Java – opaque color

The result depends on which compositing rule is specified in the graphics context using setComposite(). This utility may be useful in understanding the various modes. It may also help you in preparing an sscce that exhibits the problem you describe.

Addendum: Here’s an example that shows how one might use AlphaComposite.Src mode for this.

import javax.swing.*;
import java.awt.*;
import java.awt.geom.*;

/** @see http://stackoverflow.com/questions/7823631 */
public class X extends JPanel {

    private static final int SIZE = 300;
    private static final int INSET = 64;
    private static final AlphaComposite OVER_HALF =
        AlphaComposite.getInstance(AlphaComposite.SRC_OVER, 0.5f);
    private boolean src;

    public X(boolean src) {
        this.src = src;
        this.setBackground(Color.lightGray);
    }

    @Override
    public Dimension getPreferredSize() {
        return new Dimension(SIZE, SIZE);
    }

    @Override
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;
        g2.setRenderingHint(
            RenderingHints.KEY_ANTIALIASING,
            RenderingHints.VALUE_ANTIALIAS_ON);
        Line2D line1 = new Line2D.Double(INSET, INSET,
            getWidth() - INSET, getHeight() - INSET);
        Line2D line2 = new Line2D.Double(getWidth() - INSET,
            INSET, INSET, getHeight() - INSET);
        g2.setStroke(new BasicStroke(64,
            BasicStroke.CAP_ROUND,
            BasicStroke.JOIN_BEVEL));
        g2.setComposite(OVER_HALF);
        g2.setColor(Color.red);
        g2.draw(line1);
        if (src) {
            g2.setComposite(AlphaComposite.Src);
        }
        g2.setColor(Color.blue);
        g2.draw(line2);
    }

    public static void main(String[] args) {
        final JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setLayout(new GridLayout(1, 0));
        frame.add(new X(false));
        frame.add(new X(true));
        frame.pack();
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
    }
}