manuals in pdf (resolution, compression)

From: Paul Koning <pkoning_at_equallogic.com>
Date: Mon Jun 28 09:33:27 2004

>>>>> "Tom" == Tom Owad <owad_at_applefritter.com> writes:

 Tom> On Saturday, June 26, 2004, Al Kossow, wrote:
>> I scan at 400dpi 1bpp and save the pages using group4 compressed
>> TIFFs which is the maximum res the IS520 scans at.

 Tom> I was doing 150dpi greyscale jpegs. I've changed to 400dpi 1bpp
 Tom> tiffs, and now I'm ending up with each page about 700K... (scan
 Tom> as greyscale tiff (unknown compression), convert to 1-bit with
 Tom> lzw compression, save as pdf in Photoshop).

 Tom> Using tiffcp to change the compression from lzw to g4 more than
 Tom> doubles the size to 1.6 MB. That's for a single page.

 Tom> Surely this is too large?

Quite possibly the scan is "dirty". Ideally you'd want the letters to
be black and the rest of the page white. More typically, the scan
process will put a scattering of black pixels elsewhere on the page,
for example due to "bleed through" from the text on the reverse side.

If the pages are pretty consistent in quality and color, you can tweak
the scan settings to help this, or you can achieve the same result by
post-processing in Photoshop or a similar tool. If it has
"automation" features (as Photoshop does) you can process lots of
pages quickly.

The trick is to look for a threshold setting for the black vs. white
threshold that results in minimal pixels on the page, but not so high
that the letters lose their shape. This is a compromise -- the edge
of a printed letter is not really sharp in a scan, so as you raise the
threshold some of the outer pixels change from black to white -- your
letter gets "thinner". If you can't get both a clean page and an
acceptable letter shape, then the source material isn't good enough to
support bitonal scanning. If so, you'll need a grayscale scan and
you'll have to put up with the larger file sizes that result.

       paul
Received on Mon Jun 28 2004 - 09:33:27 BST