I was unable to figure out why Lightroom is boosting the brightness and chroma of 32-bit floating point TIFFs imported into it, so I reluctantly decided to use the correctly-exposed synthetic image with the minus one-stop Lightroom Exposure adjustment as the reference and compute errors from it.
I computed the CIEL*a*b* Delta-E stats of the differences in the Lab values of the 121 patches in each of the “underexposed and compensated stop for stop less one stop for the Lightroom processing” patches. They’re similar to what I have measured in real camera testing, but with far less noise.
The overall stats:
For reference, a 3D look at the difference between the baseline exposure and itself expressed as displacements of the original target values:
A 3D look at the image that was exposed 4-stops under and corrected four stops more than the baseline in LR, processed the same way:
A 2D look at the same image:
The one, two, and three stop under plots are boring; they’re virtually the same as the four stop under graphs above.
These are fairly small errors. Still, it’s nice to look at how they break down.
Here’s a set of Delta-H curves, the basic CIEL*a*b* difference metric that measures hue error:
And the chroma error:
There’s more chroma error than hue error (that’s a good thing) and the chroma error is mostly in the direction of increasing the chroma (could be a good thing, could be a bad thing).
Looking at the hue angle errors, we see no bias in direction:
And finally, here are the results using the CIE Delta-E2000 Color-Difference Formula:
Thanks to Gaurav Sharma, Wencheng Wu, and Edul N. Dala for the Matlab code to implement this measurement.
This color difference formula is new to me, and fairly new to the color science community. It is supposed to be more accurate than the old Delta-E formula. Assuming that it is, it is telling us that the errors we are seeing are nothing to worry about, since half a Delta-E is said to be one just-noticeable difference in the new metric (down from the 1 Delta-E which was supposed to be one JND in the old CIELab Delta-E.