The definition of 'roll' in android is:
angle between a plane perpendicular to the device's screen and a plane perpendicular to the ground
See here.
Maybe I'm missing something, but there seem to be many planes perpendicular to the ground giving many different angles.
Could someone please rephrase this definition using e.g planes parallel to ground or screen, north direction, gravity direction?
NB! I am asking for a rephrasing using different words. I know the official definition as I have stated. Please don't repost the official definition.
From the mentioned docs:
Roll, angle of rotation about the y axis. This value represents the angle between a plane perpendicular to the device's screen and a plane perpendicular to the ground.
From another doc:
Roll (degrees of rotation about the y axis). This is the angle between a plane perpendicular to the device's screen and a plane perpendicular to the ground. If you hold the device parallel to the ground with the bottom edge closest to you and tilt the left edge of the device toward the ground, the roll angle becomes positive. Tilting in the opposite direction—moving the right edge of the device toward the ground— causes the roll angle to become negative. The range of values is -90 degrees to 90 degrees.
You may also have a look at that picture
The important point is about the y-axis. The y-axis is the one that goes positive from the bottom edge to the upper edge of the screen.
----EDIT----
A may be much more explicit picture here.