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Comment by zokier | Hacker News Reader

> In a sense, the choice of red, green, and blue is arbitrary. The primary colors are not derived from some property of physical light, nor are they derived from some truth about human color perception.

What about the cone cell response curves which roughly correspond to r, g, and b? https://commons.m.wikimedia.org/wiki/File:Cones_SMJ2_E.svg#m...

> They do happen to work very well--because the space of colors we can perceive is vaguely triangular.

That is hardly a coincidence. Our perceived color space is vaguely triangular because we have vaguely three types of cone cells to detect color.

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> What about the cone cell response curves which roughly correspond to r, g, and b?

The diagram is misleading because the spectral sensitivity curves are drawn using red, green, and blue colors on the chart. Read the wavelengths on the bottom--they do not roughly correspond to red, green, and blue. This is why they are named L, M, and S instead.

The peak sensitivities are closer to green, a different green, and violet.

> That is hardly a coincidence. Our perceived color space is vaguely triangular because we have vaguely three types of cone cells to detect color.

It's a poor approximation of a triangle. It's more paraboloid.

It would be a triangle if there were no overlap between the spectral sensitivity of different receptors. Since there is overlap between the green, other green, and purple receptors, the shape is not a triangle.

> The peak sensitivities are closer to green, a different green, and violet.

This is fascinating. My first instinct is to say that the peak of those curves are not as important as where they discriminate best/where they are most different from the other curves - which does seem to give a more clear red, green and blue. But I’m sure it’s not that simple when you take into account how the brain processes these signals.

Would it be possible to create a good color screen using subpixels emitting light at the peak sensitivity frequencies?

Any three-primary system is going to cover a triangle of colors. RGB primaries cover a fairly large triangle. If you wanted a larger gamut, you might consider switching to four primaries. You can still encode colors using the RGB space and convert to e.g. RGBY in the display or whatever.

I do note that this is a limitation mostly for physical systems/models. For non-physical color spaces you can make a triangular shape cover far more of the human visible gamut by having the primaries be "imaginary", i.e. not real visible colors. ACES AP0 is one great example of that

Yes, if you had something like a RGBY display, you would probably use a high-gamut RGB space to encode the data and do a transformation prior to display.

No, because light at the L (‘red’ cone) peak still stimulates M (‘green’) around 75%, and the brain makes that yellow. To see red, you need a lower frequency that stimulates L twice as much as M.

This is not about peak sensitivities. The RGB colors are used as primaries because they provide a large response on one type of cones, and as small as possible response on the two other types of cones.

Blue is chosen to have as small wavelength as possible (S=small) and red is chosen to have as long wavelength as possible (L=long), while both being still in the visible spectrum with large enough response from S and L cones respectively.

Since M and L response profiles overlap anyway and just the ratio of the response changes, green is chosen to be as far away from the red, but still outside of the response curve of S cones that get stimulated by blue. This happens to be also around the peak sensitivity for the M cones.

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