When scanning documents, e.g. using xsane, one can set parameters for dark threshold, bright threshold and gamma.
For color scans, these parameters can be set individually for R, G and B.
1. Are there commercial scanners which control the AD conversion range by software, i.e. influence the black and the bright end of the AD conversion range?
AD converters often have input pins which influence the upper and the lower end of the conversion range and I assume that in modern designs these pins are connected to outputs of DA converters providing such reference voltages.
Of course, if the lower threshold comes close to the upper one, I expect more noise in the scanned data. But being able to adjust them directly before scanning takes place looks nevertheless advantageous. 25 years ago, one definitely would have connected these pins to adjustment potentiometers requiring their adjustment when the scanner is made. But today, I guess that adjusting them by software is cheaper than actually adjusting a potentiometer, and being able to adjust them by software even gives a chance to improve the adjustment after some drifts have taken effect in the instrument.
2. How can I find out if a particular scanner has such software controlled adjustments?
3. Do the settings of sane or xsane influence the AD conversion in the scanner (if the scanner has such adjustment properties)?
4. Or does xsane just apply greyscale transformations, e.g. look up table transformations, when one uses xsane's adjustments for the black and the bright reference point?
5. If one wants to make scans which do not impair later image enhancement, would it be best to leave all these settings at their default values, unless the settings actually influence the AD conversion on the hardware level?
I guess, if xsane just does what can be done by look up table transformations, then the best thing for my objective would be to let all these settings at their defaults, i.e. gamma=1, brightness=0, contrast=100. In the special case of xsane, this description tells us that the range for Contrast is 128 and +100% means that the difference between a value and medium gray is doubled. Enhancement default sets the enhancement values to gamma=1.0, brightness=0%, contrast=0%.