# Re: Backtracking in color scanning

Rainer Krienke (krienke@mailhost.uni-koblenz.de)
Fri, 29 Aug 1997 22:28:35 +0200

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David Mosberger-Tang schrieb:
>
> >>>>> On Fri, 29 Aug 1997 17:04:01 +0200, Rainer krienke <krienke@mailhost.uni-koblenz.de> said:
>
> Rainer> When I scan color images at resolutions of say 300dpi the
> Rainer> scan process works right, but is very slow. This is because
> Rainer> the scanner scans some line and then goes back (that's
> Rainer> backtracking if I understood it right) and then scans the
> Rainer> next lines. Looking at the scanner it seems the the way the
> Rainer> scanner goes back is just a little smaller then the way it
> Rainer> has scanned before. Scanning a paper of size DINA4 (~30cm at
> Rainer> length, ~21cm wide) at 300dpi takes about 2min and 45
> Rainer> sec. If I scan at 100 dpi the effect is basically still
> Rainer> there but the sanner then scans a larger area in one step
> Rainer> before going back only a little. So the whole thing works
> Rainer> faster.
>
> Rainer> What suprises me a lot, is that under windows doing the same
> Rainer> scanprocess at the same resolution (via the mustec-twain
> Rainer> driver) does not cause backtracking to occur and is
> Rainer> completed much much faster (not even half the time).
>
> What size scan-buffer are you using? If you're using the default size
> of 32KB, could you try increasing it to 127.5KB? See sane-scsi(5) for
> info on how to do that.
>
> If increasing the scan buffer size to 127.5KB doesn't help, the
> backtracking could be due to timing problems. If the scanner software
> is too slow to issue the next "read data" command then backtracking
> would occur. What kind of computer are you using and how much memory
> does it have?
>
> --david
>

As I already told in my mail. The scanbuffer is 127K large (in Kernel
and sane).Scanning works allrightfor me (the results are allright but
not the scantime). My computer is a 90Mhz Pentium it has 48Mb RAM
installed. So it cannot be the buttleneck. But I really think it's got
to do with data xfer since it does not happen when scanning at a lower
resolution or in Lineart mode where less data has to be xferred.

Could it be that its because the scanner and the harddisk on which the
data has to be stored is on the same controller (ADAPTEC 2940) like the
scanner so that there is a problem.

By the way I really do not understand why backtracking is needed after
all, cause I don't know whats really happening. Can someone give me a
pointer to information on this ?

I think I did not say this but it should be said: Sane is a real great
piece of software besides all problems that there are (naturally), or
has anyone seen a windows release without bugs ....

Rainer

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Rainer Krienke krienke@informatik.uni-koblenz.de
Universitaet Koblenz, http://www.uni-koblenz.de/~krienke
Rechenzentrum, Voice: +49 261 9119-641
Rheinau 3-4, 56075 Koblenz, Germany Fax: +49 261 9119-499
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