Performance problem with dashed lines

In some cases we have poor performance with rendering of dashed lines. Here is the report from the java2d mailing list:
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Hi folks,

I've found what seems like a bug in the java2d drawing code. I've
included a simple test application, at the end of this email, that
demonstrates the problem. It involves drawing dashed lines whose
ratio of line length to dash length is very large. I've tested it in
both java 1.4.2 and java 1.5 and found the problem in both cases.

Here are some results of my tests that demonstrate the issue.

Drawing a solid line seems fine no matter what the length...

ken@saturn> time java -cp . DrawDash 0 10000000

real 0m0.980s
user 0m0.752s
sys 0m0.052s

...However, when we add a dash to this long line it suddenly takes
much longer to draw...

ken@saturn> time java -cp . DrawDash 5
10000000

real 0m10.262s
user 0m9.865s
sys 0m0.080s

...however, if we simply drop the length of the line down by a factor
of 10 we are back to a reasonable length of time ...

ken@saturn> time java -cp . DrawDash 5 1000000

real 0m1.317s
user 0m1.212s
sys 0m0.016s

... The problem seems to be more based upon the ratio of the dash length to the
line length as evidenced by the following set of results. First, this
short line length with this incredibly short dash length is mostly
fine...

ken@saturn> time java -cp . DrawDash .0005 100

real 0m2.426s
user 0m2.220s
sys 0m0.044s


...while these two, with a dash/length ratio 1/10th of that above, I
had to kill because they were taking longer than 2 minutes!...

time java -cp . DrawDash .0005 1000
time java -cp . DrawDash .00005 100

...which seems to suggest that it is not entirely based upon the ratio
since if that were the case they should take around 10 or 11 seconds
as the one above with the same ratio. However, they take much longer
than that. Maybe they will never finish!

But, anyway, something is going on that it seems should be easily
fixed. Anyone have any ideas?

Cheers,

Ken


Here is the code for my tests...

import javax.swing.JDialog;
import java.awt.BasicStroke;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics2D;
import java.awt.geom.Line2D;

public class DrawDash
{
    public static void main( String[] args )
    {
        double lineSize = 1000.;
        float dashLength = 10;

        if ( args.length >= 1 )
        {
            dashLength = Float.parseFloat( args[0] );
        }

        if ( args.length >= 2 )
        {
            lineSize = Double.parseDouble( args[1] );
        }

        JDialog dialog = new JDialog();
        dialog.setSize( new Dimension( 800, 800 ) );
        dialog.show();

        Graphics2D graphics = (Graphics2D) dialog.getGraphics();
        graphics.setPaint( Color.RED );

        if ( dashLength > 0 )
        {
            float[] dashArray = new float[]{ dashLength, dashLength };

            BasicStroke stroke;
            stroke = new BasicStroke( 1,
                                      BasicStroke.CAP_ROUND,
                                      BasicStroke.JOIN_ROUND,
                                      10.0f,
                                      dashArray,
                                      0 );
            graphics.setStroke( stroke );
        }

        graphics.draw( new Line2D.Double( -lineSize,
                                          -lineSize,
                                          lineSize,
                                          lineSize ) );

        System.exit( 0 );
    }
}