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Adjust Contrast of Multi-page PDF

Posted: Fri Apr 05, 2019 2:01 am
by Brother Gabriel-Marie
XnViewMP can work with pdf files to an extent.

I have a collection of multi-page pdf files with lots of page thumbnails and bookmarks in them, but when they were scanned, they were scanned so that black isn't black - just sort of darkish gray, so it looks washed out.

Can I use XnView to adjust the contrast?

In the view toolbar, you can use buttons to navigate between pages and adjust the contrast, but when I try to save, MP tells me my "metadata will be lost" and then it replaces the entire pdf with the one page I edited, turning a 20mb file into 2mb.

Is there another way to do this - some way that actually works?

Re: Adjust Contrast of Multi-page PDF

Posted: Fri Apr 05, 2019 6:46 am
by cday
Brother Gabriel-Marie wrote:
Fri Apr 05, 2019 2:01 am
I have a collection of multi-page pdf files with lots of page thumbnails and bookmarks in them, but when they were scanned, they were scanned so that black isn't black - just sort of darkish gray, so it looks washed out.

Can I use XnView to adjust the contrast?
XnView MP batch convert (or XnConvert) will with suitable configuration on the Output tab convert a multi-page PDF directly to a multi-page PDF, so you could probably use a levels or other adjustment to get the enhancement you need...

In the view toolbar, you can use buttons to navigate between pages and adjust the contrast, but when I try to save, MP tells me my "metadata will be lost"...
I see there is an option on the Output tab to preserve metadata, so possibly not an issue, if you have metadata?

Re: Adjust Contrast of Multi-page PDF

Posted: Fri Apr 05, 2019 1:25 pm
by Brother Gabriel-Marie
XnConvert does a decent job adjusting the contrast in the pdf when I set it to Automatic Levels, and it will work on the entire multi-page pdf quite well, but any bookmark structure is erased.

I guess it doesn't know how to preserve the bookmarks structure; maybe that doesn't count as metadata.

Re: Adjust Contrast of Multi-page PDF

Posted: Fri Apr 05, 2019 1:41 pm
by cday
Brother Gabriel-Marie wrote:
Fri Apr 05, 2019 1:25 pm
XnConvert does a decent job adjusting the contrast in the pdf when I set it to Automatic Levels, and it will work on the entire multi-page pdf quite well, but any bookmark structure is erased.

I guess it doesn't know how to preserve the bookmarks structure; maybe that doesn't count as metadata.

I think the problem is that XnView and similar software are purely image processors so any other file content, such as OCR information or bookmarks, is inevitably lost...

With regard to OCR searchability, the simplest solution is probably to re-OCR the enhanced PDF file using any available software.

To both enhance the image content and maintain other content, you would probably need Adobe or similar software, possibly FineReader or OmniPage can do both??