[Digikam-devel] Colormanagement

[fj.cruz at supercable.es]

---- Peter Heckert <lists at hphsite.de> escribió: 
> 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?
> 
> greetings,
> 
> Peter

Hi Peter,

My name is Paco Cruz and I'm the developer for all de color management stuff.

In this moment, digiKam has a basic color management support that I'm going to explaint to you:

digiKam can convert from input device color profile to working or editing color profile and to output one, this is, we can convert an image from camera/scanner profile to other one suitable for image editing (e.g. Adobe RGB) or to printer color profile.

Actually, I'm working to provide digiKam with a "managed view" option, using the display (monitor) color profile.

digiKam doesn't support "assign" option, only "convert" one. As far I can remember now, with the first option (in photoshop) the image you see in the monitor stay unchanged and with the second one, the image is changed, but I'm not sure about this.

In any way, I'm not an expert in color management and for a long time I've been asking for help to get digikam a fully color managed  app but looks like nobody in developer team or mailing lists have experience on this stuff, so if you have this experience, I'll be glad to hear what you can say me :-).

PD: Excuse my poor english ;-).

Paco Cruz.