Difficulty finding correct anchor point for rotation transforms

Question

I'm animating a button's position, rotating it around a circle. This creates a UISwitch-like behavior, except it is a lot more fancy. The button rotates, but it ends up off the desired position by about 0.25 radians. I'm trying to figure out where to put the anchor point to make the button rotate in a perfect circle around its origin.

Here's the code that I use to make the button "orbit" with a 120 pixel radius from the original location.

float offsetX=120;
float offsetY=0;

enableDisableButton.layer.anchorPoint =
     CGPointMake(offsetX/enableDisableButton.frame.size.width,                 
                 offsetY/enableDisableButton.frame.size.height);

I use the following method to do the calculations. Passing an argument of 90 for degrees, I expect to see the button start at a 180˚ position and move to 90˚, still 120 pixels away from its origin

-(CGAffineTransform)calculateLabelPositionFromFixedPointWithOffsetClockwiseInDegrees:(float)degrees
{

    float rotation = degrees*(2*M_PI/360);
    CGAffineTransform transform24 = CGAffineTransformMakeRotation(rotation);
    NSLog(@"rotation: %f", rotation);
    return transform24;
}

This little 0.25 radian or so offset means that I need to visually confirm the location of each button and cannot easily adjust its location programmatically. Any help in determining the correct way to rotate the button is appreciated.

I tried other ways to rotate objects. For example, I have an arrow <--------x, and I would like to rotate this arrow in a circle around x . What is the correct anchor point placement for this operation? Is it [1, 0.5]?

An easier way to do this kind of rotations is to put an object within a symmetric UIView, center it at the desired point of rotation and assign a rotation transform. This works, but requires the view to be twice as big:

Y----x----Y < this rotates Y around center point X without any anchor point adjustments. this is a very useful method to rotate arrows within analog gauges and such.

PeakCoder PeakCoder · Accepted Answer · 2012-03-27T08:12:02

"An easier way to do this kind of rotations is to put an object within a symmetric UIView, center it at the desired point of rotation and assign a rotation transform." this way is fine.

Difficulty finding correct anchor point for rotation transforms

2 Answers