[sane-devel] Epson scanners: brightness and contrast setting

[Olivier Jolit]

Hello everybody again,
 
I am trying to control the brightness and contrast of the scan in order to provide the same capabilities than a Twain based frontend. The problem is that the extremal values I can set to brightness and contrast do not produce very distinct images.
- The maximal and minimal values that I can give to "sharpness" (the option with the closest name to contrast) are '-2' and '2' and the difference between the two outputs can only be seen by zooming several times into the image and are of a maximum difference of 35 for 8bit grayscale scan. In comparison, the Twain driver can give me an image with pixels having only 3 different values: 0, 255 and 127, which can be called maximal contrast (except maybe the 127's but that's not the point).
- The maximal value I can give to brightness, '3' is fine, the image is almost white, but the minimal value is '-4' and almost no pixel has a value lower than 127 (!) in comparison to Twain where everything is below 127 in the image.
 
I'm also a little bit surprised by theses values interval: why -4 to 3 for brightness ? at least -2 to 2 for sharpness is symetrical. I tried going outside these ranges and sane rounded my values or exited.
 
I tested all this on a Epson V700 scanner with epkowa (last release ;) ) and epson2 backends (both seem to behave identically, same values intervals, same outputs).
 
So I would like to know if I missed something on how to use these parameters. I dont know if it is useful to be noted but these parameters go from -1000 to +1000 on the Epson Twain driver. Are these parameters supposed to have better consequences on the scan or is all this kind of image processing supposed to happen after the scan on the output image ?
 
Post-processing the image is ok for me, I'm already doing it (and the Twain version also) but even so, I cant get a completely black image with "brightness" set to 0 because this means deducing 127 to all pixels in the scanned image (like xsane does) and since I have a lot of pixels with values above 127 a lot of them are not black in the end. (The result of the scan I described above are without post-processing, on sane and on twain).
 
Thank you in advance for your answers,
 
Olivier 		 	   		  
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