Jeffrey Ratcliffe wrote:
Default seems to be PNG, which isn't bad for B&W scans, and also for
scans with limited numbers of colours. As your scans were colour, JPG
would give the best size.
I did some experimentation along those lines, and also to explore your
earlier suggestion.
First of all, "a megabyte smaller than the uncompressed PNM" isn't
correct; my mistake. (Order of magnitude off; didn't count the places.)
Here's my original, done a few new ways. For gscan2pdf, I imported the
original PNM to ensure that the results would be comparable.
-rw-r--r-- 1 jeff jeff 1248907 2008-06-13 17:48 july-jpegcomp-2.pdf
-rw-r--r-- 1 jeff jeff 960727 2008-06-13 17:42 july-jpegcomp.pdf
-rw-r--r-- 1 jeff jeff 840424 2008-06-13 09:12 july.pdf
-rw-r--r-- 1 jeff jeff 4496216 2008-06-13 17:41 july-pngcomp.pdf
-rw-r----- 1 jeff jeff 26810741 2008-06-13 09:07 july.pnm
As you can see JPEG did do better than the default (PNG). The first was
done at 75% quality, the second at 90%. But neither was as good as my
command line.
For the second test, I scanned the front cover of this month's Linux
Journal, with a photo of Matt Mullenweg on it; this, I thought, would be
more "photo-like". Here's those results:
-rw-r----- 1 jeff jeff 26810741 2008-06-13 09:07 july.pnm
-rw-r--r-- 1 jeff jeff 3144781 2008-06-13 18:03 test-cmdline.pdf
-rw-r--r-- 1 jeff jeff 3558691 2008-06-13 18:00 test-jpegcomp.pdf
-rw-r--r-- 1 jeff jeff 22993707 2008-06-13 17:59 test-pngcomp.pdf
-rw-r----- 1 jeff jeff 26810741 2008-06-13 17:58 test.pnm
Again, PNG was dismal, JPEG respectable, and the command line won again.
Here's the command line I used to create that version:
cat july.pnm test.pnm | pnmtops | ps2pdf14 - test-cmdline.pdf
Note that at no time did I tell any of the utilities what kind of
compression to use. And it seems to make the right decision with all my
scans.
I don't think I'd quibble over the few kilobytes between JPEG and
command-line. But I don't think I'm the only one who will be taken
aback at just how big the PNG version is. The verdict from most people
won't be that "well, I need to tweak the settings to get it just right";
it'll be that "gscan2pdf sucks" or more likely "Linux scanning sucks".
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