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I've been trying to build my own mental model that's easy to visualize how your eyes build color. Imagine 3 people (r,g,b) standing across from each other pla

I've been trying to build my own mental model that's easy to visualize how your eyes build color. Imagine 3 people (r,g,b) standing across from each other playing a 3 way tug of war with a bowling ball in the center. They each can feed on certain ranges of radiation to help them pull the ball towards them, color is determined by where the ball ends up. If red and green pull equally the ball moves sideways out of the center to land on yellow. Now take it one step further. The ball actually starts out in a deep pit, if everyones only pulling a little bit the ball lifts up but stays in the center, you have dark grey, if everyones pulling at their max, you get white. What seemed like a flat plane tug of war is actually a 3 dimensional cone. Now take it one step further. The retina is typically composed of 95% red and green cells of various mixtures, and only 5% blue. That means the blue person has a very short rope to work with compared to the others and is not able to move the bowling ball into as many unique positions as the others. If you are red green colorblind then you only have 2 people playing tug of war. Your color model is now 2 dimensional. If you have tetrachromacy you have another person playing and capable of placing the bowling ball into positions not possible with just 3 people. Additionally there is no such thing as pure green, because all radiation that green feeds on will also feed either red or blue as well. edit/ I forgot to add the rod cells, they are another "one step further". They are one extra person but they stand directly over the bowling ball, only capable of pulling upwards, but they are very weak and can't lift it very high.

anitil 9y ago
These sorts of mental models are pure, pure gold. I spent a lot of time looking at various descriptions of color spaces and while I could do all the calculations, I never had a clear understanding of _what_ I was doing and _why_ it worked. So in 4 paragraphs you've managed to leapfrog my understanding far more than weeks of formal training.
kmill 9y ago
> Additionally there is no such thing as pure green, because all radiation that green feeds on will also feed either red or blue as well. You can exploit cone fatigue to experience a greener green: make the red and blue people play tug of war by themselves with high intensity, then if you immediately call in green's fresh strength, red and blue will be easily overpowered. (Slight model inaccuracy: the cells themselves aren't the colors, they're just cells with low-, medium-, and high- frequency sensitivity. Interestingly, the color experience you get from activating just one type of cell is different from the one you get from monochromatic light at their respective spectral peaks. I find deep (long-wavelength) red, for example, to be rather different.)
pluma 9y ago
As a "colorblind" person, this is a bit frustrating so bear with me. Few people are literally "colorblind". You don't have one cone cell of each type, you have loads of them. Colorblind people generally have fewer of them (or some of them are shifted) but usually only for one type (usually red/green). The "red" and "green" cones overlap quite a lot[1] and they also overlap with the rods. This means that even if you lack some "green" cones you will still likely perceive "green" colors, except they will be hue-shifted into red and less saturated. Simulating this effect for people with normal vision usually results in "muddier" colors. Your analogy is pretty good otherwise (except as someone else pointed out, that's apparently not at all how the signals are processed) but this seems to be a widespread misconception about color "blindness", so I felt it's important to point that out. It's also worth mentioning that it's meaningless to say "there's no such thing as pure green". Physically isolating certain wavelengths produces a "pure green" light. What someone with normal color vision perceives when they see "pure green" light is the result of multiple cones being stimulated at the same time, sure, but there's no one-to-one mapping of cones and colors except for some very small bits at both ends of the spectrum, which is why the cones are generally referred to as L/M/S (for long/medium/short wavelengths) rather than R/G/B (for the colors). A tetrachromatic person (assuming the fourth cone type's peak sits somewhere between 420nm and 564nm, i.e. "blue" and "red") likely won't enable someone to perceive colors outside that range. In your example, the 3 people are forming a lopsided triangle. Adding a fourth person makes it a four-sided polygon but doesn't add another dimension. Tetrachromacy (depending on where the cones lie) would make it easier to distinguish certain shades though by making them more vibrant -- to them, normal vision would appear as "muddy" as my vision does to someone with normal vision. [1]: https://commons.wikimedia.org/wiki/File:Cone-response-en.svg https://commons.wikimedia.org/wiki/File:Cone-response-en.svg EDIT: Because some people find that interesting, here's a page with a bunch of photos that fairly accurately simulate my color vision (i.e. looking randomly at both sets of photos I wouldn't be able to tell you which one is the original): https://web.archive.org/web/20160308043950/http://critiquewall.com/2007/12/10/blindness https://web.archive.org/web/20160308043950/http://critiquewa... -- apparently brown skin looks green to me.
- thanatropism 9y ago
  > Physically isolating certain wavelengths produces a "pure green" light. What, can we get an wavelength-energy distribution that looks like a Dirac delta? There are discontinuities in physics but you kinda need to be careful to see if what you think is a discontinuity is one.
jacobolus 9y ago
There are multiple levels of processing here, so this pure trichromatic model turns out to be a somewhat misleading mental model to fix on. The signals from three types of cone cells are processed immediately within the eye into 3 different signals, a lightness response (L + M), a yellow-blue response (L + M – S), and a red-green response (L – M), before those combined signals go through further processing. LMS signals per se don't get sent anywhere further up the chain. If you are red-green colorblind that means you aren't getting much if any useful signal from the L – M response, either because your 'L' and 'M' cones are too close together to meaningfully distinguish, or because you are missing one of the two entirely. [Calling the LMS cone types Short = 'blue', Medium = 'green', and Long = 'red' creates a lot of confusion. Stick to long/medium/short labels. A statement like 'there is no such thing as pure green' is nonsensical. As Newton figured out, light rays themselves are not colored; color is a higher-level percept constructed in the human brain. Green is no more or less 'pure' than blue or orange or purple. For that matter calling the RGB primaries 'red', 'green', and 'blue' causes some confusion also, as these primaries are seen as an orangish red, a yellowish green, and a purplish blue, nowhere near the 'unique hues' red, green, or blue. E.g. the use of these names in named CSS colors, etc. is a horrible human interface.] If you are interested in the details, this is one of the better sources online http://handprint.com/HP/WCL/color2.html http://handprint.com/HP/WCL/color2.html [I would refer you to books but those are hard to accessibly hyperlink]. As for rods, their response is pretty much ignored during the day. They are much more sensitive, so start kicking in when the visual system adapts to very low levels of light where the cone cells aren't getting much useful signal. The article under discussion in this thread is full of technicalities, and in a brief skim I didn't notice any obvious errors, but it ignores several of the most important high-level features of human color perception – and gets a bit confused about the relationship between color reproduction via additive RGB displays, color measurement by instruments, and models of human vision – and therefore isn't the most useful resource for someone new to the subject, IMO.
- i2om3r 9y ago
  Which book(s) would you recommend?
- kurthr 9y ago
  Agreed, it's a lot of different sources of information. Color perception and reproduction are deep subjects and many of the results/assumptions are in turn subjective (e.g. dependent on the person). However, this is a lot better than most of the simple presentations I've seen on-line. One of the noticeable gaps in the article was a discussion of white-point or color temperature even though it starts to discuss illumination. It's critical to realize that our perception of color is influenced by the surroundings and relative to what we accept as white. That's part of why when you're out in the snow with rose glasses on things start to look normal, until you take them off... also why people argue about the blue-black or gold-white dress.

anitil 9y ago

These sorts of mental models are pure, pure gold. I spent a lot of time looking at various descriptions of color spaces and while I could do all the calculations, I never had a clear understanding of _what_ I was doing and _why_ it worked. So in 4 paragraphs you've managed to leapfrog my understanding far more than weeks of formal training.

kmill 9y ago

> Additionally there is no such thing as pure green, because all radiation that green feeds on will also feed either red or blue as well. You can exploit cone fatigue to experience a greener green: make the red and blue people play tug of war by themselves with high intensity, then if you immediately call in green's fresh strength, red and blue will be easily overpowered. (Slight model inaccuracy: the cells themselves aren't the colors, they're just cells with low-, medium-, and high- frequency sensitivity. Interestingly, the color experience you get from activating just one type of cell is different from the one you get from monochromatic light at their respective spectral peaks. I find deep (long-wavelength) red, for example, to be rather different.)

pluma 9y ago

As a "colorblind" person, this is a bit frustrating so bear with me. Few people are literally "colorblind". You don't have one cone cell of each type, you have loads of them. Colorblind people generally have fewer of them (or some of them are shifted) but usually only for one type (usually red/green). The "red" and "green" cones overlap quite a lot[1] and they also overlap with the rods. This means that even if you lack some "green" cones you will still likely perceive "green" colors, except they will be hue-shifted into red and less saturated. Simulating this effect for people with normal vision usually results in "muddier" colors. Your analogy is pretty good otherwise (except as someone else pointed out, that's apparently not at all how the signals are processed) but this seems to be a widespread misconception about color "blindness", so I felt it's important to point that out. It's also worth mentioning that it's meaningless to say "there's no such thing as pure green". Physically isolating certain wavelengths produces a "pure green" light. What someone with normal color vision perceives when they see "pure green" light is the result of multiple cones being stimulated at the same time, sure, but there's no one-to-one mapping of cones and colors except for some very small bits at both ends of the spectrum, which is why the cones are generally referred to as L/M/S (for long/medium/short wavelengths) rather than R/G/B (for the colors). A tetrachromatic person (assuming the fourth cone type's peak sits somewhere between 420nm and 564nm, i.e. "blue" and "red") likely won't enable someone to perceive colors outside that range. In your example, the 3 people are forming a lopsided triangle. Adding a fourth person makes it a four-sided polygon but doesn't add another dimension. Tetrachromacy (depending on where the cones lie) would make it easier to distinguish certain shades though by making them more vibrant -- to them, normal vision would appear as "muddy" as my vision does to someone with normal vision. [1]: https://commons.wikimedia.org/wiki/File:Cone-response-en.svg https://commons.wikimedia.org/wiki/File:Cone-response-en.svg EDIT: Because some people find that interesting, here's a page with a bunch of photos that fairly accurately simulate my color vision (i.e. looking randomly at both sets of photos I wouldn't be able to tell you which one is the original): https://web.archive.org/web/20160308043950/http://critiquewall.com/2007/12/10/blindness https://web.archive.org/web/20160308043950/http://critiquewa... -- apparently brown skin looks green to me.

thanatropism 9y ago
> Physically isolating certain wavelengths produces a "pure green" light. What, can we get an wavelength-energy distribution that looks like a Dirac delta? There are discontinuities in physics but you kinda need to be careful to see if what you think is a discontinuity is one.

jacobolus 9y ago

There are multiple levels of processing here, so this pure trichromatic model turns out to be a somewhat misleading mental model to fix on. The signals from three types of cone cells are processed immediately within the eye into 3 different signals, a lightness response (L + M), a yellow-blue response (L + M – S), and a red-green response (L – M), before those combined signals go through further processing. LMS signals per se don't get sent anywhere further up the chain. If you are red-green colorblind that means you aren't getting much if any useful signal from the L – M response, either because your 'L' and 'M' cones are too close together to meaningfully distinguish, or because you are missing one of the two entirely. [Calling the LMS cone types Short = 'blue', Medium = 'green', and Long = 'red' creates a lot of confusion. Stick to long/medium/short labels. A statement like 'there is no such thing as pure green' is nonsensical. As Newton figured out, light rays themselves are not colored; color is a higher-level percept constructed in the human brain. Green is no more or less 'pure' than blue or orange or purple. For that matter calling the RGB primaries 'red', 'green', and 'blue' causes some confusion also, as these primaries are seen as an orangish red, a yellowish green, and a purplish blue, nowhere near the 'unique hues' red, green, or blue. E.g. the use of these names in named CSS colors, etc. is a horrible human interface.] If you are interested in the details, this is one of the better sources online http://handprint.com/HP/WCL/color2.html http://handprint.com/HP/WCL/color2.html [I would refer you to books but those are hard to accessibly hyperlink]. As for rods, their response is pretty much ignored during the day. They are much more sensitive, so start kicking in when the visual system adapts to very low levels of light where the cone cells aren't getting much useful signal. The article under discussion in this thread is full of technicalities, and in a brief skim I didn't notice any obvious errors, but it ignores several of the most important high-level features of human color perception – and gets a bit confused about the relationship between color reproduction via additive RGB displays, color measurement by instruments, and models of human vision – and therefore isn't the most useful resource for someone new to the subject, IMO.

i2om3r 9y ago
Which book(s) would you recommend?
kurthr 9y ago
Agreed, it's a lot of different sources of information. Color perception and reproduction are deep subjects and many of the results/assumptions are in turn subjective (e.g. dependent on the person). However, this is a lot better than most of the simple presentations I've seen on-line. One of the noticeable gaps in the article was a discussion of white-point or color temperature even though it starts to discuss illumination. It's critical to realize that our perception of color is influenced by the surroundings and relative to what we accept as white. That's part of why when you're out in the snow with rose glasses on things start to look normal, until you take them off... also why people argue about the blue-black or gold-white dress.