[sane-devel] GT68xx backend development

[Sergey Vlasov]

Hello!

The updated version of the program which eventually should become a
backend for GT68xx-based scanners is available at:

  http://www.mivlgu.murom.ru/~vsu/gt68xx/

Currently this is not a SANE backend, because it is easier to test and
debug it as a standalone program.  At this stage it is for developers
only.

What is working currently:

- Basic scanning on the Mustek BearPaw 2400 TA (1200 TA was also reported
  to work with the previous version).
- Line distance correction is implemented (I have determined the
  parameters for 2400 TA; the numbers for 1200 TA have to be filled in).

Calibration is still unimplemented, so the scanned images have very poor
quality.

Also there are some major speed issues at 600dpi 24-bit scans (lots of
backtracking).  The problem is that GT68xx chips have a very small buffer
(16 KB), therefore any delay in reading the data causes buffer overrun and
backtracking.  Even at 300dpi 24-bit scanning there are problems when
there are some other processes running.

I have tried to overcome the problem by forking a second process which
reads the data and passes it to the main process through shared memory.
However, this does not help much for 600dpi 24-bit scanning - it seems
that scheduling latencies are too high, and the only way to solve the
speed problems is to modify the kernel driver.

-- 
Sergey Vlasov