Re: Xsane questions and wish list

Oliver Rauch (oliver.rauch@Wolfsburg.DE)
Fri, 16 Jul 1999 15:44:56 +0200

Steve Gunnell wrote:

> Oliver Rauch wrote:
> > xsane uses the gamma bit depth the backend uses,
> > but the scanner gamma table has to be activated (normally
> > by custom gamma table in standard options, but backend dependent)!
> I'm just using the default settings. I assumed the scanner was beingchanged
> because when I closed xsane and went back to xscanimage
> my attempts to correct the white level in xsane were still being applied
> to the xscanimage previews. I had to turn the scanner off to clear them.

That sould not happen, may be there is a bug in the backend!

> > For what do you need the values? In the histogram window
> > you have the silders and the color density is given in the
> > histogram window.
> I'm scanning negatives. So I need to remove the colour mask from the
> images.To do that I need to set the white point to the value of the colour
> mask. So what
> I do is select a scanned chunk of colour mask and average it using the gimp
> generic convolution tool to give me the average RGB value for the colour
> mask.
> If I had a editable value box I could enter these values as the white point
> just like
> I do with the gimp histogram tool rather than having to mess around with
> little
> sliders that I can't control accurately. If I could set the colour mask
> scaling and
> the negative colour inversion once per film in xsane then I can save two
> gimp
> operations per frame which would be a considerable saving over the 45
> minutes per strip of 4 negatives that the process takes now.
> > If you want to use values take the brightness, contrast and gamma
> > values!
> These are no use to me because they are arbitrary values that do not
> simplyrelate to the RGB values that I need to change.

The highlitght and shadow values are also not the same, they depend on the
bit depth (8bit =256, 10 bit=1024 and so on).

The contrast and brightness are well defined:
-100% brightness moves all values in 8 bit by -128
-100% contrast makes all colors to one value,
+100% contrast doubles the range of used values

Xsane and the gimp should use the same contrast, brightness and gamma values
for the same conversion!

> If you have a better method for processing negatives that takes advantage of
> xsane's features then I would love to know about it.

I disable rgb default, activate negative option and do a preview scan,
then I disable all but one color in the histogram window and set the
highlight and shadow sliders to the used range, this is done for
red, green and blue. Then I set all gamma values to 1.0

Write down the 4 values for brightness and contrast, they are a good
start for the film (agfa, fuji,...)

In future it is planned to save different settings, then you can save a
correction for Agfa negatives, one for Fuji and so on.

To get real good results, you afterwards have to do a calibration by
hand, but that is the same as scanning positives!


EMAIL: Oliver.Rauch@Wolfsburg.DE

Source code, list archive, and docs:
To unsubscribe: echo unsubscribe sane-devel | mail