[sane-devel] data noise when scanning wide areas

[Brett (Mare) Henley]

Hello,
  I've been noticing an interesting problem that I'm having difficulty
pinning
down.
  When I'm scanning an image in, if I use a fairly high resolution I get
horizontal lines of noise their frequency increases and decreases with the
amount of data being transfered.
  The wider I make the scan window the more noise lines I see, the narrower I
make it the less frequent they become. If I keep the res and width the same
and increase the width of colors ie from 8 -> 10 bits the lines increase.
  it seems as if data is being lost or there's some kind of memory glitch
that
is messing up the data in blocks.
  Here are my system specifics:

550 Mhz PIII, running FreeBSD 4.6.2, with 512 megs of ram.
Adaptec 1542CF scsi interface.
HP C2520A scanner
sane 1.0.9pre2
xsane 0.87

Everything seems to work fine and I've tried moving the scsi card around
as in
irq's and DMA channels. I've increased the job priority. Pretty much
everything I can think of.

These noise lines or bands appear consistantly wether I'm using xscanimage,
xsane, or even hpscan.

I believe it has something to do with how sane buffers the data coming from
the scanner. But I don't have the skills to dig deeper at the moment.

The following is a basic idea of how this manifests.

using 8bit depth

300 DPI band every 100-150 vertical pixels.
600 DPI band every 30-40 vertical pixels.

using 10bit depth

300 DPI band every 20-30 vertical pixels.
600 DPI band every 4 vertical pixels.

I have tried enable-image-buffering as an option in the hp.conf file.

Any help would be appreciated.

TIA

Mare