What is the direction convention for Roll angle? This page from Wikipedia says:


  A positive rolling motion lifts the left wing and lowers the right wing.


Which means that if an aircraft has a roll of +20 degrees, it would bank towards the right. And for a roll of -20 degrees, it would roll to the left.

But when I used this flight indicators library, for a roll of +20 degrees the attitude indicator showed that the aircraft was banking left and vice versa.

Are there any conventions for the roll angle/attitude indicator?
That depends on the coordinate system, and the Wikipedia answer uses the most common convention. Flight mechanics uses a coordinate system with X in flight direction, Y sideways to the right and Z downwards positive. If you now apply the right-hand-rule for the positive direction of rotation around each axis, you will get a positive roll angle around X when the right wing moves down.



Right-hand-rule for rotation: Point the thumb in positive axis direction, then the other fingers will point into the positive direction of rotation.

If, however, X points backwards, then a positive roll angle would be as used in the linked library. Such a coordinate system is sometimes used to have Z point up, but is rather unusual for flight mechanics.