Making certain parts of an image transparent laying it on top of another

Question

I am currently doing an MSc project, where I am simulating the behaviour of ants in Java, however I have hit a stumbling block. Currently, I have an AntTerrain class that contains a paintComponent method. Every time this is called it draws a BufferedImage of the terrain which consists of all the obstacles and food, over which the ants are drawn by iterating through a 2D array. I have been trying to implement pheromones into the simulation, and although I am able to do it I want to have the functionality to be able view or not view them. Initially I had two Buffered images, one containing all the terrain information and the pheromones, and one not containing the pheromones. This became rather costly as the simulation progressed and more pheromones were laid, so recently I have been trying to create a buffered image of just the pheromones which I can overlay onto the terrain as and when the user wants to.
This has been causing me immense problems. I have had limited success with using the alpha channels of the terrain image, by making it almost completely transparent, then drawing it on top of the terrain image, by setting the compositing to SRC_OVER, however, this muddys the terrain image below.
I have been looking up various books and documentation on line to try and work out how to do it in a more elegant fashion without success. Peter has come up with the idea of using either Regions of Interest, Bitmasking or a combination of both. Neither of these techniques have been particularly well explained in the documentation I have found and I am more confused than when I started. Any help you could give me in this matter would be greatly appreciated.

Answer 1

If the pheromones can be drawn independent of the array of ants then I would use a single BufferedImage and make a new one for each user change of pheromone visibility. If this is to unwieldy of if the pheromones require the ant array then I would try an OverlayLayout with the BufferedImage of obstacles/food/etc on the bottom and the pheromones on top. The ants could go on either one; maybe they would look better drawn above the pheromones on the top layer (guessing...). If you want to draw the pheromones with an AlphaComposite and they would look okay drawn above the ants you could draw the ants below and the pheromones above. This would be easy and fast.
Here's an example of the OverlayLayout option:
import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import java.awt.image.BufferedImage;
import javax.swing.*;
public class ToggleTest
    public static void main(String[] args)
        JPanel panel = new JPanel();
        OverlayLayout overlay = new OverlayLayout(panel);
        panel.setLayout(overlay);
        panel.add(new Top());
        panel.add(new Bottom());
        JFrame f = new JFrame("click me");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setContentPane(panel);
        f.setSize(400,400);
        f.setLocation(200,200);
        f.setVisible(true);
class Top extends JPanel
    boolean showPheromones;
    public Top()
        showPheromones = false;
        setOpaque(false);
        addMouseListener(toggler);
    protected void paintComponent(Graphics g)
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D)g;
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        if(showPheromones)
            int w = getWidth();
            int h = getHeight();
            AlphaComposite ac =
                AlphaComposite.getInstance(AlphaComposite.SRC_OVER, 0.25f);
            g2.setComposite(ac);
            g2.setPaint(Color.red);
            g2.fill(new Ellipse2D.Double(w*2/5, h/3, w/3, h/3));
    private MouseListener toggler = new MouseAdapter()
        public void mousePressed(MouseEvent e)
            showPheromones = !showPheromones;
            repaint();
class Bottom extends JPanel
    BufferedImage image;
    public Bottom()
        addComponentListener(resizer);
    protected void paintComponent(Graphics g)
        if(image == null)
            makeImage();
        g.drawImage(image, 0, 0, this);
    private BufferedImage makeImage()
        int w = getWidth();
        int h = getHeight();
        image = new BufferedImage(w, h, BufferedImage.TYPE_INT_RGB);
        Graphics2D g2 = image.createGraphics();
        g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                            RenderingHints.VALUE_ANTIALIAS_ON);
        g2.setBackground(UIManager.getColor("Panel.background"));
        g2.clearRect(0, 0, w, h);
        g2.setPaint(Color.red);
        g2.draw(new Ellipse2D.Double(w/2, h/5, w/9, h/3));
        g2.setPaint(Color.green.darker());
        g2.draw(new Ellipse2D.Double(25, 16, w/6, h/2));
        g2.setPaint(Color.blue);
        g2.draw(new Ellipse2D.Double(w/4, h*5/6, 125, 25));
        g2.setPaint(Color.blue);
        g2.draw(new Ellipse2D.Double(w*5/9, h/2, 55, 60));
        g2.dispose();
        return image;
    private ComponentListener resizer = new ComponentAdapter()
        public void componentResized(ComponentEvent e)
            image = null;
            repaint();
}