main/src/georegression/geometry/UtilEllipse_F64.java
public static double computeAngle( Point2D_F64 p , EllipseRotated_F64 ellipse ) {
		// put point into ellipse's reference frame
		double ce = Math.cos(ellipse.phi);
		double se = Math.sin(ellipse.phi);

		// world into ellipse frame
		double xc = p.x - ellipse.center.x;
		double yc = p.y - ellipse.center.y;

		double x =  ce*xc + se*yc;
		double y = -se*xc + ce*yc;

		return Math.atan2( y/ellipse.b , x/ellipse.a );
	}
public static double computeAngle( Point2D_F64 p , EllipseRotated_F64 ellipse ) {
		// put point into ellipse's reference frame
		double ce = Math.cos(ellipse.phi);
		double se = Math.sin(ellipse.phi);

		// world into ellipse frame
		double xc = p.x - ellipse.center.x;
		double yc = p.y - ellipse.center.y;

		double x =  ce*xc + se*yc;
		double y = -se*xc + ce*yc;

		return Math.atan2( y/ellipse.b , x/ellipse.a );
	}
Computes the value of 't' used to specify a point's location

@param p Point on the ellipse
@param ellipse Ellipse
@return Angle from -pi to pi