Why are RGB used for additive color and CMY for subtractive color?
Asked 8/16/2021
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I'm trying to understand trichrome photography and color mixing. If I make three black-and-white separations using red, green, and blue filters, I understand that the images can be recombined with colored light to form a color image. But when making a subtractive result, cyan, magenta, and yellow are usually used instead.
Why specifically are RGB the primaries for additive mixing, and CMY the primaries for subtractive mixing? If colors on a wheel can be spaced evenly, why wouldn't another trio such as RGB for subtractive mixing, or other sets of three colors, work just as well? Can CMY also be used in an additive setup?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
4y ago
2 Answers
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At the time of taking the images you would want to use a combination of RGB filters; e.g. R+G rejection filters (or a blue colored absorptive type filter) allowing B to pass.
Then at the time of enlargement/printing you would need to use the corresponding RGB filter for that image; e.g. B for the R+G filtered negative... this is all additive color because it all has to do with projecting/recording light.
And the color paper has RGB sensitive layers which are CMY in color correspondingly (i.e. the cyan layer is sensitive to red light)... The final output is in subtractive color because the positive reflects/transmits the colors it does not absorb/subtract.
The other colors (i.e. magenta) would work at the additive stage because it is essentially a combination of the R+B positive colors (typically labeled as violet in the visible light spectrum).
At any stage involving projecting light you are using additive color (RGB). Additive colors add up to white, so a white (or colored) light falling on a reflective surface (white wall) determines the color.
Subtractive colors (CMY) have to do with reflecting light, and they add up to black. What makes something green is that it absorbs all of the other colors. I.e. it is a mix of cyan and yellow (subtracting red/blue) and reflects the remaining additive color in the light (green). I.e. you cannot mix the processes... light is always additive, and surfaces are always subtractive.
Originally by user70370. Source · Licensed CC BY-SA 4.0
user70370
4y ago
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RGB are used for additive color because human vision is effectively based on three broad sensitivities: red, green, and blue. By adding those lights in different amounts, you can reproduce many perceived colors.
CMY are used for subtractive color because each subtractive primary removes one additive primary from white light: cyan absorbs red, magenta absorbs green, and yellow absorbs blue. Combining CMY dyes or filters subtracts more of the incoming white light; ideally all three together absorb nearly everything, giving black.
So RGB and CMY are complements, not interchangeable systems. In projection or screen display you add light, so RGB is the natural set. In prints, dyes, and filters you remove parts of white light, so CMY is the natural set.
Other evenly spaced colors on a color wheel do not work the same way just because of their geometry. The wheel is arranged around how our eyes respond; it doesn’t mean any three equidistant hues can serve as equivalent primaries.
CMY can be used in additive situations, but each is effectively a combination of two RGB primaries: cyan = green+blue, magenta = red+blue, yellow = red+green. That makes them less fundamental than RGB for additive synthesis.
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