There are several apps which allow me to measure the rotation of my iPad around two axes (the X and Z axes in the image below)
What I want, however, is angles for all three axes. I am aware that it is usually assumed that the device is held parallel to the face and that without a reference, only rotation with respect to the ground or the optical axis make sense, but it could be possible to set the current measurement as the reference value for the Y axis and record the rotation relative to this value.
I am not sure if I managed to explain what I want. Basically I'd like to know how much I have rotated the iPad around the Y axis starting from some given value. (The point is recording device orientation when taking photos so I can see whether my relative camera pose estimation algorithm correctly recovers the rotation between two images, the iPad will be in landscape mode for this.)