[sane-devel] PIXMA: New option gamma

[Rolf Bensch]

Hello all,

Scanning lineart images with scanimage can be a little bit tricky. If 
the scanner has problems with the default gamma exponents from pixma.c 
or the used subdriver (e.g. pixma_mp810.c)) you should create a gamma 
table with gamm4scanimage and use it with scanimage. This is 
inconvenient if you are searching for the images's best gamma table.

For normal lineart scans my CS9000F mostly likes a gamma table from the 
gamma exponent of 0.7.

Inspired from the epjitsu backend I added a new option gamma. Each time 
gamma is used with scanimage the backend generates a new gamma table 
from this gamma exponent.

With this patch the backend uses default gamma values as follows:
--custom-gamma=no  ==> default gamma from the subdriver
                        e.g. default gamma is 0.7 from pixma_mp810.c
--gamma-table auto ==> default gamma is 2.2 from pixma.c
--gamma auto       ==> default gamma is 0.7

Please try the attached patch and give me a feedback of your scanner's 
favorite gamma values.

You can also use xsane to determine the best gamma values of your scanner.

With your feedback I can decide either to adapt the default gamma 
exponents or to create a new gamma DEVICE parameter for each scanner.

Xsane is not affected from this patch. After changing some options xsane 
always sends an updated gamma table to the backend.

Cheers,
Rolf
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