Well, i'm still undecided about how to best interpret overall image brightness. Consider this: most of us are looking at a 2.2 calibrated gamma, and L = 50.0 displays as slightly darker than 0.5 on our screen.
Do we generally wish to see images relative to gamma 2.2, or relative to L in Lab? Is that still a reference in todays imaging environments?
Most images will be viewed and reproduced on monitors, more so than on paper/print. Since most monitors are easily capable of providing significantly more than 100cd/m2, to name just 1 of the many variables, is L in Lab still a relevant parameter?
Moreover, overall image brightness is most likely not a 2degree observer problem, so perhaps that renders all the usual math obsolete to begin with.
Anyways, for a more hands-on approach:
I decided to build a piece of software to do the HDR blending with CoreImage. Also because all the available solutions tend to generate halo-ed crap. Either by intent or otherwise. Plus they all seem to offer a multitude of controls which are highly irrelevant to my needs and most probably indicative of an error in processing logic.
The software I created is meant to do the blending and nothing else. No local contrast enhancements, nor extreme highlight/shadows type of corrections. I just want a correct blend of source frames that I have prepared with some kind of faithful reproduction.
In order to do so, the software just creates luminance masks that allow it to blend the images in some reasonable matter without sacrificing original color and contrast. But, the blended result still has some overall average brightness that may (or may not) need to be corrected to some reference.
The question then is: what reference?
So for a meaningful discussion and because I could use some feedback on the software, I am providing a link to a first beta version here. The software is pretty much self-explanatory, read the ReadMe file for some pointers.
The software shows the final result in a middle-gray backing and allows you to adjust the exposure value. So the question then is this: how would you like to set the background? To what reference? Middle-gray in Lab? or in gamma 2.2? What kind of feedback parameter would be useful?
BlendEditor v1.0b1(Mac only, requires Mac OS X 10.6, less then 120 KB download)
Please note: this is beta software, do not install if you are not familiar with the concept of beta-software