[Digikam-devel] Colormanagement

[Caulier Gilles]

Le Dimanche 02 Avril 2006 04:35 AM, Peter Heckert a écrit :
> Hello,
>
> have some problems with colormanagement.
>
> I am trying to use linear RGB as a working space.
> For this purpose I am using this (homemade) colorprofile as workingspace:
> <http://home.arcor.de/peter.heckert/raw/RawSharpen/sRGBG1.icc>
>
> This is a profile that uses sRGB primary colors, but instead of
> Gamma=2.2 it uses Gamma = 1.0.
>
> My imaging software has full colormanagement and is fully 16-Bit
> capable. (Photoline32 for Windows, http://www.pl32.com)
>
> In this application I have two choices: I can /assign/ a colorspace to
> an image or I can /convert/ an image into another colorspace.
>
> In the first case,"assign",  the image data is not modified, only its
> interpretation is modified and so the visual look will change.
> In the second case, "convert", the image data is converted into another
> colorspace, the data is modified, but the display stays unchanged,
> because also  the interpretation of image data changes.
>
> Normally I do this for my RAW Images: I load the image with Gamma=1.0.
> The image will look dark then.
>
> Then I /assign/ sRGBG1.icc to the image. Then the image looks rather
> fine. (Some contrast improvements and else are necessary anyway)
>
> The image is not converted in any way. The image processor will adjust
> image's *displaybuffer* accordingly, it doesnt modify the image itself.
>
> That means, all image manipulations in follow will be done in linear RGB
> space. All filters and all blending modes work in linear RGB.
>
> That is how I understand the word "workingspace".
>
> Ok, here is another trick:
> Sharpening works much better in linear RGB, because sharpening halos are
> reduced. Denoising works better, because local noise amplitude is
> independent from local brightness in linear RGB space.
>
> Therefore I often convert my JPG's to 16 Bit and then I /convert/ to
> sRGBG1.icc before I continue with image manipulations.
>
> (I convert to 16 bit, because linear RGB cannot be used with 8 bit
> resolution, there would be too much loss of information)
>
> I tried to do the same trick in digikam.
> In digicam I cannot convert colorspaces. I can choose another working
> space. However, when I choose another working space, then the image's
> visual onscreen appearence is changed and so far I understand it, this
> should not happen.
>
> Ok, now I am unsure: Is this a bug or do I missunderstand
> colormanagement philosophy or is colormanagegement implementation still
> unfinished?

screen ICC color correction is not yet fully implemented. It's must done at 
0.9.0. Paco, another digikam developer working on actually.

Tak a look in Paco blog : http://www.digikam.org/?q=blog/5

Gilles