The source profile in my tests is sRGB - gamma 2.2 (or close) isn't it?
> So sRGB converting to Monitor profile (perceptual) should not involve a gamma
change, yet images noticeably darken when I apply the profile conversion.
Well it depends on all the other details (type of link etc.), but it's all going to be completely independent of the calibration. If you don't think what's happening there is right, then you need to investigate the profile accuracy, and what happens to the colors as they are transformed though the linking process, to decide if there is something wrong with that aspect.
I'm getting the same results with -k0 and -k1; resulting callibration curvegamma is 2.05, not 2.2.
OK, so your display has a black point with a good match in color to the white point. > Below I've listed the output from dispcal -E for
.cal files with k=0 and k=1. The gamma offset is the same (~0.08) in both cases. I don't understand why the gamma offset is low yet the gamma seems so far off.
I:\Util\Argyll\profiles>..\bin\dispwin -d2 t42pk1.cal I:\Util\Argyll\profiles>..\bin\dispcal -v -d2 -yl -E Maximum neutral error (@ 0.095629) = 1.150478 deltaE Average neutral error = 0.447931 deltaE
Right, a nice tight verification. The target curve may or may not be what you want or expect, but the calibration has made the display conform the target curve.
Here's the output from dispcal -E on a .cal file with a k of 0: I:\Util\Argyll\profiles>..\bin\dispwin -d2 t42pk0.cal I:\Util\Argyll\profiles>..\bin\dispcal -v -d2 -yl -E
You need to specify -k0 for dispcal when verifying, otherwise it will create a k = 1 target curve to verify against and will get ...
Maximum neutral error (@ 0.112249) = 6.966823 deltaE Average neutral error = 3.416810 deltaE
a poor verification fit. The discrepancy in gamma values you note is fully explained by the fact the calibration target curve is a truncated ideal gamma curve. Rather than subtract the black level (assuming view black adaptation, which is a poor assumption), the target curve is an ideal gamma curve, truncated at the point the display can't go any blacker. If the gamma number created using -r was estimated on the assumption that the curve in question is a truncated ideal gamma curve, then the agreement with the calibration target is within the tolerance of the calibration (something I've confirmed by making just those alterations). Now we can have a whole discussion about whether a truncated ideal calibration curve is a good thing or not, but at the end of the day anyone interested in color will be using profiles on top of the calibration, and this renders such details less important, as long as the calibrated display has controllable behaviour. Graeme Gill.