Color: From Hex codes to Eyeballs

So in 4 paragraphs you've managed to leapfrog my understanding far more than weeks of formal training.

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.)

[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 [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.

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

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://critiquewa... -- apparently brown skin looks green to me.

I've been wanting to do this for a while but figured out that I lack the knowledge and time to do it properly

http://www.handprint.com/LS/CVS/color.html

Deuteranomaly causes difficulty distinguishing between shades where the precise amount of green is relevant (i.e., to a deuteranomalous person, pure yellow -- equal parts red and green -- would still look slightly reddish-orangish).

Protanomaly causes a similar effect with red, and also dims the dark/deep red part of the spectrum slightly.

If someone doesn't want to use the medical terms for them, better names are "green weakness" and "red weakness" since they don't carry the misleading "can't tell red and green apart" connotation.

A good parallel would be if you ever try to take a snapshot of a vivid sunset on your phone, then compare the screen image with the sunset behind you. The sunset's colors will be far more vivid, saturated, intense, than anything your phone can display. Those are some of the colors outside the RGB gamut.

Pigments could theoretically do a better job than pixels. But those would have to be some very good pigment$. I'm not sure if there is a printer that can produce this in practice. Your cheap inkjet printer definitely won't. The most expensive printers haven't been designed to cover the full human visual spectrum either, out of technical limitations, or cost limitations. And even when you have the pigments on print, the light $ource would have to contain the entire visible spectrum at high intensity (basically you'd need a very bright white light, as bright as the sun). And then the surface cannot have any glare. etc. So not very practical to achieve.

Yes, that's correct. For maximum brightness given a limited amount of energy, you want all the light to be emitted at a wavelength matching the maximum of the eye's luminosity function. Of course, if the light is very bright, it will saturate your green cones, at which point you would want to stop adding 550nm light and switch to a wavelength that matches one of your other cones.

What is suspicious about it? This is how all response functions work mathematically, both for humans and animals, as well as for mechanical sensors.

The response function is the measured ratio of the input value to the output response (for a given wavelength in this case, but it could be for any measurement at all.) Because it's a ratio, simulating the response is a multiplication, no other operation will be correct.

> I would expect a LF to be optimal when it matches the relative human sensitivity in shape

Arriving at that idea does make some intuitive sense, but it would not be optimal. For the best possible response, you'd put all your energy into the maximum of the response function and nowhere else.

Imagine you could bet on a coin toss where the coin is known to land on heads 2/3rds of the time and tails 1/3 of the time. You can guess the tosses in advance, and for every correct guess you win $1. Should you bet on heads 2/3rds of the time and tails 1/3rd of the time, because that matches the roll probability? Or should you bet on heads all the time? It's simple to show that always betting on heads will net more money, statistically speaking.