[sane-devel] HP Scanjet 3670 bad colors

[Octavio Alvarez]

On 05/24/2014 08:25 AM, Octavio Alvarez wrote:
> On 05/23/2014 10:58 PM, Stef wrote:
>>     target_code is giving the desired target value for white. Once this
>> value is correct : not too low, and not too high to avoid
>> "solarization", you should turn to tuning gamma values. They are defined
>> in genesys_devices.c for the CCD_HP3670. Currently it is 1.0 for each
>> color channels, and this need to be adjusted. This will improve the
>> histogram.
> 
> For some reason I haven't been able to find out yet, changing gamma
> values on genesys_devices.c:350 doesn't seem to change anything, no
> matter if I set them to 1.5, 2, 3, 10 or 0.5, 0.2, etc.
> 
> Changing target_code makes a difference (just to make sure my
> compilation environment is correct).

Hello again.

Considering I wasn't able to play with gamma values in code, I took
sample scan images with a calibration sheet for all combinations of:

target_code = { 0x0000, 0x1000, 0x2000, ..., 0xe000, 0xf000 } in
descending order
resolution = { 75, 100, 150, 300 }

I uploaded the images to the following URL:

http://blog.alvarezp.org/files/scan-hp3670/calibration/

The 300 dpi scans are bigger, of course, but I found different scan
results for different dpi. I wonder if this is just a cheap scanner.

Anyway, I find 0x4000 to have the best value to have no solarization but
the higher the dpi, the darker the images.

While trying to fix the images in Gimp with the Levels and Curves tool I
find that green is more sensible to low light so its 0 point should be
moved to about 15/255 to prevent the shadows to come out greenish. Also
it needs a gamma of about 1.20 to balance the midtones out.

Blue needs a gamma of about 0.80. Red is ok.

Hope this helps.