Has anyone figured the unit and convention of PrincipalPointU and PrincipalPointV ?
For example if PrincipalPointU is -0.17037, this is probably the horizontal offset from the center of the image? but in which units, pixel, mm, inch, sensor width? Does this value go from -1.0 to 1.0 or -0.5 to 0.5 ?
Another example, what would be the principal point values if it was the top left corner of the image? -0.5,-0.5, or -0.5,0.5 or -1,-1 or -1,1 ?
Edit: I am referring to the values in the XMP file exported from RealityCapture.
Edit: Even in the UI it is very confusing, the tooltip is “Deviation of image central point in image pixels”, yet the label contains “[mm]” indicating the unit is millimeters. So is the value in image pixels, or in millimeters??
Hi Etienne,
can you elaborate please what you are looking at or where you get the info.
In the calibration report there’s only x and y and it says [mm]…
Oh, I should have mentioned that I am looking at the values in the XMP file exported from RealityCapture. The values that appear there for the Principal point are different from the ones we see in the UI.
Ah, that’s different then… 
But I’m afraid I can’t help you there.
Off topic: How did you change your avatar?
For the avatar, register at http://en.gravatar.com/ with the same email address.
Hello Etienne,
PrincipalPointU and PrincipalPointV in the exported XMP file are the relative offsets from the center of an image in x axis and y axis respectively. Principal point x and Principal point y in the application are offsets from the center of an image in [mm] w.r.t. 35 mm film format. Simply said, the values in the XMP file are the values in the application divided by 35 mm. Therefore, these values are dimensionless.
You are right about the description in the application, we will change that to make it clear.