From RGB to L*a*b* color space (2024)

Oklab is awesome and it’s such a great example of a person putting in time somewhere, where many just glanced over but still complained over the years. And it was so good that it was adopted everywhere.

This is great! The project that prompted me to submit TFA becomes ever so slightly better in OKLab (from early experiments using CSS color-mix) but I thought it would be more complicated than CIE Lab. Seems like it's not? Will try to implement it using OKLab instead!

The problem is that this will only work in one direction. You can calculate the stimulation of the photoreceptors for a certain spectrum, but not the other way around. For example the eye cannot distinguish between purple light consisting of one specific wavelength and purble light mixed by red and blue wavelengths, because both give the same stimulation of the receptors. So there is an infinite number of possible spectra for any given stimulation of the photoreceptors. All we can do is take the stimulation values (X, Y and Z) and convert from there to all kinds of color models and back.

Your approach would make a lot of sense for sensors that are full spectrum analyzers, but the eye isn't one.

Good comment. For people with physics and mathematics skills and intuition, learning or discussing about color or music theory can be a little bit frustrating experience. Ie a lot of it is about traditions and some famous people's ideas and less about how it actually works under the hood. I guess similar things exist in most fields when you dig a little bit deeper. "We always apply this fudge factor to get the correct results". Even something considered hard science.

Of course artists can be very effective with whatever the toolsets they have learned, they sort of can transcend all the obfuscations and actually can express themselves. It's a bit hard to change everything then as the "user's transformations" are then baked in.

It's also true that mathematical models are often simplifications and one has to consider the whole end to end pipeline, where a more "accurate" transformation can yield a worse end result if applied blindly. I'm always reminded of the anecdote of when in the past, at some tv channel, they switched to more accurate weather forecast models. But the audience got worse forecasts, since the resident meteorologist was used to certain errors the old model produced and could compensate those, while the new model had maybe less errors in total but they were different. Happens all the time and why some engineer thinking something is objectively better might not actually be better for the customer...

I think this is closer to that than you think. Lab has three intensity vectors. One for each of the photoreceptors in your eye. Anything more than Lab is unnecessarily conplex IMO.

Short answer

We do model it according to perception, just only when it is useful. Other colour models allow different outputs, printing with coloured ink, displaying on screen by mixing coloured lights, categorising colour, finding how bright it is etc. all need different representations in numbers.

> For example, the Physical reality is the different frequencies of light. The Biological reality is that different types of cells on our retina respond with differing intensity to each of those frequencies.

Because this isn't true. And there are multiple ways with completely different combinations of intesities of light at different frequencies to have a color that most people will see as the same color. This is because the color receptors of our eye overlap.

My understanding is that you're describing the CIE 1931 color space:

https://en.wikipedia.org/wiki/CIE_1931_color_space#Color_vis...

You'll see it is based on the physics and human biology, and is the basis for everything else.

The thing is, this color space isn't very useful for color calculations in the perceptual/subjective sense (e.g. if I just want to change one characteristic, like luminosity, without affecting the chromaticity), so we have transformations to more useful spaces like XYZ, Lab, etc.

There's also the fact human vision is a subjective/psychological phenomenon, so only frequency response is not enough: our vision can map different frequency responses to the same perceptual color (metamerism), our vision adapts color perception based on light source, etc.

Many of these, like RGB or YUV, are intermediate "simplifications", not what you're necessarily supposed to be working in to generate them. It's like a physics calculation where everyone just decides to randomly stop half way through and use that as a transport medium. RGB exists because that's how the display physically operates, it's how bright those literal subpixels should be. Thus the RGB intermediate transport. YUV (particularly 4:2:2 and 4:2:0) is then essentially a lossy-compression format that's very easy to compress/decompress, and that's why it exists.

If you're doing rendering work, like in a game, those do operate in a more physical domain. That's the so-called "Physically Based Rendering" or PBR that you might see if you hang out in game dev circles.

Most display technologies output RGB light, which means you’re already forced into an output methodology that’s relying on deception purpose-built for human biology and psychology.

Because of that, the layer of your model concerned with the physical reality of light doesn’t seem super useful for basic image input and output.

But of course this layer is incredibly useful for stuff like computer graphics, where it is indeed the case that physically based rendering is widely used for offline CGI rendering (and increasingly for realtime video game rendering).

To be more specific, RGB is a color model, and sRGB is one of the color spaces that uses this model, alongside device-specific color spaces.

> RGB on the other hand is device dependent

No, it isn't. RGB is just a color model. You cannot convert between a color model and a color space any more than you can covert between float and Euros.

In order to convert between RGB and L*a*b you need a color space. That could be an ICC profile, sure, but it could also just be any well defined color space like sRGB, Display P3, or bt2020

The colors are all roughly the same lightness? If so, for people with color deficiency, you want the lightnesses to vary because people that can't easily tell certain hues apart should still be able to tell different lightnesses apart (although, if you need a color for lots of different pie slices, the lightnesses you can pick will start getting too close together and you might have to resort to patterns).