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philb29378355
Participant
October 10, 2016
Question

Gray overlay

  • October 10, 2016
  • 1 reply
  • 3085 views

I work for the Wisconsin State Journal. We subscribe to a service that archives past issues. We've found recently that many of these PDFs have a gray overlay, aren't as sharp as they used to be, and are giant files (21 MB versus 3 or 4 typically). I assume it has something to do with how they're scanned in, but is there a way, after the fact, that we can remove what appears to be a screen over the pages? When I rapidly re-size the files (Control+wheel), the gray disappears for half a second and I see clean black-and-white text, so it must be some sort of layer. We've tried working with the folks who scan the documents to see if the problem is being caused on their end, but in the meantime can we do something as the end users?

Here are two snips: The first shows how the pages used to look, and should look; the second shows the gray overlay.

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1 reply

Karl Heinz Kremer
Community Expert
Karl Heinz KremerCommunity Expert
Community Expert
October 10, 2016

This looks like the first image was scanned in monochrome mode (black and white pixels only), whereas the second image was scanned in grayscale mode (e.g. 256 different levels of gray from white to black). This requires a lot more data than a black&white scan. You can very likely enhance the scans in Acrobat Pro using the "Enhance Scan" feature, which can create smaller files, and may be able to get rid of the background.

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philb29378355Author
Participant
October 10, 2016

That's very helpful, Karl. So where do I find the "Enhance Scan" feature? I agree with Dov, that the problem is in the scanning. We're working with the folks who do that to see if we can get it straightened out. But in the meantime, anything I can do to fix the PDFs I have would be better than nothing.

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philb29378355Author
Participant
October 11, 2016

This is very likely not a "layer" (especially if you saved to an image file and then re-imported into Acrobat in order to run OCR), but a side effect of using adaptive compression. This compression method is trying to analyze your document and then it will take the image apart and compress different pieces differently to get the smallest possible file size. It could be that it is separating the gray background from the rest.

Did you turn "background removal" on? You may have to turn it to "High" to see good results. That should remove the gray background (or at least make it a lot less obvious). Also, you should never save a document that is not a real photographic image as JPEG - you will end up with compression artifacts. Use a high-resolution (e.g. 600dpi) TIFF image instead.

The sample image you posted is not good enough to run OCR, but when I run it through the scan optimization with background removal set to high, I get this:

Keep in mind that this is still a grayscale image, and not a monochrome image, so it will still use up more space than a true monochrome document, but as you can see, the background is gone (and this was done with Acrobat X).


Thanks again, Karl; you've been very patient. I did try running "Optimize Scanned PDF" and turning background removal to high, but I get an error message that the "Page contains renderable text" and so it doesn't optimize. I get this message whether the OCR options box is checked or unchecked. It's like it's telling me that it can't optimize the document unless I also want to run OCR (which I don't). Here's what's in my optimize window. Am I doing something wrong?

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