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The concept of color spaces for human color perception is deeply and solidly anchored in the structure of the human visual system. Different kinds of color spa

The concept of color spaces for human color perception is deeply and solidly anchored in the structure of the human visual system. Different kinds of color spaces in different parts of that system. The additive "RGB" colorspace has to do with the fact that we have 3 different types of color cones in our eyes, which have different spectral sensitivities. Any color perception that we can register is a combination of these three signals. Hence a three-dimensional space. https://en.wikipedia.org/wiki/CIE_1931_color_space https://en.wikipedia.org/wiki/CIE_1931_color_space The RGB primaries that we choose for monitors do not match these color sensitivities, but also map to a space that is made to overlap with our perceptual space. > The color yellow has been an issue for many decades and diverse combinations of wavelengths evoke identical conscious yellow percepts. No. Metamerism is a general property of human color perception. For each color we can perceive there are a vast number of spectral distributions that can produce that particular color. https://en.wikipedia.org/wiki/Metamerism_(color) https://en.wikipedia.org/wiki/Metamerism_(color) One of the many reasons a purely physical approach is both too difficult and insufficient for color management. In addition to these physical color spaces, there are perceptual color spaces, my favorite one being CIE Lab, an opponent color space. With CIE Lab, the L component is overall luminosity, a is a red-green difference and b is blue-yellow difference. This corresponds to color processing that occurs after the initial signal is received and closely matches how humans actually subjectively perceive color, philosophical (im)ponderables aside. https://en.wikipedia.org/wiki/CIELAB_color_space https://en.wikipedia.org/wiki/CIELAB_color_space

canjobear 2y ago
> Any color perception that we can register is a combination of these three signals. Hence a three-dimensional space. Only approximately. In the linked article you can see that the color gamut is a horseshoe shape that cannot be fully spanned by a triangle. So there are color percepts such as responses to pure wavelengths that can’t be reproduced as combinations of real primaries. You could invent imaginary primary colors to make it work, hence the XYZ color space. In any case, the whole point of TFA is that a space is not an adequate characterization of color percepts, because there is no Riemannian geometry that matches the psychophysical data.
- mpweiher 2y ago
  >> Any color perception that we can register is a combination of these three signals. > Only approximately. No. That is precise, because three color sensors is all we (well most of us) have available[1]. >> Hence a three-dimensional space. > Only approximately. There was no claim of precision or precise matching for the space, only of dimensionality. The point was that it is a three dimensional space, and that's because there are three basic components that span it, the three color signals coming from the three types of cones. And once you have three base vectors, they will span a 3d space (unless co-planar). So quite the opposite of what the poster I was replying to claimed, which was that the idea of representing human color perception as a "space" is somehow "comical". > So there are color percepts such as responses to pure wavelengths that can’t be reproduced as combinations of real primaries. Sure, which is one of the reasons why the matching functions can have negative coefficients for some colors / colorspaces, something that is, er, more difficult with real RGB monitors. Or, as you also noted, you invent primaries that cover the space but go outside of reality. The spaces are mathematical constructs, after all. However they are not arbitrary mathematical constructs. The fact that they are 3d spaces is rooted heavily in reality. And mathematically they are actually all the same space (as distinct from specific regions in that space if you constrain the vector components, for example to range [0,1]). > In any case, the whole point of TFA is that a space is not an adequate characterization of color percepts, because there is no Riemannian geometry that matches the psychophysical data. Not really. The article is a bit fuzzy on some of the terminology, but if you read even the abstract closely, it is clear that they are talking about perceptually-uniform color spaces that make a claim about color differences in that space matching perceived color differences, our good old friend ΔE. https://en.wikipedia.org/wiki/Color_difference https://en.wikipedia.org/wiki/Color_difference I don't see the paper making such a claim for the more general concept of a color space that does not claim to be perceptually uniform. CIE XYZ for example, does not claim to be perceptually uniform. And isn't. And once again, it is well-known that "perceptually-uniform" color spaces are only approximate. Always have been. ΔE varies in different regions of the space. It also varies depending on the area of the color swatches[2]. It also varies depending on whether the color is a light source or reflected. (IIRC, CIE Luv performs better for light sources). That these distances don't add is an interesting additional wrinkle, but not surprising, given all the other oddities and also the fact that the color spaces were never intended for having the distances add. [1] Some people have reduced function in one or more of the cone types, they are color-blind, and some apparently have four types of cones. https://en.wikipedia.org/wiki/Tetrachromacy https://en.wikipedia.org/wiki/Tetrachromacy [2] The way those color spaces are tested is to give people swatches with colors that, I think, they are allowed to put next to each other. If they can tell they are different, the ΔE is greater than 1.
robwwilliams 2y ago
Interesting response and I appreciate the pragmatic utility. But reifying this space as “closer to the truth” misses the more important point that there is no one color space to rule them all. I have quantified variation in photoreceptor mosaics in humans, primates, cats, dogs, and mice. The variation in ratios of cone types is impressive. Variation of R:G ratios in health human retina is impressive (work by C Cursio). Yes, sure, humans still call green green despite the differences, but not so true of those with opsin gene variants. Here are three of my own studies: the first of which re-discovered that the nasal periphery of humans is tiled by large cones. https://pubmed.ncbi.nlm.nih.gov/1829378/ https://pubmed.ncbi.nlm.nih.gov/1829378/ https://link.springer.com/article/10.1007/s004290050319 https://link.springer.com/article/10.1007/s004290050319 https://onlinelibrary.wiley.com/doi/10.1002/cne.902970404 https://onlinelibrary.wiley.com/doi/10.1002/cne.902970404
- ForOldHack 2y ago
  8 billion Humams, a few are impaired, and many are blind, and a few, color-blind. There is an extrodinay variation. I personally cannot speak for indigenous tribes, but I do believe that the word 'green' is euro/western centric.
- mpweiher 2y ago
  Very cool that you did research on photoreceptors! I am not sure what you mean with "this space" which is supposed to be "closer to the truth". Can you elaborate? I am pretty sure I made no claims in that direction[1]. What I am saying is that modeling human color perception as a three-dimensional space is sensible, given the fact that we tend to have three color receptors with different spectral sensitivity. Similar physiological reasons apply for the CIE Lab space. Yes, it is a mathematical construct, but it is a reasonable mathematical construct. My color theory has been a while, but my understanding is that there is some pre-processing of the raw photoreceptor inputs that means the signals arriving in the brain are differential signals like the red-green difference (a) and the yellow-blue difference (b) of CIE Lab. And of course we get the luminance from the non-color receptors separately, though of course the signals are correlated. So how we interpret a specific (a) signal depends on what L is. So there are good pragmatic and scientific reasons for most of the proposed color spaces, but they are all compromises and driven by different pragmatic needs. They tend to be based on CIE XYZ, which defines a "standard observer". Is your objection this definition of a "standard observer"? To me, the very fact that such a "standard observer" is defined and called that makes it pretty clear that this "standard observer" is not the "universally true observer", but rather a useful fiction. And again, not an arbitrary fiction, there are reasons for choosing that particular one, but I don't see any claim to universal truth. I am still somewhat puzzled by your mention of metamerism as only applying to the color yellow. [1] I did single out CIE Lab, but clearly marked that as a purely personal preference.

canjobear 2y ago

> Any color perception that we can register is a combination of these three signals. Hence a three-dimensional space. Only approximately. In the linked article you can see that the color gamut is a horseshoe shape that cannot be fully spanned by a triangle. So there are color percepts such as responses to pure wavelengths that can’t be reproduced as combinations of real primaries. You could invent imaginary primary colors to make it work, hence the XYZ color space. In any case, the whole point of TFA is that a space is not an adequate characterization of color percepts, because there is no Riemannian geometry that matches the psychophysical data.

mpweiher 2y ago
>> Any color perception that we can register is a combination of these three signals. > Only approximately. No. That is precise, because three color sensors is all we (well most of us) have available[1]. >> Hence a three-dimensional space. > Only approximately. There was no claim of precision or precise matching for the space, only of dimensionality. The point was that it is a three dimensional space, and that's because there are three basic components that span it, the three color signals coming from the three types of cones. And once you have three base vectors, they will span a 3d space (unless co-planar). So quite the opposite of what the poster I was replying to claimed, which was that the idea of representing human color perception as a "space" is somehow "comical". > So there are color percepts such as responses to pure wavelengths that can’t be reproduced as combinations of real primaries. Sure, which is one of the reasons why the matching functions can have negative coefficients for some colors / colorspaces, something that is, er, more difficult with real RGB monitors. Or, as you also noted, you invent primaries that cover the space but go outside of reality. The spaces are mathematical constructs, after all. However they are not arbitrary mathematical constructs. The fact that they are 3d spaces is rooted heavily in reality. And mathematically they are actually all the same space (as distinct from specific regions in that space if you constrain the vector components, for example to range [0,1]). > In any case, the whole point of TFA is that a space is not an adequate characterization of color percepts, because there is no Riemannian geometry that matches the psychophysical data. Not really. The article is a bit fuzzy on some of the terminology, but if you read even the abstract closely, it is clear that they are talking about perceptually-uniform color spaces that make a claim about color differences in that space matching perceived color differences, our good old friend ΔE. https://en.wikipedia.org/wiki/Color_difference https://en.wikipedia.org/wiki/Color_difference I don't see the paper making such a claim for the more general concept of a color space that does not claim to be perceptually uniform. CIE XYZ for example, does not claim to be perceptually uniform. And isn't. And once again, it is well-known that "perceptually-uniform" color spaces are only approximate. Always have been. ΔE varies in different regions of the space. It also varies depending on the area of the color swatches[2]. It also varies depending on whether the color is a light source or reflected. (IIRC, CIE Luv performs better for light sources). That these distances don't add is an interesting additional wrinkle, but not surprising, given all the other oddities and also the fact that the color spaces were never intended for having the distances add. [1] Some people have reduced function in one or more of the cone types, they are color-blind, and some apparently have four types of cones. https://en.wikipedia.org/wiki/Tetrachromacy https://en.wikipedia.org/wiki/Tetrachromacy [2] The way those color spaces are tested is to give people swatches with colors that, I think, they are allowed to put next to each other. If they can tell they are different, the ΔE is greater than 1.

robwwilliams 2y ago

Interesting response and I appreciate the pragmatic utility. But reifying this space as “closer to the truth” misses the more important point that there is no one color space to rule them all. I have quantified variation in photoreceptor mosaics in humans, primates, cats, dogs, and mice. The variation in ratios of cone types is impressive. Variation of R:G ratios in health human retina is impressive (work by C Cursio). Yes, sure, humans still call green green despite the differences, but not so true of those with opsin gene variants. Here are three of my own studies: the first of which re-discovered that the nasal periphery of humans is tiled by large cones. https://pubmed.ncbi.nlm.nih.gov/1829378/ https://pubmed.ncbi.nlm.nih.gov/1829378/ https://link.springer.com/article/10.1007/s004290050319 https://link.springer.com/article/10.1007/s004290050319 https://onlinelibrary.wiley.com/doi/10.1002/cne.902970404 https://onlinelibrary.wiley.com/doi/10.1002/cne.902970404

ForOldHack 2y ago
8 billion Humams, a few are impaired, and many are blind, and a few, color-blind. There is an extrodinay variation. I personally cannot speak for indigenous tribes, but I do believe that the word 'green' is euro/western centric.
mpweiher 2y ago
Very cool that you did research on photoreceptors! I am not sure what you mean with "this space" which is supposed to be "closer to the truth". Can you elaborate? I am pretty sure I made no claims in that direction[1]. What I am saying is that modeling human color perception as a three-dimensional space is sensible, given the fact that we tend to have three color receptors with different spectral sensitivity. Similar physiological reasons apply for the CIE Lab space. Yes, it is a mathematical construct, but it is a reasonable mathematical construct. My color theory has been a while, but my understanding is that there is some pre-processing of the raw photoreceptor inputs that means the signals arriving in the brain are differential signals like the red-green difference (a) and the yellow-blue difference (b) of CIE Lab. And of course we get the luminance from the non-color receptors separately, though of course the signals are correlated. So how we interpret a specific (a) signal depends on what L is. So there are good pragmatic and scientific reasons for most of the proposed color spaces, but they are all compromises and driven by different pragmatic needs. They tend to be based on CIE XYZ, which defines a "standard observer". Is your objection this definition of a "standard observer"? To me, the very fact that such a "standard observer" is defined and called that makes it pretty clear that this "standard observer" is not the "universally true observer", but rather a useful fiction. And again, not an arbitrary fiction, there are reasons for choosing that particular one, but I don't see any claim to universal truth. I am still somewhat puzzled by your mention of metamerism as only applying to the color yellow. [1] I did single out CIE Lab, but clearly marked that as a purely personal preference.