[sane-devel] EpsoPerfection 640U: skewed scan in binary mode, 600dpi

[Christoph Ludwig]

I have an Epson Perfection 640U and use it frequently to scan documents
in binary mode at 600dpi. The SANE backend is libsane-epson.so.1.0.4.

Sometimes the resulting image is skewed. Each line seems to be shifted
to the right, relative to the preceding line. If I set SANE_DEBUG_EPSON,
I get messages similar to the following:

[...]
> [epson] sane_read: begin
> [epson] sane_read: begin scan1
> [epson] receive buf, expected = 6, got = 6
> [epson] sane_read: buf len = 551
> [epson] sane_read: buf len (adjusted) = 32509
> [epson] receive buf, expected = 32509, got = 32508
> [epson] send buf, size = 1
> [epson] sane_read: begin scan2
> [epson] sane_read: end
[...]

The problem seems to be that the backend receives one Byte(?) less than
expected, since I do not observe such a discrepancy in the log when the
scan succeeds. This phenomenon affects about half of all scans in binary
mode at 600dpi, but it is not really reproducible. Some days most of the
scans are successful, some days most fail. Neither powercycling the
scanner nor restarting the software helps. There is no problem with gray
or color scans or at a lower resolution.

Has anyone an idea what to do about the problem?

Thanks

Christoph
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