As far as the PDF file Vincent sent to me in concerned, the problem is
that Poppler tries to scale a binary image from 2566x3340 to 2566x3341.
(Which is a bit odd, because the resulting page size is 2566x3340. I'm
not sure yet if that's a bug in the PDF, or in Poppler.)
This worked uneventfully until bilinear image scaling was implemented in
Poppler:
http://cgit.freedesktop.org/poppler/poppler/commit/?id=7248da095f67d57c3999cee7d980e62fd8a7cf49
https://bugs.freedesktop.org/show_bug.cgi?id=22138
Bilinear scaling introduces lots of gray pixels, which pdf2djvu tries to
preserve, which has disastrous effect on the compression ratio.
I think I could disable bilinear scaling in pdf2djvu, although I'd have
to double check.
But perhaps Poppler should be fixed not to use bilinear scaling when
source and destination sizes are very close to each other. I'm not sure.
--
Jakub Wilk
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