Arc2D.Double may not draw correctly for tiny angular extents

Name: ddT132432 Date: 07/31/2001


Java(TM) 2 Runtime Environment, Standard Edition (build Blackdown-1.3.1-FCS)
Java HotSpot(TM) Client VM (build Blackdown-1.3.1-FCS, mixed mode)


//file: Ikebana.java
import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import javax.swing.*;

/** The following program exhibits a Bug in Arc2D.Double
 * It creates an arc with a very small angular extent.
 * Then the arc is drawn with different strokes.
 * One time the result is wrong (red),
 * one time it is o.k. (blue)
 *
 * My conjecture for the bug's source: deep inside the implementation
 * an inverse trig function (like Math.acos or Math.asin)
 * returns NaN (instead of 0, or Math.PI, or so)
 * because its argument is NOT IN [-1,1].
 * Theoretically it should be, but practically (due to roundoff errors)
 * its absolute value is greater than 1.
 *
 * Platform: JDK1.3 under Linux(i386) (Display: 17", 1024x768)
 * Reproduciblity may depend on the screen's resolution.
 */

public class Ikebana extends JComponent {

  public void paint(Graphics g) {
    Graphics2D g2 = (Graphics2D)g;

    // ==== create an arc with a very small angular extent ========

    Arc2D.Double s=new Arc2D.Double(100.0, 100.0, 240.0, 240.0, -180.0, 1.e-15, Arc2D.PIE);

    g2.setPaint(Color.red); // BAD CASE
    g2.setStroke(new BasicStroke(1)); // draw the arc, thickness 1
    g2.draw(s);

    g2.setStroke(new BasicStroke(2)); // GOOD CASE
    g2.setPaint(Color.blue); // g2.draw((Arc2D.Double)s);
    g2.draw(s);

    // ============================================================
  }

    // ==== almost literally from Niemeyer & Knudsen

  public static void main(String[] args) {
    JFrame f = new JFrame("Ikebana");
    Container c = f.getContentPane( );
    c.setLayout(new BorderLayout( ));
    c.add(new Ikebana(), BorderLayout.CENTER);
    f.setSize(400, 400);
    f.setLocation(100, 100);
    f.addWindowListener(new WindowAdapter( ) {
      public void windowClosing(WindowEvent e) { System.exit(0); }
    });
    f.setVisible(true);
  }
}
// =========== END of Sample Program ====================
(Review ID: 128180)
======================================================================