Gavin asks:

> And how do you handle trapping?  Or do you avoid overprinting colors to
>  avoid the complexities of knockouts, rich black construction, traps and the
>  like?

"Trapping" isn't necessary any longer, especially now in the era of
PDF-to-plate technology. But it really wasn't necessary even when the output
was printed to PMTs (a form of very mechanically rigid paper).

If your alignment circles surrounding the primary printing area are complex
enough to allow for registration in two dimensions easily, the pressmen now
use microscopes to align the plates on the large four-color presses. Once
that alignment is set, even two completely different colors that don't blend
into one another can coexist with no more than 1/2400 of an inch apart, with
absolutely no visible overlay or gap. Once that alignment is set, the plates
simply stay aligned nowadays on the very expensive presses.

"Trapping" was something that had to be done in the old days to accomodate
the inherent sloppiness of the presses. It generally isn't necessary any more.

Nonetheless, it is rare to really need trapping under any circumstance. For
example, yellow on a red background is nothing more than the suspension of
the magenta ink for a bit of distance. That kind of printing is always
"aligned", in the same manner than white on black reverse printing is always
aligned. This kind of reverse is simply a suspension of one ink.

However, to eliminate any possiblity of trapping, and because of the way that
QueryCalc was designed, where text is always laid down on a graphic form
background, it was extremely easy for me to design the four color separations
such that if the text were darker than some chosen threshold, the background
was printed as a solid (no holes are left in the background image for the
text) and the (generally) black text prints only onto the black plate, as an
overprint.

As you know, most graphics programs don't work that way, but due to the
manner that QueryCalc was designed to handle forms (invoices, etc.) on a
LaserJet printer, where the form is in the background and the text resides in
a foreground plane, it was not only very simple to implement this procedure
in QueryCalc, I've become very enthusiastic about the design in general.
Doing this really does eliminate the necessity for trapping.

The rules for this design are simple: if the text is above a certain gray
threshold, then it is printed on the appropriate color plate (generally
black). If the text is below that threshold, then that text is subtracted
from the opposing plates. Doing this greatly reduces (essentially eliminates)
the trapping requirements in almost all circumstances.

The only exception would be something like yellow printing on a black
background. In those few situations, you do depend on the pressmen and their
microscopes.

Wirt Atmar