Get just the rotation from a CGAffineTransform

Question

Is it possible to get just the angle of rotation from a CGAffineTransform that also has translation and scaling values? If yes, how?

Thanks in advance!

Answer 1

atan2(rotationTransform.b, rotationTransform.d)

The acos(b) solution suggested by someone is only valid in the range +/-0.5pi. This solution seems robust under combinations of rotation, translation and scaling although almost certainly not shear. My earlier answer (atan2(rotationTransform.b, rotationTransform.a) was not robust to scaling.

This duplicate was the basis for my original answer.

Answer 2

Take the asin() or acos() of the affine member b or c.

struct CGAffineTransform {
   CGFloat a;
   CGFloat b;
   CGFloat c;
   CGFloat d;
   CGFloat tx;
   CGFloat ty;
};