(v13) How automatic separation works
This page applies to Harlequin v13.1r0 and later; both Harlequin Core and Harlequin MultiRIP.
The Harlequin RIP divides the work of producing each page into two distinct phases:
- Interpretation
- Rendering
During interpretation, a much simpler representation of the page than the PostScript-language job is built up in the computer’s memory. This is called a display list. The display list can be thought of like a stencil: the interpreter makes it, and the renderer applies paint to it.
The display list is structured to make it easy for the rendering phase to read it to produce the marks on the output. This intermediate representation is necessary because the PostScript-language job can put marks anywhere on the page in any order, but the page must be delivered to the output device from top to bottom. Even for modest resolutions, especially for continuous tone output, it is not usually possible to store the whole of the final raster in memory at once.
The display list contains all of the color information given in the original job. Therefore to produce a cyan separation, the renderer reads the display list, and ignores all but the cyan information to produce tints, and uses the other colors to determine where and when to do knockouts in the cyan. Then it repeats the same process, this time for magenta, and so on. This is like taking the same stencil and applying different colored paints to it, one after another.
Where separations appear on separate media, a page throw is done in between each one, but when separations appear on the same sheet a simple displacement is added to each mark as it is placed on the page. This is like taking the stencil and repositioning it on the page before painting.
Because the separation is a function of the rendering, it is not possible to do anything other than a simple shift: separations cannot be rotated relative to each other, for example, since rotation is handled by the interpreter.
It is now easy to see why step-and-repeat is essentially the same thing as separation placement: instead of painting different colors through the stencil, we just paint the same color several times, or for color step-and-repeats several in one color and then several in another color, and so on.
There is an optimization here: when two or more copies of the same separation are located at the same vertical position, so that it is just shifted horizontally (in the fast scan direction), a simple copy of the pixels is done, rather than rendering the display list over again. This is detected automatically, but it is worth remembering when separations are being laid out.