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Comment by mncharity | Hacker News Reader

> Figure 44: The Planckian locus visualized on a CIE 1931 chromaticity diagram

Visualized... using broken code. Note the Planck curve isn't going through white. Imagine your surprise at setting your monitor to a 6500K or 5000K white point, and finding it green or yellow.

> (source: Wikipedia)

Sigh. WP has Article and Talk, but neither has served as a "writer's notebook" for long-term memory. So when there's lots of brokenness out in the world, and there's been lots of broken color code over the years, WP has difficulty remembering to avoid it.

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It’s impossible to plot colors on a chromaticity diagram in a way that isn’t grossly misleading. This particular picture is one attempt at a compromise that will give readers the right conceptual impression, not “broken” accidentally.

The best compromise if you want color might instead be to only color a narrow strip around the triangle forming the gamut of your trichromatic additive display (or, say, sRGB), with intensity of the 2-primary mixture at each point adjusted so that lightness doesn’t vary too sharply.

A large portion of the horseshoe is outside your display’s gamut; common pictures either color the outside of the gamut gray, or try to clip out-of-gamut chromaticities to the nearest in-gamut chromaticity. The former is confusing for viewers who don’t already know what they are looking at; the latter gives a false impression. For the colors that are shown, one typical way to plot a 2-dimensional picture of the gamut is “top down” with the most intense available color for each chromaticity, but this introduces substantially misleading lightness artifacts based on the display not based on chromaticities per se.

Beyond that, the xy chromaticity diagram should not be used. Stick to the u'v' chromaticity diagram.

> It’s impossible to plot colors on a chromaticity diagram in a way that isn’t grossly misleading

Ok, but...

> This particular picture is one attempt at a compromise that will give readers the right conceptual impression, not “broken” accidentally.

No, the white point calculations used to make that wikipedia diagram are simply wrong. It's a recurring problem. Not a question of handling out-of-gamut colors, but of mislocating colors within it.

For instance, this fourmilab page[1] and associated specrend code, had incorrect white-point-related math for many years (the code was fixed, the page I don't recall), which IIRC, caused 'front page of the New York Times' "Universe is green"-level professional embarrassment.[2]

Here's a less bogus diagram.[3] Note the white point being close to the Planck curve, in contrast to the wikicommons diagram.

Some of the conceptual impressions seem quite unfortunate, reinforcing common misconceptions. That blackbodies (the Planck curve) can be colorful (other than red white and blue). The associated misconception that visible candle light is blackbody radiation. The extremely common astronomy education misconception that the Sun and many stars are yellow (for white points more perceptually relevant than blue Vega). A challenge in shifting science education towards more integrated and operational understanding, is so much content being variously incorrect, and thus not fitting together.

> Stick to the u'v' chromaticity diagram.

Yes, perceptually uniform color spaces are nice. Hmm, and that raises an interesting question: might incorrect diagrams be less common there? Being younger, and perhaps with open source libraries and eyeballs, replacing coding things yourself from a book, and simply copying diagrams stumbled on somewhere without checking them. A question for google image search...

[1] https://www.fourmilab.ch/documents/specrend/ [2] https://pages.jh.edu/news_info/news/home02/jan02/color.html https://www.nytimes.com/2002/01/11/us/scientists-paint-unive... https://pages.jh.edu/news_info/news/home02/mar02/color.html [3] https://tex.stackexchange.com/questions/422862/fill-area-of-...

The u'v' chromaticity diagram has been the recommended tool since 1976, 45 years ago, long before any diagram you will see on the web was created.

The “color of the universe” brouhaha is amusing, but trying to assign a color to the universe is somewhat nonsensical, and the result is not really meaningful to laypeople.

> assign a color to the universe is somewhat nonsensical

I don't understand that - could you elaborate? Thanks.

You are absolutely right, this diagram is misleading and I've been meaning to replace it with one of my own since forever but it has fallen deep at the bottom of long list of things to do (including many, many things I would like to describe in that document, like our approach to transmission/absorption/refraction, our post-processing pipeline, etc.).

Oh, no worries. Lists of doables are infinite, and life is very not. It's merely something I reflexively note out of long habit.

Fwiw, one remediation which appeals to me, when using flawed content, is adding a "bogus" tag. As in "Figure N Mumble (source WP) Somewhat flawed." Or sometimes "Bogus <attribute or issue>". So the reader maybe gets a heads-up that there's a known issue - a "first, do no harm" thing. Modulo esthetic constraints, and I've no idea if it actually helps. And it might be phrased more accessibly. I dont know of any associated education research.

Big picture, societal-level impacts of commonly flawed content seem unlikely to improve without being addressed systemically, and so don't seem a priority focus when pursuing local excellence. For example, students are told the Sun is yellow in Kindergarten, and repeatedly thereafter, with only a few later getting an "oops, nope, our bad" in astronomy grad school discussion of common misconceptions in astronomy education content... and careful avoidance of yellow Suns in say one weather app seems unlikely to move that needle much.

Thanks for your nifty work.

Isn’t it a consequence of a post rendering creative editing, e.g. blurring of the generated chromaticities colours, rather than the code itself being broken. No display can reproduce the entirety of the chromaticity diagram, thus there will always be some form of “cheating” required to try to picture it. I do agree with you that burring is not great though and does not really serve anything but aesthetics.

Gamut limitations mostly affect the periphery, rather than the center. Three arms of lightness converge on a white point, here near the bottom of the line marked 6000. Incorrectly far below the Planck curve, pulling bogus colors down over the curve. The curve should go from red to white to blue. Very not orange yellow green cyan. Figure 45 just below looks more plausible. Earlier versions of the wikicommons diagram[1] seem less blurred, making it easier to see its error.

For extra fun, this[2] illustrates both gamut position, and the recurring brokenness of a white point label indicating where white was expected, and the rendered white point being elsewhere.

[1] https://commons.wikimedia.org/wiki/File:PlanckianLocus.png [2] https://www.fourmilab.ch/documents/specrend/figures/ciegamut...

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