
The Case of the Missing Magenta - captn3m0
http://nowiknow.com/the-case-of-the-missing-magenta/
======
jeroen94704
A slight elaboration about the way your eye works: The cones are not on-off
affairs, and they are not limited to specific wavelengths. Each type of cone
has a response curve, and these curves overlap (see e.g. the Wikipedia article
about Cone cells:
[http://en.wikipedia.org/wiki/Cone_cell](http://en.wikipedia.org/wiki/Cone_cell)
). So different colors are not really "made up" by filling in the gaps between
the wavelengths. Each color we see corresponds to a specific cone response
pattern. So for example, 500 nm would lead to responses of (roughly) 0.3, 0.4,
0.1.

This means the explanation here is slightly misleading, as it compares yellow
(which is a color that does exist as a single wavelength) with magenta, which
is by definition a mix of at least two wavelengths.

You could say that magenta is a compound color, the interpretation by our
brain of the simultaneous stimulation by at least two wavelengths (i.e. two or
more cone response patterns). This means, of course, that magenta is far from
the only color for which this is true. Any combination of at least two
wavelengths is a compound color, and the interpretation of each of these
combinations will not be found on the visible spectrum.

So, interesting article, but a bit oversimplified, in my view.

~~~
swayvil
I make a study of funny colors. For example you can get weird _vibrations_ via
certain patterns of strobed color combinations and frequencies.

This is an interesting study of such (warning! strobes!) :

[https://www.youtube.com/watch?v=dL0lNGXoP8E](https://www.youtube.com/watch?v=dL0lNGXoP8E)

~~~
jeroen94704
Wow, anyone who can watch that without becoming epileptic is immune :).

Interesting effects though.

------
fela
Another interesting color fact: many birds have four independent color
channels instead of our three [1]. Which means they can see a whole additional
dimension of colors we cannot even imagine. I like to think how it would be to
wake up one day and suddenly being able to see all those additional colors...

[1]
[http://en.wikipedia.org/wiki/Tetrachromacy](http://en.wikipedia.org/wiki/Tetrachromacy)

~~~
alphaBetaGamma
I never understood why we have three trichromatic vision and not tetrachomatic
vision.

We have rods and three types of cones, which look like four channels to me. In
very low light the cones don't fire and we see in black and white. In
reasonably bright settings I can imagine that the rods saturate and we only
have three useful channels. But there must be a level of light where both
cones and rods are within their dynamic range. What am I missing?

~~~
lnanek2
The wavelength curve the rods are sensitive to spikes in the middle of all the
wavelengths curves for all the cones. So I think the mental sensation of gray
is pretty accurate, actually, since it is close to being just a rough
measurement of all the others.

~~~
alphaBetaGamma
You are saying that the rods' spectral curve response is collinear with the
cones. That would do the trick, but I'm a bit surprised as I don't see any
reason for evolution to push you in that direction.

Do you have a reference for this? Quick googling seems to show that the rods'
response curve is not, it seems to be a most sensitive at a wavelengths
between the peak of the blue and green cones.

~~~
icegreentea
I'm not sure if this will satisfy you, but you can give this a shot:

[http://en.wikipedia.org/wiki/Mesopic_vision](http://en.wikipedia.org/wiki/Mesopic_vision)

[http://en.wikipedia.org/wiki/Purkinje_effect](http://en.wikipedia.org/wiki/Purkinje_effect)

------
Xylakant
Now I Know is probably my most favorite daily read. It's a tiny chunk of
seemingly useless knowledge, but it always makes me aware of how much is out
there that I don't know.

~~~
DanLivesHere
Hi, I'm Dan! I write Now I Know... thanks! :)

For everyone else, please consider subscribing at
[http://nowiknow.com](http://nowiknow.com)

~~~
rogerbinns
Is there any particular reason you don't have an RSS feed? That is the only
way I will subscribe.

------
thaumasiotes
> All the colors the monitors show as are actually just a mix of red, blue,
> and green light. If you could magnify your screen, you’d see something like
> this, and you’ll notice that there are only three colors there.

The easiest way to do this is to get some water on the screen. Red, green, and
blue pixels will be apparent under the water droplet.

------
TruerMadness
Violet is in the rainbow. Like magenta, it is a combination of red and blue
(#8F00FF). This is because there's a second hump in the frequency sensitivity
of the red receptor. See:
[http://en.wikipedia.org/wiki/CIE_1931_color_space#Color_matc...](http://en.wikipedia.org/wiki/CIE_1931_color_space#Color_matching_functions)

So there is a "between red and blue" in the spectrum. Just no place where
there's more red than blue.

~~~
acqq
It's not in the rainbow as in the diagram you link to ("CIE 1931 color space
chromaticity diagram"). The diagram has the

    
    
             R
    
         G       B
    

"in 2 dimensions" and the values between and the rainbow has it as

    
    
        R     G      B
    

"in one dimension" and the values between.

Edit: under "it" I wrote about magenta, as the main article writes too. The
"drawings" are about the appearance of magenta. I see you talk about violet.
Yes it is an interesting observation about how we see the colors.

~~~
TruerMadness
What happens if your eye sees pure 400nm light? There is some stimulation of
the "red" receptor and more stimulation of the "blue" receptor. I think the
fact that we DO see violet in the spectrum is more interesting than the fact
that we DON'T see magenta.

See this:
[http://en.wikipedia.org/wiki/Violet_(color)](http://en.wikipedia.org/wiki/Violet_\(color\))

Wavelength: 380–450 nm

Hex triplet: #8F00FF

------
lnanek2
I think it would have been a better explanation if it included a color space
graph, with the different directions corresponding to stimulating the
different cones. You'd need a lot less text and people would grasp a lot more
intuitively that a a line cannot stimulate 3 different cones in all the
combinations.

~~~
3rd3
So what would be the shortest explanation using concepts from linear algebra?

~~~
alok-g
The stimulus is a spectrum. Discretizing wavelengths for simplicity of
explanation, the stimulus is a vector where each element specifies the
amplitude at each wavelength.

The color bar of rainbow colors is a sub-sampling of this spectrum where only
one element is taken at a time. In other words, those are monochromatic
colors. Alternatively, each point in the rainbow is obtained by multiplying a
delta function with the stimulus and integrating over the wavelengths. This
sub-sampling cannot possibly cover the entire color space. In fact, the
majority are missing, including magenta which is not a monochromatic color.

The human eyes also sub-samples this space. However, it does not use delta
functions. Each cone has its own vector (or more precisely a function of
wavelength) and the sensed amplitude is integral of this vector multiplied by
the stimulus. This is very much a dimensionality reduction problem where given
the spectrums of natural objects around us, nature chose a smaller space that
allows distinguishing the objects as much as possible.

Quite obviously, the sub-sampling done by the human eye also leads to loss of
information. There would be spectrums which are different but would map to the
same perceived color. See metamerism:
[http://en.wikipedia.org/wiki/Metamerism_%28color%29](http://en.wikipedia.org/wiki/Metamerism_%28color%29)

Electronic-displays use RGB because those three colors then cover a good
percentage of the colors human eyes can perceive. When more than three primary
colors are used in the display technology to enhance color gamut, etc.,
avoiding metamerism issues becomes an interesting concern as we need to make
sure colors which should appear the same do not appear different given that
the cone response curves varies somewhat between humans, varies with age, and
also surrounding illumination can impact the two rendered colors in different
ways.

See also my older related comment on the topic:
[https://news.ycombinator.com/item?id=5931005](https://news.ycombinator.com/item?id=5931005)

~~~
pionar
That's not as helpful as the original article. It's all in science terms
(delta functions, vectors, stimulus, discretizing wavelengths), without really
explaining them.

~~~
ithkuil
it was a reply to somebody kindly asking an explanation using linear algebra
concepts.

------
batmansbelt
Wouldn't magenta just be red mixed with white? That's how you do it with
paint.

~~~
dietrichepp
No. You'd have to mix in some blue, and then you'd just end up with a muddy
purple.

You can prove it to yourself by looking at any painting from before about
1860. See any magenta? No. Magenta pigment was not available until 1859, and
it could not be mixed from other colors without getting a muddy result.

Edit: Here's an example: [http://en.wikipedia.org/wiki/File:Bouguereau-
Psyche.jpg](http://en.wikipedia.org/wiki/File:Bouguereau-Psyche.jpg)

The painting (Psyche by Bouguereau) was done in 1890 and could not have been
made thirty years earlier.

~~~
batmansbelt
Thanks for the interesting reply.

------
jobigoud
No violet either, can't get violet out of a mix of red, green and blue. (True
violet, the stuff that is after blue on the spectrum).

"Magenta (often called pink)"

\- Aaaaargh. What next, "purple"? Please.

------
yoha
tl;dr: magenta = blue + red

Long version: brains interprets green+orange as yellow because they are close
enough and it save one kind of photoreceptor ; blue + red is not interpred as
another color are they are far enough, hence magenta (not a simple color ray)

