Comment by randomdrake | Hacker News Reader

Comment by randomdrake | Hacker News Reader

Neat article, but I found it surprising that the author didn't go into a bit more detail about why RGB is used and where it's used. Saying that it's just how computers or televisions deal with it because of monitors only offers a bit of information.

While the explanation of additive and reductive color combining was interesting, I think it's important to note that it's because you're dealing with waves of light being absorbed or reflected.

I learned more about color theory and light in stage lighting classes than anywhere else. Teaching your brain how to combine colors correctly to get the exact shades you're looking for on the stage is hard. You really do have to forget a lot of what you learned in art classes to be effective. Understanding how colored light is absorbed, reflected, or otherwise changed on makeup, furniture, fabrics, and other surfaces added a whole different level of complexity not found in typical art classes.

Searching Google for "stage lighting color wheel" reveals more wheels similar to the one that you see at the author's really cool implementation[0].

[0] - http://bahamas10.github.io/ryb/

> Neat article, but I found it surprising that the author didn't go into a bit more detail about why RGB is used and where it's used. Saying that it's just how computers or televisions deal with it because of monitors only offers a bit of information.

It's our eyes. Our eyes are (roughly) RGB, therefore a RGB monitor is a best spectrum match for our eyes, therefore it's how computers and printers deal with it (CMY/K/ is just the inverse of RGB).

For many animals, even our best high end lifelike wide-spectrum display cuts out or inaccurately represents portions of the spectrum they can see.

> For example, while the L cones have been referred to simply as red receptors, microspectrophotometry has shown that their peak sensitivity is in the greenish-yellow region of the spectrum. Similarly, the S- and M-cones do not directly correspond to blue and green, although they are often depicted as such. It is important to note that the RGB color model is merely a convenient means for representing color, and is not directly based on the types of cones in the human eye.

It's not that easy though

http://en.wikipedia.org/wiki/Color_vision

> The cones are conventionally labeled according to the ordering of the wavelengths of the peaks of their spectral sensitivities: short (S), medium (M), and long (L) cone types. These three types do not correspond well to particular colors as we know them. Rather, the perception of color is achieved by a complex process that starts with the differential output of these cells in the retina and it will be finalized in the visual cortex and associative areas of the brain.

> For example, while the L cones have been referred to simply as red receptors, microspectrophotometry has shown that their peak sensitivity is in the greenish-yellow region of the spectrum. Similarly, the S- and M-cones do not directly correspond to blue and green, although they are often depicted as such. It is important to note that the RGB color model is merely a convenient means for representing color, and is not directly based on the types of cones in the human eye.

Also there are alternative color spaces like YUV: http://en.wikipedia.org/wiki/YUV

To elaborate on why the RGB color model is convenient for representing color...

The choices of RGB in trichromatic reproduction systems are such that the individual contribution of each minimizes the cross-cone activation in the eye, allowing greater fidelity (widest gamut) in color reproduction with only three sensors at input and three emissive colors at output. In other words, if you're going to use analog electronics and passive filters, it helps to make the selected primary frequencies as functionally orthogonal and isolated as possible.

Consider the quality of an absorptive filter and/or response profile of a pixel on CMOS sensor; so long as a it has a strong peak at the primary frequency, then we're not too concerned about leakage from other frequencies into it; nor are we too concerned that nearby colors could leak a little into the other two channels because this will mostly be correlated with overall luminance, which makes it very hard for the eye to discern upon reproduction (it looks a little more washed out).

This is why Young and Helmholtz initially identified red, green and blue as primary colors way back in the early 19th century. They tried to identify three specific color frequencies that could be used to mimic other pure frequencies through re-combination in test subjects, based on a theory about how the eyes worked. While these colors do not correspond to sensory peaks for each cone cell type, it turns out the retina/visual cortex's post processing (the "opponent-process" discovered by Ewald Hering) derives hue from the combined activation ratios, and is thus bypassed by using combinations of RGB to create hues as opposed to direct spectral activation.

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