Re: [JAVA2D] Problem with dashed strokes

[Jim Graham]

Hi David,

I haven't run your example, but I am pretty sure that this is happening
because thin non-dashed lines are rendered using a fairly fast Bresenham
algorithm, but dashed lines are rendered using a fairly complex but
accurate technique of generating "widened" geometry for the line and then
filling the "widened" geometry.
You can work around this by either turning on antialiasing or by
using a line width of 1.01 or so for both dashed and non-dashed
lines.  This is because the antialiasing pipeline always uses the
accurate line widening technique to draw even thin lines and in
the second case because a line that is greater than 1.0 pixels
in width will always go through the widening algorithm...
                       ...jim

--On Tuesday, July 01, 2003 18:26:08 -0400 David Eisner <[EMAIL PROTECTED]>
wrote:
I have a GeneralPath that I draw with two different BasicStrokes.
The two strokes are (intended to be) identical except that one is
dashed.
What I'm observing is that in some cases the two stroked outlines
do not overlap, as I'd expect them to.  Are my expectations incorrect?
I'm including a simple program that demonstrates the problem.

You can find a (blown up) screen capture here:

    http://cradle.brokenglass.com/images/dash.png

This is occurring with (at least) the following:

  Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.2-b28)
  Java(TM) 2 Runtime Environment, Standard Edition (build 1.4.1_01-b01)
Thanks in advance.

-David

----

import java.awt.*;
import java.awt.geom.*;
import javax.swing.*;


public class DashTest extends JFrame {

    class BoxCanvas extends JPanel {

        Stroke      dashStroke_;
        Stroke      lineStroke_;
        Shape       shape_;
        BoxCanvas() {

            BasicStroke ds = new BasicStroke();

            lineStroke_ = ds;
            dashStroke_ = new BasicStroke(
                ds.getLineWidth(), ds.getEndCap(), ds.getLineJoin(),
                ds.getMiterLimit() , new float[] {8, 8}, 0
            );
            shape_ = makeShape();

            setBackground( new Color( 0, 200, 0));
            setPreferredSize( new Dimension(100, 100));
        }


        private Shape
        makeShape() {
            GeneralPath p = new GeneralPath();

            p.moveTo( 20.0f, 70.0f );
            p.lineTo( 70.9f, 70.0f);
            p.lineTo( 70.9f, 20.0f);
            p.lineTo( 20.0f, 20.0f );
            p.closePath();
            return p;
        }


        public void
        paintComponent( Graphics g ) {
            super.paintComponent( g );
            Graphics2D g2 = (Graphics2D) g;
            g2.setStroke( lineStroke_ );
            g2.setColor( Color.BLACK );
            g2.draw( shape_ );
            g2.setStroke( dashStroke_ );
            g2.setColor( Color.WHITE );
            g2.draw( shape_ );
        }
    }


    public DashTest() {
        super( "DashTest" );
        JPanel canvas = new BoxCanvas();
        getContentPane().add( canvas );
        setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
    }


    public static void
    main( String[] args ) {
        DashTest app = new DashTest();
        app.pack();
        app.setVisible( true );
    }
}




------------------------+--------------------------+
David Eisner            | E-mail: [EMAIL PROTECTED]   |
CALCE EPSC              | Phone:  301-405-5341     |
University of Maryland  | Fax:    301-314-9269     |
------------------------+--------------------------+
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