Java Simple Rotation Around Point

Question

I'm trying to use this method to rotate a point:

 AffineTransform at = AffineTransform.getRotateInstance(Math.toRadians(90), 1, 2);
 double[] pt = {0, 1};
 at.transform(pt, 0, pt, 0, 1);
 int x = (int)pt[0];
 int y = (int)pt[1];

The expected result is x=0 and y=3 because we are rotating 90* clockwise, however the output is currently x=2 and y=1;

Why am I not getting the expected result?

Answer 1

The program is doing nothing wrong. By specifying Math.toRadians(90) as your theta, you are instructing it to rotate counter-clockwise, not clockwise. That is how rotation is defined in trigonometry.

Using -Math.toRadians(90) would get you the correct result.

Also, as a side note, casting the point values to int isn't the best idea for checking your code. Double-precision trigonometry is prone to very minor errors, and so a tiny bump in the wrong direction would get you the wrong integers. (I.e., 2.99999999995 would be cast to 2)

(int) Math.round(value) is much safer

Answer 2

Actually, you're rotating anti-clockwise. The Javadoc says:

Rotating by a positive angle theta rotates points on the positive X axis toward the positive Y axis

See the Javadoc