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Beware Stacked Transparency in PDF Files Across Long Documents

The Problem

I recently encountered a PDF export problem that plagued me for months. I was trying to export a book containing roughly 50 InDesign documents. The last 25 or so documents contained scanned engineering drawings placed as graphics into the InDesign documents. When I tried to export a PDF from the InDesign book file, the PDF would seemingly be exporting just fine… until InDesign reached pages with some of those scanned images. Then it seemed to randomly hang. Fortunately, I finally found the source of the problem, as well as a fix.

But first a little background…

I love Acrobat commenting. I like to use PDF comments to mark up and color-code the engineering drawings that people send me. I especially like to highlight text in the PDF using various colors of highlighters. And highlighting text works great when the documents are original digital files (not scanned), because Acrobat can only highlight live fonts. So, if you want to highlight a picture, outlined text, or scanned text, you’ll need to use a tool other than the Acrobat’s highlighter tool.

I developed a method for highlighting objects using Acrobat stamps. That technique works great, but it’s a little cumbersome to get all the stamps set up properly. So lately I’ve starting using Acrobat’s drawing markup tools with different colors. To “highlight” a scanned image, I simply draw a colored box (colored fill, no stroke) around whatever I want to highlight, and then reduce the opacity to around 60%. It makes the text a little harder to read, but it’s definitely easier than using dozens of custom stamps (which do not scale non-proprtionally; i.e. a rectangular stamp stamp can only be rectangular, and never a square).

I was working along just fine and then…

So, a few months ago, I had marked up some drawings using the Acrobat shape tools, adjusting the opacity of each shape as needed. Everything seemed to be fine… until I tried to include that commented PDF in a InDesign book project.

Now normally, I can comment on a PDF and subsequently use that PDF in an InDesign document without any problems. Because comments do not appear when you place a PDF into InDesign… or do they?

Here is the drawing as it appeared in Acrobat. I replaced the original scanned engineering drawing with flattened text of the PDF taunting me. Note the various colorful drawing markups. All the of these drawing markups are at 60% opacity.

AcrobatTauntingMe

Commented PDF viewed in Acrobat

 And here is the preview when I place the document into InDesign. Note the lack of drawing markups (PDF comments).

PlacedGraphicPreviewInInDesign

When I try to export a large book containing this placed PDF, InDesign always hangs. I can export just this page, but for some reason, InDesign cannot process this graphic when it is at the end of a large book. The culprit? Stacked Transparency.

Do you remember how I had to change the opacity to 60% so that I could see the text beneath? This is what the problematic PDF looks like in Acrobat, when zoom in really close.

StackedTransparentRectangles

PDF with rectangles made using the Acrobat Drawing Markup tools, with opacity set to 60%

Since these rectangles are all touching, the result is multiple levels of transparency, all stacked up on top of other. This stacked transparency is what was causing InDesign to hang. As an experiment, I went back into Acrobat and changed the opacity of the touching rectangles back to 100%.

StackedRectanlgesWithNoTransparency

PDF with rectangles made using the Acrobat Drawing Markup tools, with opacity set to 100%

Once I adjusted the PDF comments so that there were no more transparent objects touching one another, InDesign was able to process the PDF as part of the book file without any problem. (Note that I did not have to adjust the opacity of the small circles, because they were all free-floating, not touching one another.)

I think the reason it took me so long to pinpoint the problem was because PDF comments do not show up in InDesign. There’s not even an option for making them show up as part of the placed graphic. It’s almost like InDesign doesn’t even know they are there. At least, that’s what I thought. But apparently, stacked transparency, even in an object that InDesign doesn’t render on screen can wreak havoc on a PDF export — perhaps not in single documents, or small books, but when some larger number of these files need to be exported at the same time.

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6 Comments on “Beware Stacked Transparency in PDF Files Across Long Documents

  1. Wow!! Great heads up!
    It seems that most of my crashes or hangups have been font related…. just another thing to watch out for.

    Thanks Kelly

  2. I’ve never tried flattening the PDF, as I would lose all my PDF comments (in that they wouldn’t be true PDF “comments” anymore). I imagine that would fix the problem, though.

    • Don’t you lose the comments anyway when you process commented documents in ID? (yes, I have no clue about ID…)

      On the other hand, it is possible to transfer comments from one document to another; a little bit of JavaScripting, and you could get them back alive.

      • I was using the original scanned PDF for two purposes:

        Purpose #1. For my own research purposes: I color code all the information in Acrobat, using PDF comments. I do this so I can wrap my brain around all the content, figuring out everything I need to know about that particular mechanical system.

        Purpose #2. To be included as a graphic within a large book: Because this engineering drawing is to be included as part of a large printed manual, I then take the engineering drawing and place it into an InDesign document. That InDesign document is then included in an InDesign book. I do this so I can put headers, footers, and some other text that will allow me to include the InDesign document in a table of contents, add cross references, etc.

        So because I was really multi-purposing this scanned. I’m not concerned about losing comments when I export the scanned PDF from InDesign. Because when I export it from InDesign, the resulting PDF has a completely different purpose than the PDF from which I originally started.

  3. Impressive troubleshooting! I am VERY impressed. I get to do the troubleshooting around my workplace, and I don’t think I could have figured out this one. But I will try to keep it in mind.

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