A: There were some extensions made to PostScript so that it can be used onscreen; the new form is called Display PostScript (DPS). We have to handle the new operators (such as
\b alphaimage
\b0 ) specially whenever we send anything to a conventional PostScript printer. We do that by inserting the print package at the beginning of the data that is sent to the Window Server. The print package—/usr/lib/NextStep/printPackage1.0.ps—redefines the DPS operators that can be simulated and no-ops those that don't mean anything on the display. Other DPS operators are not redefined and will cause a "malformed postscript" message if sent to a printer.\
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The philosophy behind this is that we don't want to image something on a printer differently from the way it looks on the screen.
\b alphaimage
\b0 can't be imitated on paper, so it's an obvious one not to redefine. It probably really is a mistake to try to send PostScript code that uses it to a printer.\