alpha filter of an element

[Jim Henessey]

Hello,
Is it possible to set the alpha filter of an element (0-100%) of an element? If this is not formally implemented, is there a pseudo- way of doing this? What I am looking for is something which allows elements to be semi-transparent over an image background.
Thanks.

[Nathan Pruett]

You can call setOpaque( false ) on any JComponent. This makes the component ignore any background color set on it and it allows the background of it's parent to show through. This isn't anything as sophisticated as setting an aplha filter, but to do that correctly you'd probably have to make a look and feel, or subclass every component. I tried overriding the paintChildren() method to paint to a buffer and mess with the buffer's alpha filter before painting the buffer on the component, and this seemed to work pretty well, except if you click on the button in the example, it isn't transparent until the parent panel is updated. I think this is because the look and feel ( or the component) calls the component's paintComponent() method directly, instead of painting the whole screen again. This is good for performance, but it breaks this alpha filter method.

I'll post the code I used to test this out, maybe it will help someone work around this problem...

package nep.components;
 
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.LayoutManager;
import java.awt.image.BufferedImage;
import java.awt.image.DataBuffer;
 
import javax.swing.Box;
import javax.swing.BoxLayout;
import javax.swing.ImageIcon;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
 
public class TransparentPanel extends JPanel {
    
    private float transPercentage = 1.0F;
    private BufferedImage transBuffer;
    private Graphics transG;
    private int transValue = -16777216;
    private Image image;
    
    public TransparentPanel(LayoutManager layout, boolean isDoubleBuffered) {
        super(layout, isDoubleBuffered);
    }
    
    public TransparentPanel(LayoutManager layout) {
        super(layout);
    }
    
    public TransparentPanel(boolean isDoubleBuffered) {
        super(isDoubleBuffered);
    }
    
    public TransparentPanel() {
        super();
    }
    
    public TransparentPanel( Image image ) {
        this();
        this.image = image;
    }
    
    public void setTransparency( float percent ) {
        if ( percent > 1.0F ) {
            percent = 1.0F;
        } else if ( percent < 0F ) {
            percent = 0F;
        }
        transPercentage = percent;
        transValue = Math.round( 255 * transPercentage ) << 24;
        repaint();
    }
    
    public float getTransparency() {
        return transPercentage;
    }
    
    public void setImage( Image image ) {
        this.image = image;
        repaint();
    }
    
    public Image getImage() {
        return image;
    }
    
    public void paintComponent( Graphics g ) {
        super.paintComponent( g );
        
        // paint image in center of component
        if ( image != null ) {
            Dimension size = getSize();
            g.drawImage( image, ( size.width - image.getWidth( this ) ) / 2, 
                    (size.height - image.getHeight( this ) ) / 2, this );
        } 
    }
    
    public void paintChildren( Graphics g ) {
        Dimension size = getSize();
        if ( transBuffer == null || 
                transBuffer.getWidth() != size.width || 
                transBuffer.getHeight() != size.height ) {
            transBuffer = new BufferedImage( size.width, size.height, BufferedImage.TYPE_INT_ARGB );
            transG = transBuffer.getGraphics();
        }
        g.setColor( new Color( 0, 0, 0, 0 ) );
        g.fillRect( 0, 0, size.width, size.height );
        super.paintChildren( transG );
        DataBuffer data = transBuffer.getRaster().getDataBuffer();
        int elem = 0;
        for ( int i = 0; i < data.getSize(); i++ ) {
            elem = data.getElem( i );
            if ( elem >>> 24 != 0 ) {
                elem = elem << 8;
                elem = elem >>> 8;
                elem = elem + transValue;
                data.setElem( i, elem );
            }
        }
        g.drawImage( transBuffer, 0, 0, this );
    }
    
    public static void main( String args[] ) {
        JFrame f = new JFrame( "TransparentPanel test" );
        Image img = new ImageIcon( "image.gif" ).getImage();
        TransparentPanel tp = new TransparentPanel( img );
        tp.setLayout( new BoxLayout( tp, BoxLayout.X_AXIS ) );
        tp.setBackground( Color.blue );
        tp.setOpaque( true );
        tp.setTransparency( 0.5F );
        tp.add( Box.createHorizontalStrut( 80 ) );
        JLabel l = new JLabel( "Test" );
        tp.add( l );
        JButton b = new JButton( "Button" );
        tp.add( b );
        f.getContentPane().add( tp );
        f.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
        f.setSize( 300, 300 );
        f.setLocationRelativeTo( null );
        f.setVisible( true );
    }
}