Real World Computing

Even where trapping isn't a problem, misregistration can destroy the optical illusion on which colour perception is based, and produce fuzzy and inaccurate colours. This is bad enough for full-colour images, but it would be more disastrous still for solid black text, which, when printed via CMY, would require perfect alignment of the solid cyan, magenta and yellow versions of the type. Even with perfect registration, the inherent imperfection of the subtractive ink-based colour model means the resulting text still doesn't look a perfect black, but rather a very dark brown. Both these problems can be solved at one stroke by adding a fourth colour black (K) with its own plate and an extra printing pass. This CMYK four-colour process print underpins the entire design and publishing industry.

Introducing an extra black plate provides two other advantages, so long as you know what you're doing. To begin with, 100% K can look a bit flat, but by adding, say, 30% each of CM and Y you avoid potential trapping problems and produce a much darker rich black. You wouldn't add 100% of CM and Y because then you'd be applying 400% ink coverage and the paper simply wouldn't be able to absorb it or would fall apart. Even with dark shadow areas in full-colour photographs, this overinking can be a problem, but one that the black plate can solve. The technique called Under Colour Removal (UCR) moves all neutral grey shadows from the CMY image onto the black plate, so slashing the amount of ink applied to the paper. With Gray Component Replacement (GCR), shared CMY percentages get moved to the black plate even from coloured areas, so that, for example, a colour represented by 95% C, 60% M and 75% Y could be reproduced as 35% C, 0% M, 15% Y and 60% K, by removing 60% of each colour onto the black. However, to avoid flat-looking results from UCR and GCR, you might decide to move only 50% of shared greys to the black plate - or maybe even only 20%.

Once you introduce UCR/GCR, translating from an RGB full-colour image to the printed CMYK version is clearly not a simple 1:1 conversion process, but more of a complex balancing act involving multiple criteria and multiple possible routes. In fact, this goes far deeper than just deciding what moves onto the black plate. The bottom line is that the CMYK colour space can only approximately duplicate the RGB colour space, so even when you know exactly what hue an RGB colour is meant to represent (for which you need colour management), there's no guarantee it can be replicated on the printing press. And just as there are many RGB colours that can't be accurately printed, equally there are CMYK colours, such as those rich blacks, that can't be accurately reproduced onscreen. Given no fixed 1:1 mapping between colour models, multiple matching approaches and, ultimately, context-sensitive colours dependent on the medium on which they're displayed (pixels or paper), RGB-to-CMYK conversion becomes very much an art or craft rather than a science.

To successfully produce a run of commercial print, you first need to transfer your PostScript-based digital artwork to your chosen printers, avoiding any preflight and compatibility problems. You and your printers working together then need to make sure your imageset colour separations, complete with halftones, are accurately produced and transferred to the printing plates, then to the inked image itself - two inherently uncertain chemical processes. Next, you need to understand and choose between the different types of paper and ink, and know how these will interact. You also need to understand how your final colour print depends on the two optical illusions of halftones and colour mixing, and also on the mechanical registration of each printing pass. And finally, you need to understand and adjust to the fact that press and computer work on two entirely different colour models and spaces.

Continued....