I know that the primary colors of light are RGB, and the primary colors of pigment are RYB. I also know that it's because of "additive" vs "subtractive". I just don't understand why the primary colors of pigment are different from light's. Or, why do they both include red and blue, but have a different third color?
Posted Aug 22, 2012, 10:26 PM UTC
The primary colors of light are RGB because the photoreceptors in human eyes have three types (short, medium, long cone cells), and are sensitive to red, green and blue light, approximately. Thus, if one wanted to quanify color perception, it made sense to do so in terms of the stimulation of those three types of cells (as that would map nicely to distinguishable sensations), in turn in terms of RGB.
Pigments, however, function by absorbing light that you shouldn't see. The commonly used pigment primaries (CMY) absorb red, green, blue light, respectively. So there's commonality here.
Oh, I see. So the primary colors "RYB" are just close enough to "MYC" to mimc the effect there? That makes a lot of sense to me. Especially why printers use CMY instead of RYB. Thanks!
Mimicry is not quite the right term, but in the end that is functionally correct. Basically, the space of colors (as far as human perception is concerned) is well-approximated as being three-dimensional, and as long as you choose three independent colors, you should be able to mix them (either additively or subtractively) and recreate the palette you want. It just happens that for additive color, RGB is a very reasonable choice, and for subtractive, CMY is. If you used RYB for additive or subtractive, you could also get by, but you'd have to mix the colors in different proportions to get a target color, and it may limit the range of colors you can reproduce.
(e.g. In an extreme case, if your primaries were red, purple and orange, you would be hard pressed to create the sensation of pure green. This is also true of RGB as well.)
That actually makes a lot of sense to me. I'm familiar with the idea of a basis, from Linear Algebra. I believe it's similar to that. It seems like in your example, in order to create pure green, you would need a negative concentration of one or more of the colors (Red, I think). It's amazing how much sense that makes to me. Thank you!
Ah, I think you've grasped it well. It is equivalent to basis, minus the non-negative constraint and homogenization.
To depart a bit from ELI5: so, with three-dimensional basis, the range of colors that can be created as a mixture is a convex combination of the three---it would be a triangle in some high-dimensional space, because you could normalize the coordinates. e.g. (r,g,b)=(2r,2g,2b) if you're concerned only about the color.
The enclosing high-dimensional space happens to be 3D (=three types of cone cells.) Unfortunately, the set of distinguishable colors in this space is shaped like a horseshoe, so no matter what choice of three primaries you pick, you end up getting a triangle that's enclosed in this horseshoe. The goal in picking primaries would be to minimize the area outside the triangle (but inside the horseshoe.)
because of "colors of light" and "colors of pigment"
light is its own light source. pigments only REFLECT light.
ADDITIVE - the more light you add, the brighter it gets (towards white)
SUBTRACTIVE - the more pigment you add, the darker it gets (towards black)
A thing is red because it absorbs all the non-red frequencies. A thing is yellow because it absorbs all the non-red or non-green frequencies, and the reflected red and green appear yellow.
Similarly if you have a source of red light and a source of green light, the result is yellow.
Technically the pigment colors are CMY (Cyan, Magenta, and Yellow). Cyan and Red are opposites, Magenta and Green, and Yellow and Blue.
Oh, that makes a lot of sense to me. It's actually CMY, not RYB, that corresponds to RGB. I get it. Thanks!