On Thu, 5 Jun 2003, Henning Meier-Geinitz wrote: > Hi, > > On Thu, Jun 05, 2003 at 02:59:04PM -0400, m. allan noah wrote: > > with the adf fujitsu scanners, in grayscale or color, we get fine, lightly > > colored lines as a part of the scanned image, > > Vertical lines?
yes, as if some small portion of the diode array always reads, even when there is just white paper. leads to occasional pink or cyan lines, etc. > > > and the 'background' behind the document when the scan window > > exceeeds the size of the paper looks mottled gray, as reported on > > this list. > > That may be the result of the selected gamma, brightness and contrast. > seems to be no way to mess with gamma/brightness/contrast in the hardware, there is only an 8bit or 10bit single-color LUT. > > i can find no way in the interface manual to prevent this in hardware, > > though it has been reported that the same scanners under twain do not > > display these problems. > > > > hence, i assume that this can be corrected in software. are any of the > > existing sane backends doing this internally? i would like to do something > > similar as an option in the fujitsu backend... > > The gt68xx backend does calibration completely in software. I think > mustek_usb does this also, other backends of the more low-level > scanner may use the same scheme. All scanners supported by the gt68xx > backend have a white calibration strip at the top of the scanning > area. It's visible if you open these scanners. > this scanner is adf only, but the backer behind each read array is very bright white plastic. i could use that, if i could figure out how to get the scanner to send me data when there is no document in the scanner i may have to hook it up to a windows box (ack!) and usb snoop on it. thanks for the data below, i will look at these drivers and see if i can see this being done... allan > Calibration works about like this: > > a) Coarse calibration: The scan bar moves to the calibration strip. > The gain and offset values of the analog frontend are setup so the > brightest white of the calibration strip is defined as white (255) > and the darkest black is 0. You get black by either turning offf > the lamp or by scanning a small black mark on the strip. > b) Fine calibration: The scan bar scans some milimeters of the white > strip in full width. Once with the lamp turned on and once with the > lamp turned off. So you get two values per pixel per color, one for > max white and one for min black. > > If you do a real scan you can calculate the resulting pixel color like > this (from my memory, untested): > > real_color = 255 * (scanned_color - min_black) / (max_white - min_black) > > Bye, > Henning > > _______________________________________________ > Sane-devel mailing list > sane-de...@www.mostang.com > http://www.mostang.com/mailman/listinfo/sane-devel > -- "so don't tell us it can't be done, putting down what you don't know. money isn't our god, integrity will free our souls" - Max Cavalera
