[ptx] Display gamma (was:Vignetting correction in nona)

[Pablo d'Angelo]

Hi Doug,

>>Wouldnt Display gamma be a toppic to do together with colour management, as
> 
> Douglas Wilkins allready planed?
> 
> I would have thought so :-)
> And now that 0.5 has been released, I think it is time to put some thought into
> the colour management in hugin.
> 
> As I see it, there are two areas that need to be addressed. Firstly, ensuring
> that embedded ICC profiles are passed through correctly. Unlike an normal image
> editing application, there is no need for hugin to modify the embedded ICC
> profile or add one if there is no profile. 

This is the most important thing, I guess. Enblend already supports this. 
I'd like to add this soon, but I have both enblend and hugin contain their 
own, forked version of the vigra library, especially of the image loading 
functions. I've started to get the modifications back into the main vigra 
distribution, and hopefully I'll have enought time to do so in the next 
month. Then it should be quite simple to add support like that.

> Secondly, to colour manage the actual images in hugin for accurate display.
> Modifying the display gamma "on the fly" would fall into this area I guess.
> Again, hugin is slightly different and would probably only require an input and
> display profile since we are not doing any colour manipulation, but I welcome
> input on this.

Actually since hugin is not really a photo editor it is not terribly 
important that the color display is 100% accurate, but if it can be done 
easily (and without performance issues), why not.
I guess it would mean that the displayed images need to be transformed with 
lcms while loading. Shouldn't be too hard to add.

I'm not an expert in color management, but for stuff like the vignetting 
correction it would be great to work with a linear (intensity proportional 
to gray values) image.
It would be nice if the image could be transformed into a linear colorspace 
for this. I'm not sure if this can be done if images with good input 
profiles are given.

> Finally, I was thinking of using lcms for the colour management, mostly because
> I am familiar with it ;-), but also because it is reasonably mature and
> complete. It will add an additional dependancy though.

Hmm, shouldn't be that bad, lcms is not an exotic library.

ciao
   Pablo