Comment by ignifero
15 years ago
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).
You are missing my point.
From a physical standpoint, it makes no sense to add two RGB colors.
It makes sense to add two lengths. (1 inch) + (1 inch) = (2 inches).
It does not make sense to add two RGB colors. The result is not a meaningful physical quantity. In the same spirit, (7.8 earthquake) + (8.3 earthquake) != (16.1 earthequake).
This is a big problem for those of us who are trying to reproduce how nature looks on the screen of a computer. (In my case, realtime graphics.)
For the case of the combining beams of light, the only time that the hue wouldn't change is if you have two lightbulbs that emit precisely the same EM waves --- the same frequencies and the same intensities for all emitted waves. This is practically impossible. Therefore, the hue of the combination is never exactly the same. Just because you can't perceive the difference, doesn't mean there isn't a difference.
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Sorry if i am being dense but you got me confused. It makes sense to add 2 rgb vectors from a perceptional standpoint: you add a red color on a green color you get a yellow. It also makes sense from a physical standpoint: the resulting frequency spectrum will have peaks near both red and green, thus what we perceive as yellow. If instead you mean that "red" is not a physical quantity, i totally agree, it's just a name for a range of frequencies.
It seems to me in computer graphics you also have to deal with other color problems such as light dispersion, the blue shift due to light scattering in the atmosphere, maybe even doppler effects :), but it's still valid to combine colors linearly for short distances, for example a green and yellow lightbulb would look yellow from a reasonably far distance.
In the example you give below, RGB 128 can be considered halfway between black and white, just not in terms of frequency, but in terms of freq. distribution (also, RGB is a 3dimensional space). As for the earthquakes, you 're right, the amplitude of the resulting wave would be the product of the original waves. Sorry for the rambling, i think it's a fun subject.
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