Impossible color (opens in new tab)

Anecdotally, in my first year of university a tutor of mine did a demonstration on Fechner Colour. We walked into the lab and he had a spinning disc on a motor, that was apparently pink in colour. He asked the class (*first year students) what colours we thought each half of the disc were to create the pink shade. Most people guessed pink and white, while a couple of people had correct guesses..

When he stopped the motor, the disc spun down and it was yellow on one side, and blue on the other.

Seems like you're well positioned to add that to the article‽

My local science centre has had a Benham's top that you spin yourself in one of its permanent exhibits for probably over 15 years now and it's always really cool to see. It fascinates me that you can see yellow, purple, and green on a disk of only black and white.

Wow, I often as a child (still now) would see vivid purple and greens when I closed my eyes or in a dark room. I never thought about it but I wonder what the cause is.

The thing though (if I got the article right) is that these "reddish green" and "bluish yellow" colours are "virtual". It is almost like they're the result of a glitch in the brain. So I guess standard colour mixing rules don't apply.

Isn’t brown just a reddish green. Or a greenish red.

There are three kinds of cones in the eye, and each frequency of light is absorbed at a different rate by each of them. So when your eyes see light of a given frequency your brain gets this information as three numbers, which you can visualise as points in 3-dimensional space. One of the directions in this space just represents brightness, so we can throw it away if we're only thinking about colour.

So the different colours are all points in a 2-dimensional space. Each frequency corresponds to a given point in this space. If you plot all the frequencies from the spectrum they form a curve in this space, which is a semicircular shape. Red is at one end of the arc, and violet is at the other.

But the possible colour aren't just the ones on this arc. We can also see colours that correspond to mixtures of different frequencies. These fill in the inside of the semicircle, creating the full space of possible colours, which looks like this (https://pbbhandarkar.files.wordpress.com/2016/10/colorspace....). The spectral colours are on the arc around the outside, and all the colours on the interior can only be made by mixtures. The line joining red and violet is called the "line of purples". It joins violet back up to red to make a circle, but it consists only of mixtures rather than spectral colours. The spectral colours don't go around in a circle, just a semicircle. We need the line of purples to join it back up.

Red receptors are somewhat sensitive in violet part of spectrum too. In standardized response curves it is omitted (I am not sure why), more details: https://midimagic.sgc-hosting.com/huvision.htm

My completely made up intuitive understanding of it is that it's the same as musical pitches; there are many versions of each named pitch e.g A through G that are each an "octave" away from each other. The difference between "Middle C" and the one above it is that the frequency of the higher version is exactly double that of the lower. As a listener the two notes sound extremely similar, the higher one just feels more "energetic" or something. I imagine colors would be the same if our eyes could see more than one "octave" but we are only sensitive to approximately one "octave" of light, ~ 480Thz-750Thz.

Given that, violet appears to be between red and blue in the same way the pitch of G appears to be between the pitches F and A.

For anyone looking to save skimming through the page:

https://upload.wikimedia.org/wikipedia/commons/5/56/Chimeric...

Some interesting work around this research paper:

Zhang, Haimo, Xiang Cao, and Shengdong Zhao. "Beyond stereo: an exploration of unconventional binocular presentation for novel visual experience." Proceedings of the SIGCHI Conference on Human Factors in Computing Systems. ACM, 2012.

http://www.shengdongzhao.com/wp-content/uploads/2012/06/chi2...

My vision seems to oscillate between both eyes in the example picture shown (the yellow-blue one) - in the overlay, the colour changes from yellow to blue and back, with an overlay inbetween. If I concentrate I can see yellow over a blue background consistently though.

Does anyone have the same or a very different experience?

I imagine something similar would happen with your proposal, and possibly make people sick.

(Mixing vision between eyes is really interesting. My eyes are really different (one is good for close-up, one better for distance) so I have some experience with this.)

Traditional displays (such as what would be used in vr goggles) do not produce yellow light. You only get reddish-green with rgb colors

Artist Sterling Crispin has a VR painting program [1] that explores this allowing you to paint different colors to each eye simultaneously.

[1] http://www.cyberpaintapp.com/

Reminds me of Pratchett's Octarine. And seems reasonable to assume that impossible colours are only visible to wizards and cats.

You joke, but that’s a great example: brown is really dark orange; neon is fluorescent, and so brighter than your eye thinks should be possible for reflection, so sadly, no neon poo emoji.

So do it then.

No, it's a different bug in the human optical system.

Impossible colors are derived from the frequency-response curve of the three standard pigments in the retina. (There are people with 2 pigments and thus have a reduced color space, and people with 4 pigments who have an increased color space.) Since the pigments are not evenly distributed, the brain synthesizes some colors from incomplete information.

The dress problem stems from luminance-color correction in the brain itself. The eye has quite an amazing dynamic contrast ratio, but a much narrower static contrast ratio. If you have picked up on clues that the photograph was over-exposed in one way, your brain interprets that as a signal that the colors are washed out. If you have picked up on clues that the photograph is undersaturated, your brain says that the colors should be brighter than the pixels are. The photograph in question gave ambiguous clues.

https://en.wikipedia.org/wiki/The_dress

Nah. Not Han Solo coat on Hoth, either.