Date   : Mon, 16 Aug 2004 21:30:13 +0000
From   : Jules Richardson <julesrichardsonuk@...>
Subject: Re: compression of scans

On Mon, 2004-08-16 at 16:39, W.Scholten wrote:
> Jules Richardson wrote:
> 
> > I can probably get a 20-30% reduction in storage space needed if I
> > adjust the black / white thresholds on the greyscale images - because
> > some of the source paper was pretty thin there's some text bleed through
> > from the opposite sides of pages; adjusting the thresholds should get
> > rid of that
> 
> Not quite, usually. For this you need to use black background when
> scanning, or an image analysis program.

Actually, some initial tests are looking pretty hopeful - setting the
black threshold to 50% and the white to 90% seems to do a good job of
boosting dark text without introducing jagged edges, whilst getting rid
of any bleed-through from text on the reverse side.

File size seems to end up about 60% of what it was but text still has
nice smooth edges. Obviously I can't do that for colour pages (such as
manual covers and advertising), but it seems to be OK for text without
any loss of apparent quality.

> I only use unix (FreeBSD/OpenBSD) so if you're interested, you'd need to
> compile it for whatever OS you use.

95% unix here too...

>  It also only handles gif as it was
> easier to use the IO routines and as tiffs are stored by most program
> uncompressed. I use a simple script to convert all tiffs from the
> scanner in a given directory to gif (Image magick), then enhance them.

Yep, I'm using imagemagick for sorting these images out as I can just
script the whole lot - fantastic bit of software (there's a Windows port
for anyone needing such a utility, but of course scripting anything
under Windows is next to impossible :-)

Much easier than having to load every single image into a graphics
package to alter them anyway (particularly as for A5 booklets I've
scanned 2 to a image, as though they were an A4 sheet - with a script I
can just rotate them, crop as necessary, and split them down the middle
into two seperate files)

Only issue with using gifs rather than tiff is the limitation on colour
depth, so they're not the best for colour advertising and the like. Fine
for greyscale images though... (and anything's better than jpeg!)

cheers,

Jules