[sane-devel] Insights from a newbie backend writer...

Fred Odendaal freshshelf@rogers.com
Fri, 08 Apr 2005 23:39:14 -0400

So, I'm thinking "I've got a degree in comp. sci. How difficult can it 
be to write a scanner driver?" I leaf through the sane documentation - 
looks simple enough! "Okay", I decide, "I'll do it!" Then I start 
looking for information about my scanner - a Lexmark X1185. I start 
with  Lexmark. They've got a Linux developer's kit. "Oh wow! This is 
going to be easier that I thought...".


The developer's kit only works with specific models of Lexmark scanners 
- not mine, and is only compliant with really old versions of RH Linux. 
I'm using FC3. The kit includes libraries for the low level drivers 
(they won't supply the source code) and C++ code to interface between 
the low level drivers and the sane backend code. "Yuck!"

Okay, back to the drawing board. I go back to Lexmark and read their 
spec. sheet. Later, I find you really can't trust the information in the 
spec. sheet, which I find out after 4 rounds of emails with their help 
desk - I don't think they really understood  the difference between 
resolution and colour depth. The spec. sheet says 48-bit colour and 
18-bit gray scale. Well, the scanner never returns enough data to 
support this. Lexmark's claim is that the scanner does this bitdepth 
internally and picks the best results from the sample, but only sends 
24-bit colour or  8-bit gray scale back to the computer.

Oh, as another interesting side bar - black and white scanning is not 
really black and white. It's a gray scale scan that gets converted to 
black and white. Also, the Windows driver lets you select a myriad of 
resolutions from 72dpi up to 9600 dpi. The scanner really only does 
75x75, 150x150, 300x300, 600x600, 600x1200 dpi scans.

So, eventually I find enough information from various sources. I found 
the part number (rts8852C) of the scanner chip from the sane pages and 
the best usb sniffer to use from this mailing list. As I dug a little 
further I found there has been a lot of work already done on the 
predecessors to the rts8852, the rts8801 and rts8891. I'm not sure why 
this isn't part of the sane backends yet, but there is a sane web page 
at http://hp44x0backend.sourceforge.net/ for several HP scanners that 
use these chips.

Finally after many Windows scans and log files at various different 
sizes, resolutions, and colour or gray  scale mode I start writing the 
backend. It only took me about 2 weeks before I could scan on Linux, but 
this is the easy part! What I soon discovered is something called 
CALIBRATION. At 600x1200dpi colour resolution the Windows driver 
actually does ~20 scans off to the side of the glass flat bed. I have no 
idea what its doing. By looking at the usb logs it appears to be setting 
the gain and the dc offset in a gradual fashion over several calibration 

Now I'm faced with a dilema. I ask myself - "Do I continue and try to 
reverse engineer the Windows driver?" "Is this even legal?" I really 
can't see progressing any further without taking a peak, but I'd really 
rather not get sued. I've already looked at the .ini file used to 
configure the driver. I wish I'd found it earlier. It's got a lot of 
valuable information in it, which confirms what I've said above...

I'd really like to continue. It's actually become a bit of an obsession 
with me, but I'm also a realist.
Anyone else faced a similar conundrum?
Fred Odendaal