Generating Composite, Trapped PDF from InDesign
Well created, Composite PDFs are the most commonly requested format for printers in this part of the world. So, how exactly do you generate a composite, trapped PDF?
InDesign 1.5.x, 2.0.x and CS have support for Application Built-in trapping. The trapping engine inside InDesign matches many of the features of Adobe’s InRIP trapping engine; usually found in higher end RIPs from Creo, Heidelberg, Agfa etc.
Trapping is the process of changing the shapes of certain objects in a printed output to reflect the way the ink will run when printed. Each of the inks are laid down on the paper by different cylinders of the press. As the paper (substrate) are impressed by these cylinders, there might be a misregistration – where the inks don’t quite line up. In this instance, the plates contain extra overprinted areas of dots to cover up any unsightly gaps that may be left in the final output.
Highend PDF workflows today are bedevilled by the “Who is trapping this?” question. In controlled workflows, this is easy to coordinate. Prepress know which press a job is running on, and apply some visual checks on the job and may make trapping changes to the source file prior to output. However, when there is a hand-off to an unknown press and printing environment, the creator will not know the press conditions. Today’s digital workflows largely assume that the final RIP (imagesetter/platesetter) will trap the document.
In some workflows, this trust may not be enough. We need to make composite PDFs for onscreen viewing, simplicity and size; yet trap them so there are no ugly white patches at print time.
With InDesign 1.5, 2.0 and CS, the Adobe trapping engine takes InDesign created object (text and vector objects) and traps them to placed images, text and vector shapes. InDesign does not trap placed PDF or EPS objects. These are assumed to be trapped. InRIP trapping solutions will trap the entire page stream. To use Adobe In-RIP Trapping, you must use In-RIP separations. Built-in trapping limits trap widths to 4 points, regardless of the value you enter for the trap widths. For larger trap widths, you will need to use Adobe In-RIP Trapping.
From QuarkXpress, the trapping information as setup by the Trap palette only comes into play when printing separations. With QuarkXpress 4.0 and above, overprinting and knockouts as set by the palette are retained in composite Postscript output. Saving Pages as EPS from QuarkXpress 3.32 and higher does result in overprints/knockouts being retained. (see QuarkXpress, PDF, Trapping and Overprint)
InDesign 1.5.x, 2.0 and CS preserve knockout/overprint attributes in Composite Postscript output. As you would expect, Adobe’s Applicaton Built-In trapping works when printing separations.
Back to the topic at hand: A significant workflow difference with InDesign is that it can also apply Application Built-In traps when printing InRIP separations.
What are InRIP separations? Essentially, programs like InDesign send composite Postscript to the RIP with some extra commands telling the RIP to take a completed page and produce n-plates based on the colourants on the page. For simple process jobs, this would result in a page each for C, M, Y and K.
The implementation of the Postscript
showpage and copypage operators have the capability of producing pages for each colourant. If the output device doesn’t have this colourant, then the alternativeSpace colourant is used; failing this, DeviceCMYK is used. There is a Postscript engine inside the Distiller, and it yields DeviceCMYK colourant output.
Acrobat Distiller has a Postscript engine inside: what does it do with InRIP separation-marked Postscript? Distiller 4.0x and 5.0.x ignore the operators for producing pages for each colourant, and produce a composite PDF. The colours stay as CMYK (or spot colours if used) if the Distiller options are left to “Leave Colour Unchanged”
Now InDesign 1.5.x and 2.0.x allow you to apply Application Built-in trapping to the composite Postscript when printing as InRIP separations. Yes, I know its a little mind bending! If you then send the resulting Postscript to Acrobat Distiller, the Distiller discards the commands to separate, but it does retain the extra “trapping” Postscript commands. The final PDF holds these trap commands, and they can be viewed in Acrobat 5.0 by turning on Overprint Preview.
The final outcome is a Composite, Trapped PDF.
1. Ensure that your Trapping settings are correct. Please consult the InDesign CS Printing Guide, the manual and your Prepress/Printer before assuming the defaults are correct.
2. Assign a Trap Style to the page. In this instance, I am using InDesign’s [Default] Trap style as viewed above.
3. Here is the File>Print dialog box. In Output, Color is set to In-RIP Separations, and Trapping is set to Application Built-In
4. To make it easier to use the same settings on the next document, you can save a Printer Style for later use.
5. This is the source document in InDesign 2.0/CS
6. The final PDF inside of Acrobat 5.0 or 6.0, (available here: Example CompositeTrapped PDF)with Overprint Preview turned on. The darker areas around the type etc. display where overprinted strokes have been applied based on the inking requirements.
For more information about Trapping, and specifically the Adobe trapping engine:
Thanks to Steve Amerige, Matt Phillips for their assistance with some of the finer details.