> 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.
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.
CU,Andy
-- Andreas Beck | Email : <becka@sunserver1.rz.uni-duesseldorf.de> ======== GGI - The Right Thing To Do : http://synergy.foo.net/~ggi/ ========
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