[sane-devel] CIS scanners: to calibrate or not to calibrate :) this is the question (my scanner is mad and lazy :)

[Pierre Willenbrock]

Rene Rebe schrieb:
> litlle girl wrote:
>> AFE settings looks more stable:
>>
>> grep "\[gt68xx\] afe" ./*
>> ./color-1.log:[gt68xx] afe 0x0f 0x04 0x10 0x05 0x0f 0x04
>> ./color-2.log:[gt68xx] afe 0x0f 0x04 0x10 0x05 0x0f 0x04
>> ./color-3.log:[gt68xx] afe 0x0f 0x04 0x10 0x05 0x0f 0x04
>> ./gray-1.log:[gt68xx] afe 0x0f 0x04 0x10 0x05 0x0f 0x04
>> ./gray-2.log:[gt68xx] afe 0x0f 0x04 0x0f 0x04 0x0f 0x04
>> ./gray-3.log:[gt68xx] afe 0x0f 0x04 0x0f 0x04 0x0e 0x03
>> ./lineart-1.log:[gt68xx] afe 0x0f 0x04 0x0f 0x04 0x0e 0x03
>> ./lineart-2.log:[gt68xx] afe 0x0f 0x04 0x0f 0x04 0x0e 0x03
>> ./lineart-3.log:[gt68xx] afe 0x0f 0x04 0x0f 0x04 0x0f 0x04
> 
> Aside exposure and AFE the backend probably uploads a
> scaling value for each image sensor element (white / dark
> shading etc.?) that should be pretty constant.
 >
> (And no, I still have not read the genesys backend source
>   to verify what it's doing, this is just from other
>   educated experience.)

I don't know what the gt68xx uses for calibration data,
but the highly experimental genesys/calibration-cache code stores(in 
memory only) for each combination of color mode and resolution:
* exposure time
* AFE offset and gain
* per pixel offset and gain(actually just the black/white levels, but 
the offset and gain are fully defined by those)

> I bet the Windows driver also does not re-calibrate every
> scan, at least the Mac driver for the Canon Lide 70 does
> only calibrate once after unpacking.
> 
> Yours,
> 

Regards,
   Pierre