Question

I am writing a java application using swing in which I need to draw a grid above a square. In order to do so, I am using the drawLine(...) method provided by the Graphics class.

Everything works fine except that it takes a lot of time to draw each line (more than 20 sec for 50 lines…). I can even see the lines being drawn in real time. One weird thing is that the horizontal lines are drawn way faster than the vertical lines (almost instantly).

I might be doing something wrong. Here is the code for the grid.

public void drawGrid(Graphics g){
    g.setColor(new Color(255, 255, 255, 20));
    int width = getWidth();
    int height = (int) (width * Utils.PLATE_RATIO);
    int step = pixelSize*gridSpacing;
    Color bright = new Color(255, 255, 255, 100);
    Color transparent = new Color(255, 255, 255, 20);
    for(int ix = insets.left + step;                        
            ix < width; ix += step){
        if(((ix - insets.left) / step) % 10 == 0){
            g.setColor(bright);
        }
        else{
            g.setColor(transparent);
        }
        g.drawLine(ix, insets.top, ix, height+insets.top);
    }
    for(int iy = insets.top+step;
            iy < (insets.top + height); iy += step){
        if(((iy - insets.top) / step) % 10 == 0){
            g.setColor(bright);
        }
        else{
            g.setColor(transparent);
        }
        g.drawLine(insets.left, iy, width + insets.left, iy);
    }
}

Answer 1

The code you have posted is fine, there is no problems in it.
Here is a working example of a component using your method (a bit simplified):

public static class MyGrid extends JComponent
{
    private int step = 10;

    public MyGrid ()
    {
        super ();
    }

    public Dimension getPreferredSize ()
    {
        return new Dimension ( 500, 500 );
    }

    protected void paintComponent ( Graphics g )
    {
        super.paintComponent ( g );
        drawGrid ( g );
    }

    public void drawGrid ( Graphics g )
    {
        int width = getWidth ();
        int height = getHeight ();
        Color bright = new Color ( 255, 255, 255, 200 );
        Color transparent = new Color ( 255, 255, 255, 100 );

        for ( int ix = step; ix < width; ix += step )
        {
            if ( ( ix / step ) % 10 == 0 )
            {
                g.setColor ( bright );
            }
            else
            {
                g.setColor ( transparent );
            }
            g.drawLine ( ix, 0, ix, height );
        }

        for ( int iy = step; iy < height; iy += step )
        {
            if ( ( iy / step ) % 10 == 0 )
            {
                g.setColor ( bright );
            }
            else
            {
                g.setColor ( transparent );
            }
            g.drawLine ( 0, iy, width, iy );
        }
    }
}

I guess there is some problem outside that piece of code.

P.S. A bit an offtopic but…

I suggest you to calculate the visible part of the painting area (using either JComponent’s getVisibleRect () method or Graphics g.getClip ().getBounds () method) and limit your paintings with only that area.

That small optimization could speedup component’s painting in times if it is really large (for example with 10000×10000 pixels component’s area).