Wide Format Imaging

Since you're reading this publication, no doubt you've got a good idea that the transmissive—or additive—primary colors are red, green, and blue; and that their opposites are the reflective—or subtractive—primaries: cyan, magenta, and yellow.

Of course additive means that if you add the three transmissive primaries together you get white, and subtractive means that it you subtract the three reflective primaries you get white—or at least the white of ambient light as reflected by the media. And the reverse is also true: Subtract the additives and you get black; add the subtractives and you get black. Except, of course, as you might also even be able to recite by rote, "pigments are never 100 percent pure and because of that the combination of cyan, magenta, and yellow comes up to not black but a muddy brown, so black is added as a fourth color; giving us cyan, magenta, yellow, and black color separations."

But do you know how black is "added" and when, and who decides how? Black is added at the moment the "color separation" is made, and it's added per the black generation information in the CMYK color space to which the originating RGB is converted (or in the CMYK creation space, if the file is created in CMYK.) Who decides how is whomever it was that put the black generation information into that color space at the time the color space was created.

So what's that mean?

A "color separation" is what we commonly refer to as a CMYK file. Back in the old days, you sent transparencies or reflective photographs to a "color separator" and the separator would convert them into printing negatives (or positives depending on process) for the four process colors. The black was generated by techniques either known as Grey Component Replacement—GRC—or Under Color Removal—UCR.

These days, the instant you convert an image from RGB to CMYK you just did all that.

RGB is always device dependent, as is CMYK. You can't just convert from RGB to CMYK generically. You convert from whatever RGB space the file is in into whatever CMYK space you've asked the application to convert it. And you have to ask it; if you don't specify, you're asking it to use its default. Included in every CMYK color space is black generation information. If there wasn't, it would be a CMY color space.

So what is black generation information? Here's what it boils down to: Black start point; black start value; black end point; black end value; black ramp.

We still use the terms GCR and UCR, but in reality, they're not mutually exclusive and they're not descriptive of anything that goes on in the process as it's done in our industry today.