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When scanning documensts, 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 seetings 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 tramsformations, 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 setings 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%.

1: Yes there are/were. Scanners where the gain is controlled via the analogue circuitry however, are almost all expensive, They are not your average scanners that you buy in a computer store. Most of them will also be film scanners.There is a reason for this: if you use prints (even with the higher quality version of the old fashioned chemical process), you loose resolution (in bits/color).

2: As far as I know, no-one compiled a list of such scanners. So going through manuals and any other documentation you can find is the only option. Note that it is not just the A/D conversions, but also the brightness of the lamp that might be changed.

3: Scanners are controlled by the SANE back-end. The options are determined by the back-end. If a back-end supports the options, they should be visible through XSane. It has been a while for me, but I believe that, for example, the Nikon Coolscan back-end supports it.

4: What is done most of the time, is to scan in the maximum bits/color possible, and then do a transformation. This works well if, for example, you're scanning JPEGs (8 bit/color) and your scanner is 12 bits/color. Theoretically, you get 4 stops of correction.

5: The optimum would be to use the analogue gain and lamp settings that create the maximum dynamic range (again in bits/color) for each color separately and use TIFF16 as output. That gives the maximum information about the picture. However, the picture might not look like what you want and you may need an additional scan to get the color balance information.

Note that, for most users outside the professional domain, this discussion is not really important. Although the human eye has a theoretical resolution of over 20bits/color, most people do not even see it if you flatten an image to 6 bits/color. The same goes for printers and displays. Indeed: most displays are in the range of 8-10 bits/color.