Re: Drawing images on a JPanel

In article <4a031806$0$25069$b9f67a60@news.newsdemon.com>,
 Knute Johnson <nospam@rabbitbrush.frazmtn.com> wrote:


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Well, they _did_ misspell invoke[r]. :-)


In this context, I thought opaque meant "the component paints every
pixel within its bounds." But now I wonder.


I _used_ to see it happening when I'd set opaque to true but fail to
paint every pixel. In retrospect, I was probably painting with a
partially transparent color.


On my implementation, the example's background is gray with no yellow at
all, I think due to multiple-buffering. Graphics#clearRect() says,
"Beginning with Java 1.1, the background color of offscreen images may
be system dependent. Applications should use setColor followed by
fillRect to ensure that an offscreen image is cleared to a specific
color."

With or without super.paintComponent(g), I get interesting artifacts
with non-opaque colors as I resize this window:

<code>
import java.awt.*;
import javax.swing.*;

public class OpaqueTest extends JComponent {

    public OpaqueTest() {
        this.setBackground(Color.yellow); // no effect
    }

    @Override
    public void paintComponent(Graphics g) {
// super.paintComponent(g);
        g.setColor(new Color(255, 0, 0, 128));
        int w = getWidth();
        int h = getHeight();
        g.fillRect(0, 0, w, h);
        g.setColor(new Color(0, 0, 255, 128));
        g.fillRect(w/2 - w/4, h/2 - h/4, w/2, h/2);
    }

    @Override
    public boolean isOpaque() {
        return true;
    }

    public static void main(String[] args) {
        EventQueue.invokeLater(new Runnable() {

            public void run() {
                create();
            }

            private void create() {
                final JFrame f = new JFrame();
                f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                f.setLayout(new GridLayout(0, 2));
                for (int i = 0; i < 4; i++) {
                    f.add(new OpaqueTest());
                }
                f.setSize(240, 180);
                f.setVisible(true);
            }
        });
    }
}
</code>

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