Comment by palish
15 years ago
Very informative, thank you!
Something I've been mulling over for awhile now is... what does it mean to represent a color with numbers?
That is, every pixel on our monitor is RGB 0 through 255. But what about in physics? Well, "color" is a photon whose wavelength falls between 400nm (short wavelength; intense energy; "ultra"-violet; and violet = blueish; therefore 400nm = "the blue side" of the EM spectrum; this is how I remember that) and 700nm (red).
So on one hand, we have three numbers, R, G, and B; on the other hand, we have a single number, wavelength.
Obviously, the answer is that R, G, and B have some significance to the human eye. But wait a minute; every human eye is different. Some people are color blind, for example. So these RGB values can't really claim to have a physical basis.
Now, what is the meaning of a combination of colors? Let's say, RGB(50,50,0) + RGB(0,50,50). Is the result RGB(50,100,50), like you'd expect?
I think not.
RGB is not a linear space. In the same way you can't add two points on the Richter scale, you can't add two RGB values and get a meaningful result, I think.
So then we have something called "sRGB", which claims to be a linear space. But really, sRGB is merely RGB space "warped" to conform to the gamma curves of standard monitors. It still has no physical meaning, as far as I can see.
I believe perhaps the correct answer is...
When a light source emits light (for example an incandescent light bulb), it's really emitting many different wavelengths along the EM spectrum. The final color is the result of our eyes combining those wavelengths.
So, to add two colors A and B, we must first convert them both from RGB back into the EM spectrum (think of it like a histogram, where the X axis is wavelength from 400nm to 700nm, and the Y axis is the intensity of each wavelength); then add the two spectrums; then finally convert the resulting spectrum back to RGB.
Will this produce the same results as (A + B)? I don't think so, but I haven't tried it.
Food for thought...
EM radiation can have any wavelength. Color is the visible spectrum in which our retinal cones and rods respond when EM is shone upon them (actually, they cease responding, but that's another matter). We have 3 kinds of cone cell receptors sensitive to different parts of the visible spectrum. Color is the perception generated further down in our brain from the combination of these incoming signals. Screens were designed to be visible, that's why it makes sense to have 3 colors of LEDs. Light wave frequencies do not add up, i.e. you can't add 2 green beams to create an ultraviolet.
My point was that you can't add two RGB values and expect to get a meaningful physical result --- you can't multiply them either, as far as I know.
This has vast implications for computer graphics. Video games, for example.
"Light wave frequencies do not add up, i.e. you can't add 2 green beams to create an ultraviolet."
The light wave frequencies do add up --- if you add 2 "green" beams, then you get a more intensely bright green beam. The hue changes only slightly.
RGB values can be added to create other colors, just like painters mix colors (just divide the sum by 2 to avoid saturation), because the R, G and B colors were arbitrarily selected from the beginning to match what the artists were familiar with.
In the case of combining beams, what changes is the intensity of the beam, the frequency spectrum does not change at all (there is two photon excitation, but that's a completely different phenomenon).
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