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

Olaf Meeuwissen olaf.meeuwissen at avasys.jp
Tue Nov 23 23:26:25 UTC 2010

Hash: SHA1

On 2010年11月23日 22:06, Olivier Jolit wrote:
> 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.

Sharpness controls how much edges in your images are (de)emphasized,
quite something different from contrast.

> 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.

The values you can set here are what the device can handle.

> 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 ?

I'm not familiar with the TWAIN driver internals, but it is most likely
that that is all post-processing in software.  The epkowa backend only
gives you access to what the device can handle.

Hope this helps,
- -- 
Olaf Meeuwissen, LPIC-2           FLOSS Engineer -- AVASYS CORPORATION
FSF Associate Member #1962               Help support software freedom
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