OPI is a nowadays outdated technology that allowed the embedding of low-resolution versions of images in a file. This lead to savings in storage and network bandwidth. The smaller images would be swapped out with the original images whenever the high-res data are needed.
PDF supports OPI since version 1.2 (Acrobat 3). From PDF 1.3 onwards, both OPI 1.3 and 2.0 comments are supported. PDF files containing OPI-images are commonly referred to as ‘thin PDF files’. PDF files containing only high-resolution images are sometimes called ‘fat PDF files’.
I haven’t used OPI in the past 20 years so all of the information below is old and just kept for sentimental reasons.
Using PDF in an OPI workflow with InDesign
Apparently InDesign releases up to CS2 allowed you to place an FPO PDF in an InDesign document and get that document processed properly by an OPI system. From CS3 onwards InDesign strips the OPI-comments for any placed PDF file. You have to use EPS FPO’s to get OPI working again – something I learned in this B4print thread.
PDF support in OPI systems
As far as I know, there are no OPI-systems on the market that provide true support for PDF, meaning that they can pick up a PDF from a hotfolder, perform image replacement on all OPI-referenced images in the file and write a fat PDF to an output folder. The systems I know are only capable of performing image replacement within PostScript files.
In case you receive a PDF-file that contains OPI-images and you also get the highres images, it may be worthwhile to know that there is a plug-in on the market that allows you to relink the images within the PDF file. This plug-in is called OPI-Doctor.
Why use OPI within PDF?
I have had a lot of discussions with people regarding the advantages and disadvantages of using thin PDF. Its only advantage is file size. Thin PDF files can be extremely small and quick to process.
But there is a long list of disadvantages of using OPI within PDF-files:
- One of the advantages of PDF is that you can visually check the files using Acrobat Reader or Exchange. But if you have an OPI-workflow using ‘omit images’, the PDF file will only contain OPI-references and you will not be able to view the file properly.
- PDF files can be quite small due to the excellent compression options it offers. So using OPI to limit the filesize of PDF-files is often not necessary.
- Preflighting software or PDF-editing tools are often useless with thin PDFs because the files don’t contain all the final data.
* Using Distiller a great way of eliminating possible PostScript errors early on in the production chain. This advantage disappears when an OPI-solution can still insert corrupted images or incorrect PostScript code after having processed the clean PDF-file.
All of these arguments prove that thin PDF is not always a great idea. In a lot of cases, it makes more sense to keep using OPI but to have the OPI-system deliver fat PostScript to Distiller so that a fat PDF can be processed in the workflow.
When OPI is not OPI
Some applications like QuarkXPress 4.x tended to add OPI-comments to their PostScript data, even if no OPI is used. This was typically done for any TIFF-files placed in the document.
Why such applications would do this, is completely beyond me. But this approach does have a nasty side effect: some pre-flight applications like PitStop consider it useless to flightcheck images that contain OPI-comments. This means that they will not pre-flight those TIFF-files even though the full image data are available.
To avoid this problem, you should always deactivate the ‘Preserve OPI Comments’ option in Distiller if no OPI is used. This way such OPI-comments are deleted while creating the PDF-files and post-processing tools are not confused by irrelevant OPI-comments.