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paintComponent() Graphics2D versus panel Graphics2D

 
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gmseed



Joined: 01 Dec 2008
Posts: 38
PostPosted: Thu Nov 18, 2010 2:31 pm    Post subject: paintComponent() Graphics2D versus panel Graphics2D Reply with quote
Hi

If I place the panel below onto a frame using the argument Graphics2D object then I see the centred circle.

However, if I use the Graphics2D object of the panel [via this.getGraphics()] then I do not see the circle.

I'm trying to draw several objects on a panel accurately and found that the argument Graphics2D object refers to the overall frame graphics object and performing any affine transform operations are with respect to the frame and not the panel. Thus, if I wanted to use the frame Graphics2D I would to calculate offsets to add to panel object transforms. With the frame having a menubar, toolbar, status bar etc, calculating such offsets is error prone.

Clearly, it is much simpler to use the Graphics2D object of the panel itself. However, it's refusing to make the panel circle visible?

Does anyone know a solution to this problem?

Graham

Code:


public class TestJPanel extends JPanel
{
    protected void paintComponent(Graphics g)
    {
        super.paintComponent(g);

        Graphics2D g2 = (Graphics2D)g;

        //Graphics2D graphics = g2;
        Graphics2D graphics = (Graphics2D)this.getGraphics();

        Rectangle   panelBounds   = this.getBounds();
        int         panelTopLeftX = panelBounds.x;
        int         panelTopLeftY = panelBounds.y;
        int         panelWidth    = this.getWidth();
        int         panelHeight   = this.getHeight();

        // circle
        double circleRadius = 250.0;
        double circleDiameter = 2.0 * circleRadius;
        java.awt.geom.Ellipse2D.Double jcircle = new java.awt.geom.Ellipse2D.Double(0,0,circleDiameter,circleDiameter);

        // save original transform
        AffineTransform origTransform = graphics.getTransform();

        AffineTransform tx = AffineTransform.getTranslateInstance(panelWidth/2-circleRadius, panelHeight/2-circleRadius); // alternative static method of creating transform

        graphics.setTransform(tx);

        graphics.setRenderingHint(RenderingHints.KEY_ANTIALIASING,      RenderingHints.VALUE_ANTIALIAS_ON);
        graphics.setRenderingHint(RenderingHints.KEY_TEXT_ANTIALIASING, RenderingHints.VALUE_TEXT_ANTIALIAS_ON);
        graphics.setColor(Color.BLACK);
        graphics.draw(jcircle);
        graphics.fill(jcircle);

        // restore original transform
        graphics.setTransform( origTransform );
    }

} // class TestJPanel
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gmseed



Joined: 01 Dec 2008
Posts: 38
PostPosted: Thu Nov 18, 2010 3:57 pm    Post subject: Reply with quote
Hi

Gotya!! The affine transform of the argument Graphics2D has to be concatenated with the panel affine transform - see below.

The tutorials never tell you that!

Graham

Code:


    private void paintCircle_Test(Graphics2D g2)
    {
        // panel properties
        Rectangle   panelBounds     = this.getBounds();
        int         panelTopLeftX   = panelBounds.x;
        int         panelTopLeftY   = panelBounds.y;
        int         panelWidth      = panelBounds.width;
        int         panelHeight     = panelBounds.height;

        // circle
        double circleRadius = 250.0;
        double circleDiameter = 2.0 * circleRadius;
        java.awt.geom.Ellipse2D.Double jcircle = new java.awt.geom.Ellipse2D.Double(0,0,circleDiameter,circleDiameter);

        // save original transform
        AffineTransform originalTransform = g2.getTransform();

        // total affine - initialising with g2 affine
        AffineTransform affineTotal = new AffineTransform(originalTransform);

        // panel affine
        AffineTransform affinePanel = AffineTransform.getTranslateInstance(panelWidth/2-circleRadius, panelHeight/2-circleRadius);

        // concat
        affineTotal.concatenate(affinePanel);

        g2.setTransform(affineTotal);

        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,      RenderingHints.VALUE_ANTIALIAS_ON);
        g2.setRenderingHint(RenderingHints.KEY_TEXT_ANTIALIASING, RenderingHints.VALUE_TEXT_ANTIALIAS_ON);
        g2.setColor(Color.BLACK);
        g2.draw(jcircle);
        g2.fill(jcircle);

        // restore original transform
        g2.setTransform(originalTransform);
    }
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