How do I extract the angle of rotation from a QTransform?

Question

I have a QTransform object and would like to know the angle in degrees that the object is rotated by, however there is no clear example of how to do this:

http://doc.trolltech.com/4.4/qtransform.html#basic-matrix-operations

Setting it is easy, getting it back out again is hard.

Answer 1

Assuming, that the transform ONLY contains a rotation it's easy: Just take the acos of the m11 element.

It still works if the transform contains a translation, but if it contains shearing or scaling you're out of luck. These can be reconstructed by decomposing the matrix into a shear, scale and rotate matrix, but the results you get aren't most likely what you're looking for.