Why RGB Color Separations for Tricolor Gum?

To make color separations to print CMY from the RGB file, invert the RGB file, then split the channels and print them. This may seem counterintuitive to people who were taught the simplistic idea that RGB is for light and CMYK is for printing, but bear with me; inverting RGB is actually the most direct and accurate way to separations that print the CMY layers in a tricolor gum print. (To see why converting to Photoshop CMYK may not be as accurate)

If you've taken any color photography classes, or adjusted color curves in Photoshop, you know that the additive primaries and substractive primaries exist in a complementary relationship to each other. Each is at one end of a continuum that its complement is at the other end of. So the more yellow, the less blue; the more blue, the less yellow and the same for the other two pairs of primaries. As my Color Photography 101 teacher drilled into my head again and again, Cyan = minus Red; Yellow = Minus Blue, etc. If you look at the values for a particular pixel in info, invert the pixel and see what value you get, you will find that these are the same values you would get if you subtracted the values in the pre-inverted value from 255. In other words, R inverted = minus R = Cyan. By extension, RGB inverted= minus RGB=CMY. This is the most straightforward way to make CMY color separations for gum printing. The separation obtained by inverting the red channel prints the cyan layer, the separation obtained by inverting the green channel prints the magenta layer, the separation obtained by inverting the blue channel prints the yellow layer.

Let's work through a simple example to see how RGB converts to CMY by inverting the RGB file:

The first column of numbers gives the RGB values for this color from the info palette. The second translates those values into percentages to make them more comparable with the CMYK and separations values which are given in percentages. The third column is the value that results when the file is inverted, in other words this is minus RGB, or CMY. The fourth column gives the densities of the separations that result from the inverted file. The final column simply converts back to CMY to show what the resulting color should be in the final gum print if the world were perfect and gum behaved perfectly and the negatives were perfect and all. In other words, if 63% of light is masked from the cyan layer, then the cyan layer should print at 37% density, etc.

To help you see that the inversion is a true conversion from RGB to CMY, note what happens to the yellow as RGB is inverted. The color could be described as a light green with a lot of yellow in it. When the mode is RGB, the yellow is expressed in the R and G channels, because in light, yellow light is created by the combination of red and green light. But when the file is converted to CMY by the inversion, then the yellow is expressed in yellow pigment, which is printed with the yellow separation, created by inverting the blue channel in the RGB file.

Copyright Katharine Thayer, all rights reserved.

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