# Re: Gray Mode of MFS12000SP

Wolfgang Jung (woju@keep.in-berlin.de)
Wed, 10 Sep 1997 22:50:37 +0200

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On Wed, Sep 10, 1997 at 09:05:18PM +0200, becka@sunserver1.rz.uni-duesseldorf.de wrote:
> Hi !
>
> > But if I tell the sane to use 1 line/buffer the scanner scans for a 10x10nn
> > area an awful lot more (about 16 times as much :( ).
>
> > It seems to me that the the follwing happens.
> > ....READ_DATA(lpb)
> > Scanner scannes lpb plus some additional lines (estimate of 16)
> > Scanner delivers data (maybe it delivers all the data)
> > sane just read lpb from the buffer
> > ........
>
> > between buffers the Scanner stops for a moment.

Sure, but at least in gray & lineart it does no backtracking as it does
with color mode.

While testing I was able to get the Scanner to backtrack in gray mode
as well. but it seems the PAUSE reading needs to be long enough for the
backtracking to take place.
in normal Gray operation the scanniner just stops for a moment and
goes movving again. But if it stops without doing backtracking it seems
to do some funny things with the scanned data (I supect it does some sort
of postscanning, but sane does not know about it. maybe it is an option
which can be set with the MODE SET command)

>
> This can be right. The scanner works in the following manner (this drawing
> should go to the FAQ):
>
> -snip-snip-snip-
>
> Q: Why does my scanner backtrack and what is backtracking ?
>
> A: Backtracking are those little backward steps the scanner makes when a
> SCSI buffer has been filled. You recognize them a "stops" in the scanning
> process and if you watch closely, you should see the slider move back a
> bit. This has technical reasons :
>
> R L P The scanner has 3 CCD Elemets, 1 for each color. According to my
> \ E / A measurements they are 1/72" apart optically "on the paper".
> G---*---P Thus when the scanner wants to acquire a 1"x1" area, it needs to
> / N \ E travel yet another 2/72" to get the whole image in all colors.
> B S R Effectively 3 images R/G/B with a displacement of 1/72" are
> scanned simultaneously.
>
> At least for the MFS 12000 both possibilities to correct this artifact seem
> to be used, depending on the firmware.
>
> For FW < 1.02, nothing is done on the scanner side, and the "alternate LD-
> correction" algorithm is engaged which effectively displaces two channels
> by 1/72" and 2/72" via ring-buffers.
>
> For later FW the scanner corrects the effect itself for each block read
> by mixing up the data in a strange manner which causes only the data
> needed for the currently "logically scanned area" to appear in the
> output. This correction scheme makes "backtracking" necessary. I.e.
> the scanner's slider will back up a little bit after each transfer (2/72")
> to have all data from the currectly transferred area available.

Yeah! this is sure be noticable.
But anyway wht does it need to do with Gray and Lineart mode ?

Gruss
Wolfgang

Is there someone whoo has the Dokumenation for those MFS 12000 SP
Scanners, so i might find a clue of the reason for my observations ?

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