
How do our colour-blind cones achieve colour vision? - Audiophilip
http://theneurosphere.com/2015/12/07/why-are-all-the-colours-we-experience-composed-of-three-primaries/
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logicrook
I'm always surprised to see barely high-school level science articles get on
the front page.

An alternative title to this article could be "how spaceless numbers achieve
defining space"? Like you just have a basis, and this defines space. Mind
blown. Really, how do these magnets work?

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pluteoid
It's not even an article I'd want to present to a high school class. An RGB
pixel on your monitor is not emitting red, green and blue monochromatic
(single wavelength) light. Same goes for the light reflected from a CYM
pigment. Metamerism is at play, just like in the "real world". The author
confuses the need to choose three primaries with a requirement those primaries
be monochromatic. And since when have painters stuck to three pigments?

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neurosphere
Painters don't stick to just three pigments, but all the pigments they use CAN
be created by mixing three primaries in different proportions. I'm quite sure
that's the whole basis for Thomas Young's colour mixing experiments performed
in the 1800s, finding that every colour we see can be made with mixtures of
three lights. Any more than that is redundant. Similarly, while an RGB pixel
on your monitor may not emit all single-wavelength light, it doesn't negate
the validity of the statement that three single wavelengths can be used to
recreate all colours visible to the human eye. I'm quite sure that some RGB
pixels are indeed single wavelength, though I would love to hear from those of
you who know better about this because I don't know for sure...

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logicrook
>Painters don't stick to just three pigments, but all the pigments they use
CAN be created by mixing three primaries in different proportions.

No. Paint is not light, and has physical properties that constrain what you
can do with them. Some colors just don't mix well together, so you'll find
recommendation on what exact type of pigment work best for specific mixes.

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neurosphere
Yeah, that's true, paint is subtractive, as opposed to light. But isn't it
still the case that all pigments can be achieved by mixing three primaries?
Whereas in light mixing the primaries are considered to be red, green, blue,
in pigment mixing they are considered to be cyan, magenta, yellow. Obviously
please correct me if I'm wrong (no sarcasm implied), links would be useful.

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rspeer
No. No matter whether you're doing additive or subtractive color, there will
be perceivable colors that you can't reach just by mixing a finite number of
primaries. In fact, CMYK printing has a rather small gamut, with more colors
that are clearly unreachable than RGB has.

Perceivable colors are a three-dimensional space, and colors you can make out
of three primaries are a three-dimensional space, but they're different
shapes. Mixing primary colors is a way of interpolating between them linearly,
and the space of perceivable colors doesn't have convenient linear edges.

Here's an example. Your printer has cyan, magenta, and yellow ink (and also
black for convenience). So it can print any color, right? It should be able to
print bright lime green, something that looks like RGB #00ff00, right?

No, in fact, you can't print that color without "spot color" ink. When you mix
cyan and yellow to make green, it will necessarily get darker. If you use
enough cyan and yellow to get full-saturation green, it'll be too dark. If you
use less ink, it won't be saturated enough.

Lime green is outside of the CMYK gamut. Similarly, the deep cyan you'd get by
printing with lots of cyan ink is outside of the RGB gamut.

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KingMob
As a former neuroscientist who studied perceptual awareness, there's lots of
little things wrong with this article.

1) The RGB palette used in monitors most certainly does not span the whole
gamut of human vision. Not even the wide-gamut monitors used professionally
can do that yet.

2) The diagram showing each cone type's peak spectral sensitivity is a bit
misleading, too. The M- and L-cones are colored red and green, but if you
notice carefully, their peaks lie in the green/yellow area, so characterizing
them as red/green is misleading. And "each of our cones seeing a primary
colour (blue, green, or red)" is flat-out wrong. Every cone fires in
proportion to its overlap. The author clarifies this later, but it's best to
tell people the caveats up front. Continually referring to "primary" colors is
adding to the confusion here.

3) WhIle they did cite an article pointing out that ratio-computing cells
exist in the retina, it does NOT follow that "it is the main source of
information that the brain has to help it identify the wavelengths of light".
The paper itself only talks about spectral separation in retinal cells pre-
brain. I don't know where the author got the idea that ratios are the main
source of info sent to the brain. Amplitude information is sent, as well.

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neurosphere
I'm the author of the article, and I'm quite happy to see a response from a
neuroscientist who studied this topic in the past. I wanted to ask you about
point 3, because obviously I could have made a mistake. From what I understand
(and what I have studied) retinal ganglion cells computing the S vs M and S vs
M+L dimensions are the critical ones underlying coding of colour information
along the blue-yellow and red-green axis. The additive axis, featuring
amplitude information, from what I understand primarily concerns luminosity
rather than colour discrimination. Without any of the ratio information,
colour discrimination would not exist. Am I wrong? Would genuinely love to
hear from you on this.

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tantalor
> A printer generates all the colours we see just by blending three coloured
> inks in varying quantities

I thought printers typically use CMYK (4 inks) or CMYKOG (6 inks).

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foolrush
Correct. Or more.

OP is incorrect; no tristimulus based (RGB or CMY) combination can reproduce
the spectral locus without resorting to imaginary colour primaries. Imaginary
primaries cannot be reproduced in reality, and as such, “all the colours we
see” is “all the colours we may see from the printer.”

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chatwinra
I enjoyed reading this article and found it interesting, even if I was re-
learning some of the stuff mentioned.

If the errors pointed out by above posters are valid, they should really go
over to the OP's blog and post them so she can correct the post.

