Originally Posted by krasmuzik
Why is RGB barchart useless? As we discussed the % maybe meaningless - already it looks like Ursa, ColorFacts, and umr are using different methods to figure the %. So a % chart across softwares may be meaningless. What if one guy zooms the chart - and the other does not - (this one really annoys me - as the default in ColorFacts is non-zoomed in which everything looks perfect - the first bar is 20% off!) So you label the chart (but the labels are not ever going to be intelligible on a jpeg or 2" magazine offset print).
I do however use the RGB histogram - solely for telling me when I have reached my RGB clipping points - because the lines will clip.
Actually, for my $0.02, the RGB Bar chart should merely be a summary of the RGB histogram. At least, it is in my little corner of the universe. Since I could not reverse out several of the calculations that ColorFacts does, I gave up pretty quickly on mirroring those too closely. However, the RGB histogram can be calculated in several ways:
1) Calculate the light output contribution of each primary, then normalize each color (100% Stim == 1.0). Then, take an average of the primaries at each measurement point. The end result is that there is no error at 100% stim (Red = Green = Blue = 1.0), and there is a scaling error throughout the grayscale.
2) Calculate the light output of each primary, then create a set of three idealized white lights using one of the primaries from the measured set. Then relate each of these to a CCT graph. I believe that this is how ColorFacts does its histogram. I do not know how this then translates into the RGB% bar chart, but I doubt it is directly related to the CCT primary histogram chart.
3) Calculate the light output contribution of each primary at each measurement point. I then average the RGB blend, and use this for the histogram. Since the output of an XYZ->RGB transformation is normalized under ideal circumstances, deviations from an even "mix" are meaningful, as Kevin has noted. Of course, getting the right XYZ->RGB matrix is a bit of an issue (Poynton is too focused on Rec709, so Bruce Lindbloom's sight is a good second source)!
4 - n) There are several other ways to do this (xyY directly into RGB using a mixed matrix for both XYZ and RGB conversion, etc.), but others can post how they do this.
As a second line of defense, I also calculate an idealized output for each primary using D65 and the luminance measured at each point, giving an absolute deviation from ideal for each primary at each measurement point. It would be pretty easy to convert my crude percent measure into a dH/dC number if it was more useful.