[sane-devel] Fwd: HP scanjet IIcx /t resolution

[abel deuring]

George Georgalis wrote:

> Thanks to some help on irc I've got my HP scanjet IIcx /t working.
> 
> It's spec says 600 dpi but somewhere between 300 and 400 dpi it starts
> to do double passes (exactly when it starts depends on the image). I
> think I understand what's happening, though I don't know the
> vocabulary to explain it (software gets a higher resolution than the
> optical quality requiring multiple passes).
> 
> Anyway what I was wondering is:
> 1) why it would do this double take at anything less than the rated
> optical resolution? -- which is 600 dpi
> 2) why would the image affect at which dpi the double take starts?

Do you mean the image width?

> 
> I came across this determining the highest dpi I could use to do a
> scan quickly, ie no double takes.
> 
> I should add, it doesn't  scan everything twice, it just backs up and
> rescans as little as 5% of the whole area; at resolutions above 300
> dpi, more % at higher resolutions.

George,

while I've never used an Scanjet IIcx, your description doesn't sound 
like "real" double scans. For higher resolutions, the scanner must 
transfer larger amount of data per scan line to the computer, and on the 
path scanner -> SCSI cable -> SCSI adapter -> SCSI driver -> sane 
backend -> sane frontend are a number of possible bottlenecks, i.e., the 
scanner may aquire more scan data per time unit than can be sent or 
processed. If the internal buffer of the scanner is full, the scan head 
must be stopped. In order to prevent "image distortions" caused by 
mechanical margins (??? is this correct English? In German, I mean 
mechanisches Spiel or Toleranz), most scanners move the scanhead a bit 
back before continuing the scan.

Most scanners move the scan head with the same speed, independent of the 
scan width, hence you get also a larger data transfer rate for wider 
scans. If the scan head stops occur above a certain transfer rate, you 
can get this same data rate by doubling the scan width, while halving 
the resolution. This could explain your second quetsion.

Abel