It's always entertaining to try to understand these things from first principles. Color and skin color in particular are very complex things to measure and model. It's not just a physical quantity, it's also a matter of human perception based on lighting and a host of factors.

It seems like some work was done to research existing approaches, but I don't see any reference to Pantone Skin Tones [0]. Without intending to, I've worked on 3 different projects which involved measuring skin tones for makeup applications. The Pantone range is not 100% perfect, but it's pretty well done and easy to understand. There are two axes: a light/dark one, and red-yellow one [1].

[0] https://www.pantone.com/na/en-us/skintone

[1] https://cosmeticsbusiness.com/pantone-skintone-guide-80945

There's also the Monk Skin Tone Scale by Dr. Monk and Google (available under a CC attribution 4.0 international license). It was developed for machine learning, so it's not exactly the same use case, but it provides 10 colors in a variety of formats, including hexadecimal. https://skintone.google/get-started

well its not a free as in beer color space

This is beautiful work! First, I thought this would involve using PCA to go from 3D to 2D, which would result in an easier selector to use, but at the expense of representing every person.

Then, I thought this would stop at using the U-space vectors (the ones that form the basis of the PCA image) and their corresponding ellipse to form our color space, but no, the function fitting is a very slick idea, even if it was executed by hand.

Lastly, I love the presentation of sampling from different r values. Whether you sample from a fixed r value or a range of r values, I bet this has great applications in game design or animation.

I still don't quite get the manual data labeling process at the beginning? It seems like it would encode some bias, but the consistency of the first point cloud and the results certainly speak for themselves.

If you look at my doctoral thesis, in fact skin colour can be parametrised using melanin and haemoglobin concentration; which it means it is indeed (roughly) two dimensional. This allows you to invert the colour of a pixel and identify someone’s chromophore concentration. This is based on previous work of others cited in the thesis.

https://discovery.ucl.ac.uk/id/eprint/10093345/

Oh goody, an expert in a question I've always wanted to ask.

Once upon a time, I encountered a sort of interactive museum book called "Earthsearch". One of its "exhibits" was a color wheel, made of two spinnable pieces of tinted acetate, that purported to be able to generate any skin tone. I suppose this is three degrees of freedom, since for any position of the two wheels you could access a range of colors around the edge - but the printed page underneath was simply a grayscale gradient, so one degree of freedom was simply luminance.

The book was released in 1994, and was very confident. It even went as far as to encourage the reader to see a doctor, should their skin tone not be achievable! What color model were they using?

That’s sounds very interesting. I’m not sure. In the thesis, I generated spectral curves and transformed them using the light and camera sensitivity curves. So, in theory it could be used in any color space.

But I can’t vouch for the accuracy of the book, but I suspect it was an approximation.

The haemoglobin and melanin are roughly in separate layers of the skin. There is an approximation known as Beer-Lambert that models the skin as two semi-transparent layers. This ignores subsurface scattering, but is roughly correct.

So, in theory two acetate sheets might replicate this. That’s my intuitive opinion. It has been many years since I did this work.

Oh man, that's a blast from the past, I think I had that book! Was that the one with an aluminum cover?

That's it! They (rather dubiously) claimed it came from a recycled Russian coke can, if I recall.

I've heard that the so-called "yellowish" skin color that non-tanned East Asians tend to have, is due to relative abundance of subcutaneous fat. If that's true, is that captured by the haemoglobin dimension or is it a yet another degree of freedom?

We used the approximation that the hypodermis was a perfectly white reflector; so the thickness wouldn’t matter. The collagen fibrills scatter light in such a way to create a yellower tint. So a person’s skin might appear this way in the absence of melanin and haemoglobin. I’m not an expert on population statistics, and so cannot give an opinion on specific skin types. Everything was based on ranges.

There is a separate effect of increased bilirubin content, that would appear if someone was ill.

This brings up an interesting topic of atypical compounds that can visibly affect skin coloring -- whether temporary or more long-lived (such as elevated methemoglobin levels in the Blue Fugates of Kentucky).

It's fascinating to learn that melanin and haemoglobin are the two primary factors (and cover the vast majority of cases), but I'm curious how many other illnesses / disorders can produce compounds with visible effects.

Thank you for your posts on this thread -- I appreciate you knowledge-sharing on this!

In theory, you can construct a skin model of any condition you want. It would be a case of knowing the concentration, scattering profiles and reflectance spectra of any chromophores. For the thesis, I wrote a Monte Carlo skin simulator using OpenCL, when it has just come out. You could plug anything in there.

Not an expert, but when I was learning 3d modeling, there was a good chapter on subsurface scattering which affect the human skin rendering. It’s what give it liveness instead of looking like a doll.

I didn't know chin/neck are slightly bluish, can we make it more explicit without side effects?

It is a classic artist starting off point to first drop a red/blue gradient as a base mixing layer, and add skin details/masks for each additional texture layer/feature.

The "The Ultimate Blender 3D Sculpting Course" covers several sculpting techniques, and while this course has a number of challenges it does cover several basics. Probably wise to wait for a sale event if interested.

https://www.udemy.com/course/blender-3d-sculpting-course/

Best of luck =3

> People only turn yellow if they have kidney or liver damage.

This is not true. Roughly 5% of people have Gilbert's Syndrome which is genetic, metabolic abnormality where the bilirubin from dead blood cells are conjugated slower than the baseline patient leading to yellowing of the skin. It is not due to liver damage or disease and is a benign condition. Not to mention there is carotenemia from eating too many carrots or squash -- again not kidney/liver damage related.

Thank you! I call out the manual labeling bias and also that a future work improvement would be to be scientific about that labeling process (multiple labelers, expert labelers), and then to feed that into the equation generation steps. Part of where the "good enough" naming comes from - I was hoping that using my art brain and keeping a bunch of reference images in mind (shout out to the Humanae project) would maybe offset the bias a bit. But 100% right on the bias being there.

> What colors are we? The short answer is maybe something like “brown” and the long answer is very, very long.

Pete Shirley (author of Ray Tracing In One Weekend) is also a part time color scientist, and he clued me into the fact that if you crank the saturation to 100% of pictures of people of any race, the skin color comes out orange. I think I heard that some face detectors may work on this principle, but I don’t remember where I heard that.

And brown is just orange... with context.

https://www.youtube.com/watch?v=wh4aWZRtTwU

and brown is just dark orange.

Yes, true! Though cranking saturation doesn’t brighten colors (much… at least it’s not supposed to), and maybe interestingly, light brown isn’t quite orange. There might some asymmetry in color names.

So it might not be that surprising that dark skin people turn orange when sat is cranked, but perhaps slightly surprising that light skin people do.

Anyway, I suspect it’s computationally easier to turn everyone orange than brown. Plus, given that brown skin is already a category, orange could be useful terminology/category.

And orange is just a yellow red.

I read the “limitations” section’s list of things that affect perceived skin tone and it didn’t mention lighting! This is pretty important as the available light determines the shades that can be perceived.

Love it!

I was looking at this a little while ago, and used some of The Pudding's data on makeup/foundation shades (https://pudding.cool/2018/06/makeup-shades/) and plotted it into the Oklab colorspace (https://observablehq.com/@55th/foundation-shades). The shades form themselves into that same crescent as seen in the article.

Woah I LOVE both of these! When I get a chance these are 100% getting linked from the writeup.

Nice. Most of these colors look right to me, but I'm also seeing green, blue and purple? If you're curating colors from the space manually then it's not a problem.

"Only realistic colors" would be a different problem. And I'm not sure if there's a way to solve that problem without excluding some real colors too.

(Maybe if the colour selector was a different shape? I don't know much about color spaces.)

Yes absolutely right on - I mention much deeper in the page that in a picker UI you can use a bigger range, but that in a generation context you might want to be a bit more selective. I go into it a bit more when I explain what R^2 is.

"not sure if there's a way to solve that problem without excluding some real colors too" - 100% right on that too. A big part of this work was to define a way to tweak/improve the equations in order to approach a solution to these issues - part of my hope in sharing is to get more eyes and brains thinking about this and proposing improvements!

I would have "show sphere" on by default--this kind of threw me at first, too.

This is a really nice project. Great work and a great write-up. Thanks for sharing.

> blue

You'd be surprised: https://en.wikipedia.org/wiki/Argyria

Reducing the R^2 value to ~1.5 excludes skin colors that are extremely blue, green and purple. That seems to be a happy medium between including real outliers and excluding colors like green.

If you drag the bottom slider all the way to the right you only have plausible skin tones. Drag it all the way to the right and you can match skin tones for orcs, Na'vi and djinn

Love it. I especially thought the introspective “aside” section on related resources was great, and I wish more people would show this kind of reflection.

Thanks! I hope to send lots of love those ways, they're basically all things that I found outside of this work that I liked enough to remember (I think Humanae was the one that I found while doing this).

I definitely don't speak for all non-white people when I say this, but personally I find people obsessing over it disturbing/offensive independent of whether their intentions are racist or anti-racist. I did find modeling/data-driven perspective cool though

I'm using an iPhone Safari plugin called Noir to force darkmode and your tool is making white the absence of color. In other words, I think white is just an alpha channel.

This is super cool! I have always loved projects that try to categorize color space. Usually my work has gone in the other direction - isGray, isCloseToWhite, isPastel - but it's the same insofar as it's defining shapes like your matplot visual.

Wish I had this on hand like this three or four years ago when I was working on VR avatar software - we had hard-coded skin tone options at the time - would have been a cool addition!

In sharing this around I've found more people than I thought who've done things to semantically chop up the color space - definitely a more interesting area than I first thought of when working on my narrow use case. Feels like it'd be fun to play with something that has a whole bunch of different regions of color space labeled - like "here's where IsPastel overlaps with IsGreen".

But absolutely the hope is that this is a tool that stays accessible to people for whatever they're building and everyone can slow down on reinventing the wheel here for every character creator/digital art application.

I really like the idea of treating a color space as a task-specific optimization rather than something universal. It makes me wonder if we're underusing domain-specific color spaces in procedural generation in general.

I think it’s worthwhile to note this is under a light with a color temperature similar to sunlight.

When you’re actually painting, skin can really be any color. Blue is a perfectly valid (and fairly common) skin color - just look at someone in the moonlight. It takes two to tango, and the other dancer is the light source.

Interesting though that the 1.5 setting (at least on my older iPhone) doesn’t really ever seem to cover the range of skin tones of indigenous Australians (“aboriginal” and Torres Strait Islanders), who apparently get their skin tones from a somewhat different expression of genes than other peoples with darker skin.

The 2.0 “outliers” cover those skin tones.

Masai and some Somali skin tones, similarly, look better represented by the outliers.

The full range of possible human skin tones is perhaps a bit of a challenge for technical colour representation, partly because diffuse reflectance seems to vary so much and that is hard to approximate on screen (e.g. as any website designer could attest, the difference between representing silver and grey is the use of highlights), though the representation of skin tones is a lot better than the early colour film era.

Yes, 2.0 is the value I tuned the equations around - but I think there's definitely room for improvement in terms of squashing the space along the fair/deep axis in order to get better results at lower R^2.

The issue I ran into was what happens when the space moves beyond RGB, but I think the fix is to just add clamps to the end R, G, B values.

Related, pencils for more accurately drawing skin tones: https://hautfarben.org/en/products/skin-tones-pencils

So glad to see Nyma getting recognition! I went to elementary and middle school with her, happy to see her succeeding.

Skin tones are particularly difficult to pin down, besides the pigmentation there are two effects at play: 1) sub surface scattering (light goes thru and bounces under the skin a bit) and 2) some infra-red emission (the responsible for making some people appear magenta on digital photography)

So even when you get the palette right, to paint it like a believable material in a convincing way is still super hard!

The U and V axes seem roughly correspond to the tint/temperature (green-magenta/blue-amber) axes that you can find in a white balance tool in photo editors and also as an in-camera setting.

Nb. I'd advise against calling the PCA'd space XYZ because that's a well known standard color space: https://en.wikipedia.org/wiki/CIE_1931_color_space#CIE_XYZ_c...

This part ( https://toneyalexander.github.io/inclusive-color-space/#pick... ) is especially cool.

Would love to see this 3-D picker ability in the many character designers out there, instead of the lame 8 swatches or whatever.

I think the ideal would be to use this picker to select a few tones, and then gradient map those across different parts of the body according to a grayscale map. Then you could fine-tune things like palm color, cheeks, facial tone, etc. individually.

I love this! Such a beautiful use of some real nerdy math to benefit humans in a way non-nerds would appreciate.

Nice. This is relatively old work (2020) but Pinterest had/has skin tone features, for example if you're searching for cosmetic products. Worth a check if anything might be useful for your work. https://medium.com/pinterest-engineering/powering-inclusive-...

Very neat project. Could I use this code to generate skin tones in my own project? Or more meant to just be educational/inspirational? I guess I'm wondering if you've considered adding a license.

Good catch, added MIT license - feel free to use it! On top of that, my understanding is that math/equations are not subject to copyright, so the core part of this work (the equations/ideas) I think legally has 0 limitations - even if there were a restrictive license. But IANAL so might have some misconceptions.

Procedural generators are often free if you look around for a bit.

https://kuhantilope.gumroad.com/l/ultimate-skin

https://extensions.blender.org/add-ons/mpfb/

https://github.com/alessandro-zomparelli/tissue

ymmv, as some paid plugins actually speed up the funky blender workflow (the auto-Bake plugin by the same author is important for procedural generated textures.)

https://sanctus.gumroad.com/l/SLibrary?layout=profile

Have a great day =3

For anyone confused, turn off your dark mode extension. :)

Oh wow, didn't even think of how that sort of thing might affect the page.

On the movie Dolemite by Eddy Murphy, they go over the fact that black people absorb light and white people reflect, and how that changes the motion picture shooting process totally. Also how one size does not fit all. Cool project! I am not much into the humanities part of the project. For me this is as natural as having a localization framework, just in this case for skin tones.

Setting R² to various large values like 50, 500 and 5000 then moving the slider around seems to give a decent and simple visual representation of what's happening.

Never seen anyone #E6FFF8 (bottom right corner).

An albino that consumed too much colloidal silver for years?

Me after I spend a day at the beach: #BA3A17

This is excellent work! You’ve got a 49 year old and a 10 year old equally into it. That’s impressive - nice work.

Not my domain though I followed the data science enough to nod as if I really followed it,

but this is just lovely and inspirational work. A unique marriage of hacking in the celebratory sense, thoughtfulness, and meaningful contribution. Outstanding.

Wow what a touching compliment, thank you so much!!

Skin is so interesting as a surface to think about how light passes through and bounces off of.

There is at least one article published by a technical artist at Pixar who describes modeling a skull within a human scan so that subsurface lighting is more accurately rendered.

I suspect you could combine this with dr_scully’s comment and GolDDranks’ to reach the nuance involved in long tail realism details.

There's a lot of really good research papers that touch on this area from what I saw - would definitely be cool to see if that extra nuance could be added without dramatically increasing the math complexity; in its current state, the code could be used in any programming language without a library.

Cool work!

If you're ##CCFFFF please go to the hospital

Under-represent relative to what, exactly?

Turn off your dark mode reader extension

I just think it's highly amusing (in the absurd sense) that you consider a visual artist to be a political ideologue simply for applying color theory to render realistic characters in their art.