(v13) setscreen and pre-separated jobs

This page applies to Harlequin v13.1r0 and later; both Harlequin Core and Harlequin MultiRIP.

The approach of most applications producing color separations is to send the same page once for each separation, with a small wrapper around it which changes the behavior of the color operators. A common feature of this wrapper is a change of screen angle, which is often done like this:

currentscreen exch pop newangle exch setscreen

Early impressions of [RB2] stated that “if the topmost operand is a halftone dictionary instead of a procedure, setscreen performs the equivalent of sethalftone and discards the other two operands”. However, this would clearly cause examples like the one above not to work with Level 2 interpreters. New impressions of the reference manual correct this, and there is a separate list of errata for the early impressions (see [RB2ERRATA]). There is also a technical note about compatibility of Level 2 screening [ATN-L2SCREENS].

This appears to have been simply a mistake in the language specification. Indeed, Adobe implementations always seem to have taken notice of frequency and angle when the third operand is a dictionary. Certainly, the RIP has always done so. However, in both cases, this applies only to setscreen, not setcolorscreen.