Get only turn from CGAffineTransform

Question

Is it possible to get only the rotation angle from CGAffineTransform, which also has translation and scaling values? If so, how?

Thanks in advance!

Answer 1

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

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

Answer 2

 atan2(rotationTransform.b, rotationTransform.d) 

The acos (b) solution proposed by someone is only valid in the +/- 0.5pi range. This solution seems reliable with a combination of rotation, translation, and scaling, although it almost certainly does not move. My previous answer ( atan2(rotationTransform.b, rotationTransform.a ) was not scalable.

This duplicate served as the basis for my initial answer .

Answer 3

You can try using the acos () and asin () functions to undo what CGAffineTransformMakeRotation does . However, since you have a scale transformation matrix multiplied by it, it can be a little difficult to do.

Answer 4

In the past, I used the XAffineTransform class from the GeoTools toolkit to extract rotation from an affine matrix. It works even when using zoom and shift.

This is a Java library, but you should easily convert it to (Objective-) C. You can see the source for XAffineTransform here . (The method is called getRotation.) And you can read the API here .

This is the basis of the method:

 final double scaleX = getScaleX0(tr); final double scaleY = getScaleY0(tr) * flip; return Math.atan2(tr.getShearY()/scaleY - tr.getShearX()/scaleX, tr.getScaleY()/scaleY + tr.getScaleX()/scaleX); 

You also need to implement the getScale / getShear methods. Not difficult. (For the most part, you can simply copy Java code as is.)

Answer 5

asin and acos are incorrect when there is a scale.

correct formula:

 CGFloat radians = atan2f(self.view.transform.b, self.view.transform.a); CGFloat degrees = radians * (180 / M_PI); 

or

 CGFloat angle = [(NSNumber *)[view valueForKeyPath:@"layer.transform.rotation.z"] floatValue];