Question

I have the following code…

public class Point {
    private double x;
    private double y;
    static private final double RADTODEG = 180.0d / Math.PI ;
    static private final double DEGTORAD = Math.PI / 180.0d;

    /**
     * Rotates the point by a specific number of radians about a specific origin point.
     * @param origin The origin point about which to rotate the point
     * @param degrees The number of radians to rotate the point
     */
    public void rotateByRadians(Point origin, double radians) {
        double cosVal = Math.cos(radians);
        double sinVal = Math.sin(radians);

        double ox = x - origin.x;
        double oy = y - origin.y;

        x = origin.x + ox * cosVal - oy * sinVal;
        y = origin.y + ox * sinVal + oy * cosVal;
    }

    /**
     * Rotates the point by a specific number of degrees about a specific origin point.
     * @param origin The origin point about which to rotate the point
     * @param degrees The number of degrees to rotate the point
     */
    public void rotateByDegrees(Point origin, double degrees) {
        rotateByRadians(origin, degrees * DEGTORAD);
    }

    /**
     * Rotates the point by the specified number of radians about the axis' origin (0,0). To rotate about a specific origin point, see rotateByRadians(Point, double)
     * @param radians Measure of radians to rotate the point
     */
    public void rotateByRadians(double radians) {
        if(isEmpty()) // Since we're rotating about 0,0, if the point is 0,0, don't do anything
            return;

        double cosVal = Math.cos(radians);
        double sinVal = Math.sin(radians);

        double newx = x * cosVal - y * sinVal;
        double newy = x * sinVal + y * cosVal;

        x = newx;
        y = newy;
    }

    /**
     * Rotates the point by the specified number of degrees about to the axis' origin (0,0). To rotate about a specific origin point, see rotateByDegrees(Point, double)
     * @param degrees Measure of degrees to rotate the point
     */
    public void rotateByDegrees(double degrees) {
        rotateByRadians(degrees * DEGTORAD);
    }

The problem arises when given a point, say 0,200. Calling the rotation (about axis origin 0,0) for 180 degrees it should be (0, -200). The x coordinate shouldn’t have changed. However, it ends up being (-2.4492935982947064E-14, -200). I tried using strictfp but it doesn’t make a difference. This only affects the result if the coordinate being rotated is zero. Nonzero values work fine. Any ideas why this is not accurate?

The code is below:

   Point p = new Point(0.0d, 200.0d);
   p.rotateByDegrees(180.0d);
   System.out.println(p);

Gives output:

shapelib.Point Object {x: -2.4492935982947064E-14 y: -200.0}

Answer 1

Float arithmetic is not fully accurate. Error of 10^-14 power is enough in most cases.
If you calculate Math.sin(Math.PI) you’ll get 1.2246467991473532E-16. Why do you need to get precisely 0 in your case?