Can the UniWB histogram be considered at all accurate beyond what it shows in terms of clipping on the right side of the histogram?
I can't express it very well but it always gives me the feeling that it is using a "linear" calculation to simulate what may not be linear (the white balance process) - kind of a "brute force" method, if you will.
I thought through the preview color space issue, and my gut feeling appears to have been wrong. I'll post something about that soon, but first let me deal with your issue. I will try to restate it:
are the intermediate points on a real raw histogram accurate? You'll notice that I changed your UniWB histogram to the real raw histogram. I did that because I didn't want to get into trying to figure out exactly how a proper UniWB histogram differs from a real raw histogram; answering that question for an arbitrary camera is beyond my pay grade.
Before I do that, let's talk about the x-axis on a raw histogram, and the size of the histogram buckets. One way to present a raw histogram is for the x axis to be linear, and all the buckets the same size. This has a certain mathematical purity, since the raw data is linear, but is not very useful to photographers. For one thing, it means that the entire right half of the histogram is the top stop of the dynamic range of the image, and the other 13 or so stops are all crammed into the left half. It also means that half the buckets are devoted to that top f-stop.
One way to deal with this situation to provide something more photographically useful has been to present the logarithm (base 2) of the input values as the x-axis. That's appealing, because one-stop intervals are all the some distance on the plot. In one of those happy accidents associated with the way beam current translated to brightness on a now-ancient cathode ray tube, you can get a rough cut at the logarithmic axis by scaling the raw values into the range [0,1] and raising each one to the power 0.45. In the arcane terminology of digital imaging, that's referred to as "gamma 2.2".
This may seem strange, but it's the same way that Lightroom, which uses a linear color space as its working space, presents its histograms. sRGB JPEG uses a gamma of 2.2 as well. So does Adobe 1998 RGB.
And now, we get to a key word.
"Mr. Clinton, are the histogram values thus presented accurate?"
"It depends on what the meaning of the word 'accurate' is."
Yes, those intermediate values are accurate, in that they're derived from the raw image in a repeatable, mathematically-well-specified manner. But are they useful? IMHO, not very. But then again, again, IMO, the intermediate values in and sRGB histogram don't tell me much. Sure, I can see that there are or aren't a lot of midtone values, but does that help me select the exposure? If we were working with 8-bit images, I could see histogram depopulation from too-aggressive tone curve moves, but those days are well behind us.
One thing you can do with the intermediate values is, once you decide to let something clip (say there's a sunlit window behind your subject), you can look for a little bump that indicates the brightest part of the scene that's not the window.
Does any of that help?
Jim
