[sane-devel] how should you determine a good gamma correction to use through the backend?

[gerard klaver]

On Sun, 2004-02-08 at 20:07, Christopher Marshall wrote:
> I have been using Sane for about two months now with a CanoScan LiDE 80 and getting great results.
> 
> I have been wondering, however, about how best to enhance my images after scanning.
> 
> They have been coming out fairly dark with no corrections.  I have dealt with this in two ways.
> 
> For scanning text documents, I set the backend --contrast argument to 30 and have no complaints.
> 
> For photographs, however, setting --contrast to 30 is a poor choice compared to using the GIMP to
> brighten the midtones with the image->color->curves tool.  I add a control point in the middle of
> the "value" curve and move it up and to the right and the image gets very close to the original
> with this.  However, I can still see differences in the colors when comparing the image on the
> screen to the photo I scanned.
> 
> I just read up on gamma correction and it has hit me that I was approximating a gamma correction
> the way I was using the gimp curves tool in the value channel.
> 
> So what I am wondering is this.  What is a reasonable procedure for figuring out a good gamma
> correction to give to the backend when scanning photographs?
> 
> Might it make sense to print a test page with solid blocks of red, blue, and green colors spread
> across the 0,255 range, scan that page with sane,  and then use the Gimp to get the exact values
> of the colors, then calculate the best gamma correction from that?
> 
> Has anyone ever done this?
> 
> Chris Marshall

Calibration your scanner with a scanner calibration target see for 
example http://www.targets.coloraid.de/
Example for profiler program see 
http://www.littlecms.com/profilers.htm


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m.vr.gr.
Gerard Klaver