[sane-devel] Ideas Wanted [Acer ScanWit]

[Max Ushakov]

Hi,

I am still playing with Acer ScanWit 2720S film scanner and I
found out some protocol details that allow me to scan. I am
about to start writing a quite restricted SANE backend for this
thing. Though, there are some things that are still quite
unclear to me. I wonder if some of you have any ideas of what
could be happening. So, I list these problems here.

1. Scan speed/mode. When I issue a SET WINDOW command and set
the resolution to 2700 (maximal optical resolution of the
scanner), it scans at speed comparable to what I get with
existing drivers (windows and VueScan). Then I reduce the
resolution to 1350, and the scan is almost as slow as before.
Why?

2. (somewhat related to 1) When I reduce the resoultion even
more -- to 675 -- the scan *still* takes almost as much time as
the scan at 2700. Moreover, the resulting file, while having
proper dimensions, contains only part of the image; plus, it
seems to have been stretched in vertical direction (every row
doubled or like that). Did anyone experience similar effects on
some other scanner?

3. I traced VueScan's SCSI dialog, and figured out just about
every command it issues, except for one part. Maybe it is
something common for scanners, I just have little experience
and/or knowledge. The protocol is like this:

   1. RESERVE
   2. Set focus
   3. SET WINDOW
   4. Upload gamma tables 
   5. INQUIRY
   6. [strange part]
   7. SCAN
   8. Set frame holder to desired frame
   9. READ....
  10. RELEASE

The ``strange part'' 6 looks like this:

   READ with DT=0x80 and DTQ=0, 61056 bytes
   READ with DT=0x80 and DTQ=0, 0 bytes

   READ with DT=0x82 and DTQ=0x10, 7650 bytes
   (repeats 15 times)
   SEND with DT=0x82 and DTQ=0x01, 7650 bytes

The data sent and received had little meaning to me -- it
contained a block of bytes near to 0xff in one place and a block
of bytes near to 0x3c in another.

What could this be?

This could not be the autofocusing sequence because I already
know how to do autofocus.

These are the main problems I have. I also have one question:
where should I start writing the new backend? It seems a good
idea to take some existing one as a template -- but I don't know
what...

Thanks for reading all this,

--Max Ushakov.