# Re: Backtracking in color scanning

Rogier Wolff (R.E.Wolff@BitWizard.nl)
Tue, 2 Sep 1997 18:07:43 +0200 (MET DST)

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shendrix@gypsy.atlantic.infi.net wrote:
>
>
> In message <19970901104750.53420@drawbridge.in-berlin.de>
> woju@keep.in-berlin.de (> ) writes:
>
> > Just calcualte the Data the scanner needs to transfer.
> > As Far as I could find the Mustek Scanner justs uses async transfer
> > to the HOST which could be as slow as 1.5MB/S (even if you Adapter
> > could get up to 7MB/s )
> > An tiny Area of 1" times 1" brings you with 24 Bit and 1200 dpi Color
> > into 4320000 Bytes of Imagedata. if the scanner is able to scan that within
> > 1 Second the scanner needs to stop to get rid of the
> > Data.

I did some calcualations, but my scanner never hits the data transfer
rate problem. It seems to get slower at higher resolutions all by
itself.

>
> Well, under Windows95 it usually never did backtracking. When it did,
> it only backed up half the number of times it does with sane. This
> was with the little cheap controller it came with too.

So, they are using a buffer that is twice as large.

> There had got to be a way to get it as fast as under Windows. My scanner
> is backtracking every half inch sometimes. Under Win95 it always moved at
> least a couple of inches before backtracking.

I'm suspecting that a Windows 95 driver may be able to do some tricks
like scatter-gather that are not fully implemented under Linux yet.

If W95 allows scatter-gather, the application allocates a megabyte of
memory, requests a read from the SCSI device, and the scatter/gather
will get the data into the user-pages in the right order.

Under Linux, there is a sg_buffer in kernel space, which is contiguous
so that devices may DMA into that area, and then the data is copied
over to userspace, using the MMU for the pseudo-scatter.

Microsoft operating systems have a vague distinction between
user-space and operating system. Because of this, there are also
some barriers that don't need overcoming under MS software.

Roger.

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