[sane-devel] HP Scanjet 3670 bad colors

[Stef]

On 27/05/2014 18:13, Octavio Alvarez wrote:
> 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.
     Hello,

     many thanks for your work. Once I fixed an intermittent issue in 
LiDE 210 head parking, I'll use all these information to improve HP3670 
color scan quality.

Regards,
     Stef