[sane-devel] [PATCH][RFC] gt68xx backend: sheetfed calibration WIP

[stef]

	Hello,

	thanks for another hardware donation from Jack McGill (an OpticSlim M12), I 
have started to work on adding sheet fed scanner calibration for gt68xx based 
scanners. Since to my knowledge there is no active maintainer for this 
backend, I submit the first patches done so I can get feedback before 
committing anything.
	The appended patches bring only calibration for the M12 for 100 dpi full 
width scans for now. It works by adding a gt68xx_sheetfed_calibrate() 
function. It is called whenever the OPT_CALIBRATE button option is set. It 
then find a white area on the inserted document and runs the existing 
calibration code. Once the corresponding image GT68xx_Calibrator  are created, 
all subsequent 100 dpi scans can be done with image quality corrections.
	The new code is guarded by a device specific flag, so existing device aren't 
impacted and only tested device can use this calibration.
	In its current form, it only works for 100 dpi full width scans, and white 
area detection is picky. The next steps I plan are:
	- extend the GT68xx_Scanner struct to keep  per dpi calibration data
	- improve calibration function do calibration for all available resolution in 
	  one call
	- build needed GT68xx_Calibrator with these data before each scan 
	- make the calibration persist in file

	I don't expect to have all this finished soon, so I don't think anything 
should bet committed before next release.

Regards,
	Stef
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