Nemo, try to do an "autopsy" of cited IA pdf by pdfimages (xpdf) that recovers raw pdf images into its pages. You'll find that pages are exotically segmented into a full color background, a strange image, and an inverted image of thresholded image (I presume, used as a mask). Just negating the last one, you can get a decent, light BW image of the page. I could build from the last one a decent BW djvu image: https://it.wikisource.org/wiki/File:Paolina.djvu , but it.source users didn't like the idea https://it.wikisource.org/wiki/Wikisource:Bar#Pensiero_in_libert.C3.A0_sulle...
I presume that this complex structure is somewhat similar of djvu background/foreground segmentation into djvu files, and artifacts are similar.
So, pdf images are not only "compressed", but deeply processed and segmented images.
Anyway: IA image viewer doesn't use at all pdf (nor djvu) but uses jpg from jp2 files; so, if you need a djvu similar, for details, to what you see into the IA viewer, you have to download and process jp2 images to build a decent djvu file.
Is something of this complex IA image processing path documented anywhere? I got my conclusions simply by "try and learn" from IA file "necropsy".
Alex
2016-05-12 20:10 GMT+02:00 Federico Leva (Nemo) nemowiki@gmail.com:
Andrea Zanni, 12/05/2016 19:38:
[1] https://it.wikisource.org/wiki/File:Tarchetti_pdf.png [2]
https://commons.wikimedia.org/w/index.php?title=File%3ATarchetti_-_Paolina.p... [3] https://it.wikisource.org/wiki/File:Tarchetti_pdf.png
That was meant to be https://it.wikisource.org/wiki/File:Tarchetti_alex_djvu.png
I don't think this has anything to do with DjVu or PDF, the problem is very clear just by looking at https://archive.org/download/digitami_LO10534041 : the JP2 conversion compressed the images 30 times, the PDF compression 5 more times.
The first step in such cases, as documented in https://en.wikisource.org/wiki/Help:DjVu_files#The_Internet_Archive , is to add/increase the fixed-ppi field. I don't understand what was used in https://catalogd.archive.org/log/487271468
Nemo
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