Wow, the color difference stuff is fascinating. How on earth do they measure the space of perceptual difference?

This might be a good start: https://en.wikipedia.org/wiki/MacAdam_ellipse

Describes it exactly. Thank you!

Not sure, but maybe you could test by showing people two similar colors and asking them to decide if the are identical.

With enough samples (and enough individual participants) you establish how many "statistically-significant distinguishable differences" exist along a given path through the color-space.

Then combine those and get a sort of topographic grid...

Yeah, something like that was my best guess—either that or letting subjects adjust a second color until it matched the first. Turns out that's exactly what they did: "MacAdam set up an experiment in which a trained observer viewed two different colors, at a fixed luminance of about 48 cd/m2. One of the colors (the "test" color) was fixed, but the other was adjustable by the observer, and the observer was asked to adjust that color until it matched the test color." - from https://en.wikipedia.org/wiki/MacAdam_ellipse

That would actually be an interesting research topic because the results might be influenced by culture/language:

https://en.wikipedia.org/wiki/Blue%E2%80%93green_distinction...

It would be easier to measure the photoreceptor response of the eye and mathematically extrapolate.

Man I have used this thing so many times, thank you

In the first article you posted there is a section called

> HSV and HCL

Should't it be "HSL" instead of "HCL"?

Yes, he means HSL (or sometimes HLS), though the abbreviations used in the wild for these models are wildly inconsistent.

Here’s the first footnote in the Wikipedia article I wrote about HSL/HSV:

In Joblove and Greenberg’s (1978) paper first introducing HSL, they called HSL lightness "intensity", called HSL saturation "relative chroma", called HSV saturation "saturation" and called HSV value "value". They carefully and unambiguously described and compared three models: hue/chroma/intensity, hue/relative chroma/intensity, and hue/value/saturation. Unfortunately, later authors were less fastidious, and current usage of these terms is inconsistent and often misleading. http://dl.acm.org/citation.cfm?id=807362

[Worth noting: Joblove and Greenberg’s terms weren’t really consistent with standard color science definitions, but they were at least well defined at the top of their paper.]

EDIT: misunderstood, thought it was referring to original article not grandparent.

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No. HSV and HSL are two very similar physically-based color models that only loosely map to perceived brightness and perceived grayness: https://en.wikipedia.org/wiki/HSL_and_HSV

They are nice cylindrical shapes because they are straightforward transformations of the RGB cube-shaped color space.

By contrast, HCL is a biologically-based close approximation of actual perceived brightness and perceived grayness: https://en.wikipedia.org/wiki/Lab_color_space

It has a funky shape because the frequency responses of the color-sensitive pigments in the cones of our eyes are complicated: https://en.wikipedia.org/wiki/CIE_1931_color_space#Color_mat...

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Yes, but in davidjohnstone’s link which yoodenvranx is talking about, what that Wikipedia page calls HSL is being called HCL, whereas what you are calling HCL is being called Lch. (CIE LCh or CIELCH, optionally with a subscript 'ab' to distinguish it from the cylindrical version of CIELUV, would be a better, relatively unambiguous name. By convention, the C should be capitalized.)

Oh my god you're so right