PDF image covers are inverted in thumbnails

Hi all,

One of my colleagues batch uploaded a number of PDFs of our university magazine, but the thumbnails show the negative of the cover. However when we go to the direct file, the PDF displays normally in each record. Is this a case with having to manipulate something within the PDF files themselves or is my thumbnail plugin acting up? I’ve never come across this issue so far, so I’m wondering if anyone else has and if they have any easy fixes.

Thank you all for your help!


I met the same issue here today. After some research, I think it’s due to how ImageMagick handles the pdf file with the color model in CMYK. As long as the pdf file’s color mode is RGB, the generated thumbnail will be fine. So the solution can be either convert the original pdf’s color mode to RGB or switch the setting of “fileDerivatives.strategy” in “application/config/config.ini” to a different value. BTW, The “Omeka_File_Derivative_strategy_GD” can not generate thumbnails for pdf files properly.

My recollection is that the last time this came up, I wasn’t able to reproduce the problem. So it’s at least partially just an issue of versions of ImageMagick and/or Ghostscript fixing this problem.

I’m happy to try again if you have an example problematic PDF.

We started having this issue after upgrading to 2.7.1 late last year. Any PDFs uploaded since then (all greyscale) have the inverted thumbnails. Here’s an example: https://digitalarchives.sjc.edu/items/show/7515.

This is another one where the ImageMagick setup I have just does it correctly, hmm.

2.7.1 didn’t really change anything about this so it may be some other change on your server or otherwise (or with the PDFs themselves) that’s caused the difference for you.

Not sure whether it helps you, but I’ve tried to upload the same PDF and I got your same result (inverted thumbnail). Since all my other PDFs are showing correctly, I would vouch with some kind of issue with the PDF itself (since John did not experience the same issue, it could be that something in the PDF is triggering something else in ImageMagick specific version…).

I think the common factor is that the “problem” PDFs are using CMYK color, but what I can’t figure is what’s the difference in server setups that causes some people to get this inversion and not others.

It’s probably either ImageMagick or Ghostscript… I would probably guess ImageMagick first but Ghostscript is also possible.

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