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_sull…
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(a)gmail.com>om>:
Andrea Zanni, 12/05/2016 19:38:
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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