[sane-devel] Avision: Non-standard papersize requires additional left offset?

[Crypto]

Hi,

I came across this while using gscan2pdf to scan a number of postcard-sized 
sheets.

Because the format is unknown to gscan2pdf (and the underlying scanner 
backend) at first I have entered the dimensions of the sheets manually.

The sheets are 105mm x 148mm in size.

My scanner is an Avision AV200 and I have noticed that the sheets are not 
scanned properly unless I add an additional left offset in the "Change 
Papersize" form.

The paper guides of the scanner's sheet feeder are mounted in such a way that 
all paper sizes that fit into the scanner sheet feeder are centered in the 
sheet feeder, i.e. that the center of the sheets always is positioned at the 
same vertical line in the middle of the available scan area.

BUT: I have noticed that the underlying scanner backend seems to assume that 
instead of a centered paper position the left side of the paper always is 
"line 0". I have found that because when I try to scan the postcard-sized 
sheets  I have a certain area of the sheets that is not scanned (the right 
half of it) and therefore I keep getting only about half way scanned sheets.

In the "Change Papersize" form I can enter an additional left offset of about 
56mm and this "moves" the sheets to the left but leaves an area of that size 
which seems to be unused yet scanned. It means that I can now scan the full 
area of the sheets but I get an area which is scanned unnecessarily and is 
wasted.

So why do I need to enter an additional left offset at all to scan smaller than 
A4 sized sheets? Why does the backend not automatically adjust the offset (if 
it is one, or bug?) to scan smaller than A4 sized sheets?

Thanks for any hint,
Crypto.