[sane-devel] Canon 9000f MK2 48bit Color Gamma Too Dark

[Rolf Bensch]

Hello Roger,

Sorry for the late response.

I can still remember that the scanner doesn't accept gamma settings for
48 bits color scans. So you need to post process the images if they are
too dark. This is what you're doing within Gimp.

Cheers,
Rolf


Am 02.04.2016 um 06:39 schrieb Roger:
>> On Thu, Mar 31, 2016 at 09:33:12PM -0400, Roger wrote:
>> I think I've found a bug (when using a Canon 9000f MK2), scanning at 16 bits 
>> color (AKA "Color") produces an apparent proper image gamma of 2.2, while 
>> choosing to scan at "48 bits color" produces an image too dark, or an image 
>> having an approximate gamma of less than or equal to one.
>>
>> Both scanimage and xsane, as well as enabling and disabling Color Managment 
>> exhibit this problem.
>>
>> A temporary work around when scanning at "48 bits color", would be to open the 
>> scanned image within Gimp, opening Levels and moving the middle slider on the 
>> first slider to a value of 2.2.  (Hence, first selecting Auto Level, then 
>> adjusting the gamma to 2.2.)
>>
>> Feedback on this issue?
> I think I can answer my own question here, as this appears to be a standard.
>
> VueScan PDF User Manual, page 89, nine paragraphs/sections down.
>
> "The image gamma value is 1.0 when there are two bytes (16-bits) per sample, 
> and 2.2 when there is one byte (8-bits) per sample. Raw files saved with gamma 
> 1.0 will look dark, but this is normal."
>
> Also on page 76, maybe relevant for some situations:
>
> "Note that if you use the Apple RGB, ColorMatch RGB, ProPhoto RGB or ECI RGB 
> color space, the image gamma will be 1.8. If you use any other color space, the 
> image gamma will be 2.2."
>
>
> So I'm guessing in order to work with 16 bit/channel images (ie. 48 bit color), 
> we'll need to bump the gamma to 2.2 when their viewed on our sRGB displays, 
> else they're too dark?
>
>