Understanding flattening and production issues
Since PostScript has no concept of transparency, flattening is the process of simulating transparency by altering page elements to produce the intended design. Flattening occurs only in the print stream — as items are fed to the print engine — so your QuarkXPress layouts are never actually modified. In QuarkXPress, flattening works as follows.
First, boxes are decomposed, transparent elements are identified, and relationships between discrete shapes (including text outlines) are deconstructed. Regions that do not have to be rasterized are filled with a new color that is created by merging existing colors. (None and 0% opacity areas do not need to be flattened except when used for blends and pictures.)
Regions that need to be rasterized result in clipping paths. (Semi-opaque pictures, drop shadows, semi-opaque blends, and semi-opaque items that overlap page elements must be rasterized.)
The settings in the Transparency pane of the Print dialog box (File menu) control the output resolution of page elements that are rasterized due to transparency effects or drop shadows. For more information, see “Transparency pane.”
In general, when working with transparency relationships, trapping is not necessary. When trapping does come into play, overprints of opaque items are inherited by paths created through decomposition; chokes and spreads set for transparent elements are ignored. All other items created by decomposition are set to knockout and sent through the default QuarkXPress trapping during host-based separations.
All transparent page elements are always flattened in PDF output.