Spirograph.java

/*************************************************************************
 *  Compilation:  javac Spirograph.java
 *  Execution:    java Spirograph R r a
 *  Dependencies: StdDraw.java
 *
 *  Draw a curve formed by rolling a smaller circle of radius r inside
 *  a larger circle or radius R. If the pen offset of the pen point in
 *  the moving circle is a, then the equation of the resulting curve
 *  at time t is
 *
 *     x = (R+r)*cos(t) - (r+a)*cos(((R+r)/r)*t)
 *     y = (R+r)*sin(t) - (r+a)*sin(((R+r)/r)*t)
 *
 *  % java Spirograph 180 40 15
 *
 *  % java Spirograph 100 55 20
 *
 *  Credits: idea suggested by Diego Nehab
 *  Reference: http://www.math.dartmouth.edu/~dlittle/java/SpiroGraph
 *  Reference: http://www.wordsmith.org/~anu/java/spirograph.html
 *
 *************************************************************************/

import java.awt.Color;

public class Spirograph {

   public static void main(String args[]) {
      int SIZE = 600;
      StdDraw.create(SIZE, SIZE);
      StdDraw.setScale(-SIZE/2, -SIZE/2, SIZE/2, SIZE/2);
      StdDraw.clear(Color.black);
      double R = Double.parseDouble(args[0]);
      double r = Double.parseDouble(args[1]);
      double a = Double.parseDouble(args[2]);

      for (double t = 0.0; t < 100; t += 0.01) {
          double x = (R+r) * Math.cos(t) - (r+a) * Math.cos(((R+r)/r)*t);
          double y = (R+r) * Math.sin(t) - (r+a) * Math.sin(((R+r)/r)*t);
          StdDraw.go(x, y);
          StdDraw.rotate(-Math.toDegrees((R+r)/r)*t);
          StdDraw.spot("earth.gif");
          StdDraw.rotate(+Math.toDegrees((R+r)/r)*t);
          StdDraw.pause(20);
      }

   }
   
}