Date   : Fri, 03 Aug 2001 20:13:47 +0000
From   : "W.H.Scholten" <wouter.scholten@...>
Subject: djvu quality, scan resolution etc.

Mark Usher wrote:

> OK, I have put the images up now.
> There does seem to be some loss by djvu, so that seems to be out of the
> running to.

Not really. Scanners are not perfect, pages yellow with time etc. It's a
matter of making images that are 'good enough'.

Considering the djvu filesize, the result is pretty good.

> I did try the PNG format back at the start of my investigations, but wasn't
> really getting any smaller file sizes than with LZW compressed .tiffs

Right. With my (grayscale) PCW scans that I reduced the palette of, PNG
in some cases provided smaller, in other cases bigger files.

Regarding resolution: I found that 600 dpi is about the minimum to show
all the detail that's present in PCW and MU pages (some very small
letters on a cover from IIRC nov. 1983 that I tested with were not
readable at 300 dpi). At 600 dpi the filesizes are pretty big. About 1MB
grayscale LZW tiff using a reduced palette (smoothing the images out a
tiny bit, also I find B&W to be too ragged) and 2.5 to 3MB for jpg at
90% quality (which gives almost no visible jpeg artifacts). This means
just about 2 to 3 issues would of MU would fit on a CD...

Perhaps that's the way to go for now? (that's what I've done for the
sections I've scanned from PCW, around 100 pages or so).

Btw, I'm not going to disect my mags Mark! If anyone is willing to
donate mags for this purpose then that's the way to go, but who'd want
this :)
A bit of a problem is that the scanner I'm using has a bad depth
perception (canon canoscan 630), which means the inner edges get a bit
blurry. Well, you'll be able to see all this when I send another CD to
Chris (8bs). I'll make some small images for my webpages too.

Chris Thornley alread mentioned that magazines use just a few colours.
This is clearly visible with high res scans (red/yellow dot pattern ->
orange for example) so a way to reduce the filesizes would be to
'prepare' them before png/giff ing them. I.e., remove the slight
rotation always present, and then let colours only have one of a small
palette (say 8 colours? haven't actually counted the number of colours
used in MU). This could give enormous compression like that PNG of the
exile map...

A crude version of this is saving in gif format....

Of course someone would have to write such a program, as I don't think
such a program exists (or does it?).

Wouter