[Digikam-devel] CMS beginner question

[fj.cruz at supercable.es]

---- Thorsten Schnebeck <thorsten.schnebeck at gmx.net> escribió: 
> Am Donnerstag 09 Februar 2006 23:21 schrieb F.J.Cruz:
> > A Color Management Module (we are using Little-CMS) is a conversion engine
> > which translates the pixels colors from a color space to another color
> > space. A color profile is a set of tables of parameters and mathematical
> > matrix coefficients or raw color values describing the target gamut in a
> > standard format.
> [...]
> > - Inputs profiles (cameras and scanners): an input profiles describes the
> > color gamut of a camera or scanner. All cameras and scanner "read" the
> > color in RGB. These are "one direction" profiles, this is,  we can
> > translate from device coordinates to absolut coordinates. The profile can
> > also contain one dimension curves that can be linears or non linears,
> > altough the purpose of theses curves are not specified and depend on the
> > app which buids the profile.
> 
> Ok, I make a new example. Lets focus on two colors, black and white :-)
> 
> My cam produces a jpeg, 8-Bit per channel so white is 
> RGB8=(255,255,255) and black is RGB8=(0,0,0)
> If I use the RAW mode white is
> RGB12=(4095,4095,4095) and black is RGB12=(0,0,0)
> When I view a 16Bit PNG the color white is
> RGB16=(65535,65535,65535) and black stays RGB16=(0,0,0)
> 
> So now I want to start using CMS. The cam's  JPEG are calibrated in sRGB. When 
> using RAW I have to apply an external profile. Like you said in your 
> description a input color like RGB12(white) will be translated to a target 
> like e.g. RGB16(white) . Thats my problem in understanding this stuff. The 
> color white is well defined in all color spaces, so when I use CMS why do I 
> have to manually adjust the target color range. White should be white, or?
> 

Well, are you sure all your photos have a pure white? because you have to remember that the "white" color depends on external factors, maybe the most importat is the ambient light: incandescent, fluorescent, sunlight, flash, etc. Your white color, or i have to say what your eyes see as a pure white color, is not the same from a color space to another. In any case, when you correct the white level you are doing something like this: "ok, let's me see what 'white' is the more white....ahm, this one, so all the rest have to been adjust from this".

> Maybe the problem is, that we did not talk about format conversion so far but 
> only about device color ranges. Is a PNG-16bit file handled like an output 
> device?
> 

You have to forget all about file formats. A color transform using an ICC color profile and a Color Management Engine has no relation with the image file format. None of the image file formats are handled as any kind of devices, they are only data containers. When we talk about devices we mean physical devices: cameras, scanners, monitors, printers or film recorders.

> > > Ah! So, what about showing at least the filename when the "vendor
> > > tag" is empty?
> >
> > I guess you are talking about info messages in dialog setup. These messages
> > show the path to the profile.
> 
> I attach a screenshot of the setup dialog. Please look at the monitor profile 
> (Monitorprofil) its set-up, as you see in the the info box but not shown in 
> the dialog.
>

Ok, now I see what you want to say. I'll look for a solution :-).
 
> Thanks for your patience :-)
> 

No problem ;-).

> Thorsten