[sane-devel] Setting black/white threshold on ScanJet 7450C with avision backend

[TomF]

My ScanJet 7450C scanner now works on my Redhat 8.0 system with avision
backend.

I found http://www.xsane.org/doc/sane-xsane-doc.html and learned what
line-art mode means.  When I scanned in line-art, I got the positive
black and white image that I wanted.  

However, the black/white threshold does not look right, and I could not
find a way to adjust it.  The doc file mentions enhancement options, but
I could not find a way to set the options to make the threshold
adjustable.  I executed scanimage --help -d avision:/dev/scanner, but
found no options which appear to be related to the black/white
threshold. 

Is there any way to do this?