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.

Welcome to Adobe InDesign CS

For more indepth articles: Adobe InDesign: Prepress Techniques

This is written in a similar manner to: Acrobat 6.0 Professional: Graphics, Print, Prepress Overview

Since writing this introduction, Adobe has released: Adobe InDesign CS Printing Guide for Service Providers

InDesign CS, the third major revision of InDesign, contains many new prepress features that solidifies my belief that InDesign is the premiere desktop prepress tool on the market today.

Having worked between many pre-release testers and the Product Team and Engineers for nearly two years – it is such a relief to be able to talk publically about all the “new stuff”

What is this Adobe Creative Suite?

The Adobe Creative Suite is a new application that combines the full desktop versions of Photoshop CS, Illustrator CS, InDesign CS, GoLive CS and Acrobat 6.0 Professional with a new piece of technology called Version Cue. This new application installs with a single serial number, comes on single CD and is a application suite. This InDesign CS that comes with either Creative Suite Premium or Standard is the same as the single copy version.

System Requirements

InDesign CS (and the other CS applications) requires Windows 2000sp3, Windows XP Home or Professional. On the Mac, InDesign CS like Acrobat 6.0 requires at least MacOS X 10.2.4. That’s correct: no MacOS 9 support. If you are receiving InDesign CS files, you are going to need a MacOS X to run the files out. From a high quality print perspective, saving backwards is not an option.

New Prepress Features

Separation Preview

Having been exposed to this particular piece of engineering since prior to InDesign 2.0’s announcement, this has to be my favourite feature. It alone will change prepress perceptions of InDesign’s status as the best tool to work with on the desktop.

Until the advent of Quite Revealingfor Acrobat 4/5, Acrobat 6.0 Professional (Acrobat 6.0 Professional: Graphics, Print, Prepress Overview)- the only way to preview the plates that would appear at some stage of the print process was to print separations as Postscript and Distill.

InDesign CS adds a new feature called Separation Preview that is a “mode” for layout. You can work completely in this mode; placing images, changing swatches, editing text if you like – and see how the final plates will appear whilst still editing the document.

The above shows Cyan and Black plates, with a Ink Density count on a per-plate basis.

This shows a separation preview highlighting one spot colour, with the black text knocking out correctly.

I remember first seeing this feature and being on cloud 9 for hours. It has to be experienced. Thanks Matt.

Flattener Preview

Transparency, the ground-breaking set of features added in InDesign 2.0, provides designers scope to create eye catching layouts. When it comes to output, however, some of the print aspects require finessing.

To aid the print side, the Flattener Preview will show what elements are going to be effected by transparency, and in which way. The Transparency Flattener is still required in Postscript 2/3 and PDF/X workflows.

The areas highlighted in red above are Transparent Objects that will result in some transparency flattening at output.

Ink Limit Preflight

Common in newsprint and other print applications where the total ink density is tightly controlled, InDesign CS will now permit a preview of a layout – and highlight elements that are above to total ink limit as specified.

In the image above, an Ink coverage limit of 280% is specified: the areas highlighted in red on the page have more ink coverage than this percentage.

Bleeds and Slugs

No, this is not going postal on the evil garden pests. InDesign 2.0 added the ability to print with independent bleed-per-side in a document. In InDesign CS, documents can be created with predefined bleed and slug areas:

These predefined Bleeds and Slugs can be used when printing, without retyping the appropriate values.

In this image, Print Preview with Bleed has been requested.

To make life easier when creating documents in InDesign CS, page dimensions including Bleeds and Slugs can be saved.

Another commonly requested feature from longtime QuarkXpress users is the ability to see the ‘page edge’ when placing elements. Guess what, its here:

In the above screen dump, the black line is the trim size of the page, clearly shown through the image.

Word Count

Not strictly a Prepress feature, but I am going to incorporate it here! Yes, there is a word count in InDesign CS:

Not only a word count: InDesign CS also counts sentences, lines and characters. No more need for InDesign 2.0: Word Count using Visual Basic! The above image depicts a text frame that contains a certain number of characters/words etc, and the “+61” indicates that there is overset text.

Info Palette

In PDF delivery of final for-print documents, the two major errors that cause prepress headaches are RGB elements and low resolution images. InDesign always converted RGB elements in bitmaps to CMYK (if printing CMYK). InDesign CS adds the conversion of elements in RGB that are inside placed PDF elements to CMYK. (InDesign 2.0 and CS have a technique that will force EPS into CMYK or Greyscale: InDesign 2.0: Printing Output Choices and Flattener Tricks (including force Greyscale export!))

On the matter of DPI, however, there has been a reliance on the designer on “guessing” the print DPI (otherwise known as effective DPI) by calculating the percentage scaling by the original DPI. InDesign CS has a new palette known as the Info palette that previews the DPI of a placed image element:

The Info palette above shows that the placed image is a JPEG in the RGB colourspace, and due to scaling of the image, its print (effective) resolution is 288dpi in both dimensions.

Mixed Ink Support

An ink swatch in InDesign CS can be what is a Mixed Ink swatch containing spot colours and process colours.

InDesign CS also adds a new type of swatch known as Mixed Ink Group which eases the mixing of two spot colours into a varying combination of percentages.

Support for Duotone Photoshop files

DCS is the thorn in the side of the Prepress professional. It forces print workflows into separated output at a very early stage – and is a legacy of QuarkXpress. In our modern, composite workflows – DCS is a legacy that would be rather forgotten.

InDesign CS changes the scene in rather a dramatic way. DCS 1 and 2 files created from Photoshop (bitmaps only) placed into InDesign CS are recombined into composite for composite PDF/Postscript output. DCS1, for the sake of clarity, is a preseparated format where each plate is broken into a file: one each for C, M, Y and K (there is not spot colour support in DCS1). DCS2, in comparison, is a single file containing each plate – and can support spot colours.

For Photoshop files and designs that contain vector elements and transparency, this technique still applies: InDesign 2.0: Photoshop with Spots, InDesign and Composite PDF

InDesign CS also includes support for TIFF with spot colour channels, Photoshop PSD with spot channels (including Duotones, Tritones and Quadtones) and Photoshop EPS.

PDF/X Support

Like Acrobat 6.0, InDesign CS supports exporting PDFs are PDF/X compliant. More than just a version PDF, compliance also involves ensuring the elements used in the PDF match the strict ISO specification.

What is PDF/X? From the FAQ on the PDF/X site: “PDF/X is not an alternative to PDF, it’s a focused subset of PDF designed specifically for reliable prepress data interchange. It’s also an application standard, as well as a file format standard. In other words, it defines how applications creating and reading PDF/X files should behave.”

PDF/X is a set of international standards: PDF/X-1a:2001 (ISO 15930-1:2001) and PDF/X-3 (ISO 15930-3:2002). PDF is a very broad format: it permits the creation of documents ready for web delivery through to very high quality book production. PDF/X simplifies what can be in a PDF to a known range of parameters. This known, and generally acceptable range therefore gives other software in the workflow a known target. If a PDF is PDF/X compliant, there are two keys added to the PDF file.

Print Workflow Changes

A topic deeply exposed here InDesign 2.0 Prepress Tips & Techniques, there has been a fundamental change in the mechanism InDesign CS uses to print placed PDFs. Placed PDFs (and therefore placed native .ai files, too) pass through a different print mechanism similar to printing through the transparency flattener. A side effect of this print mechanism is that elements are converted to the Print colour space (CMYK, Greyscale) plus a new side effect. The placed elements are Trapped.

With InDesign CS, placed PDFs pass through InDesign’s inbuilt Trapping engine. Now you can trap composite, untrapped PDFs from various sources (like QuarkXpress) and generate a composite trapped Postscript file, and therefore PDF. This technique still applies: InDesign 2.0: Generating Composite, Trapped PDFs

A small change, and probably not documented anywhere, is the ability scale in the “decimal point” range when printing. InDesign 2.0 had a restriction of scaling at print time in whole number increments (100%, 101%, 102% etc) whereas InDesign CS supports percentages such as 100.1%. This is especially required in packaging style printing on flexographic presses.

Summary

If you are into laying out documents with great design, InDesign CS raises the bar for its competitors. From a Prepress perspective, InDesign CS is distinctly ahead of the crowd.