curriculum ("things noticed in passing provide insight and invite exploration"), and am leery of science education graphics' traditional "some aspects done with great care for correctness, mixed with others of utter bogosity, with students unable to tell which is which". And so wanted to avoid the high-profile "no, that too is merely a model artifact, not reality" of say CIE 1931 chromaticity diagrams ("such tiny sRGB coverage!", "so much green!", "a prism shaped solid!"). And also its frequently sloppy graphics (incorrect colors, misleading handling of out-of-gamut colors, misplaced white point not intersected by blackbody curve). I had hope for linear-hue absolute-physical-brightness Jzazbz, but meh[1]. Perception-optimized wide-gamut recent CAM16 seems unsurprisingly closest to this vision, but for some "hmm, is that bit real or model?", and "how does this behave with changes in absolute illumination?". A tweaked CAM16-UCS can end up a seemingly unobjectionable swelling blob. The scattered instances of intensive use of Munsell in art instruction suggested at least some hope of clarity and utility. Thanks for the thoughts.

[1] https://user-images.githubusercontent.com/181628/61452533-43... [png] from https://github.com/coloria-dev/coloria/issues/41