The *best* resource I’ve read happens to be the InDesign 2.0 (and CS) manuals on trapping.
The whole central idea of trapping is that it corrects for physical errors (such as misregistration of plates) inherit in the printing process. This is either the Printing press itself, or the inks and substrates that are being used. You are adding or subtracting ink
(read: spreading or choking) of elements on the imaged page to ensure there are no horrible white gaps (ie: the substrate showing through)
With Computer-to-Plate, Web presses – printers are not trapping 4 colour documents at all.
5- and 6-colours, where you are using one or two spot colour inks, trapping is imperative. This is especially the case as the special inks or foils are being used. These inks have different “coverage” characteristics.
Digital presses (or just big old photocopiers such as Docucolor devices) don’t really need trapping, but I’ve seen instances where it would have helped…
How do you determine what colour to choke/spread into another, automatically? Mathematically, as there is a software process that is applying the trap. It is overcompensating the size of an element to make it bigger.
There is where the Neutral Ink density is key. It measures the relative ink “coverage” characteristics of a particular colour relative to another. So, if a light colour (with a smaller ink density) isn’t going to “run” enough on the substrate into a darker element (with a high neutral ink density) on the page right next to it — you have to
“spread” the light into the dark colour. If all you remember is
“spread light into dark” you’re on your way there.
On the press, the idea is that the traps are “cancelled out” and effect will be the elements will look great — no horrible white gaps, and hopefully not too overprinted. (with a horrible muddy colour as a
result)
In Postscript, when printing with On Host trapping out of InDesign, extra elements are drawn that “overprint” this light colour into the dark colour (if the element knocksout, the objects are abutting and therefore not trapping). This “overprinting” is why some people confuse trapping with overprinting. Overprinting is just the mechanism used to apply a trap based on the inks used.
How big should the traps (trapwidth) be? This all depends on the press, inks and substrates. That’s why (I think) the printers should be doing trapping.
There are a whole bunch of complexities that I haven’t discussed here: sliding traps (think gradients), trapping text, trapping images — but I’ll leave that as homework for the reader.
For those prepress operators who are from the good ole’ film days, they usually like to hand create the traps on certain elements — and not rely on InRIP trapping or OnHost trapping.
For those who don’t understand trapping, and rely on Brand-Q’s features
— “real” prepress operators snigger behind their backs. I know — I’ve worked with some of them!
The more you use InDesign, and especially CS; then compare it to the high end systems that many organisations think as “professional production engines” that cost more than US$20-30,000 a few years ago: the more you realise what an excellent prepress tool the InDesign engineering team have developed. At a fraction of the price.
Hopefully this is correct (there might be some little errors!) and helps.