Compositing Graphics and transparency question

Question

Hi there
I am drawing a floorplan on a jpanel. The floorplan consists of several "layers", meaning, I draw the floorplan and other geometrical things (representing real world things such a batch of plants).
Those things drawn on the floorplan are partially transparent. I did this is to get a more intense color if some things are drawn on top of eachother. However, the background of the floorplan I don't want to see when those obects are drawn on a particular location. So for that reason I drew the background (not to confuse with the background of the JPanel) later with an AlphaComposite "DST_OVER", hoping that I would not see this background where the previously drawn object were. This worked partially, meaning, because those objects were semi transparent to begin with, I can see the background also.
Does anyone know how to solve this?

Answer 1

I think we're in agreement: try it!
import java.awt.*;
import java.awt.event.*;
import java.awt.image.*;
import java.io.*;
import java.net.*;
import javax.imageio.*;
import javax.swing.*;
public class Composing {
    public static void main(String[] args) throws IOException {
        URL url = new URL("http://today.java.net/jag/bio/JagHeadshot-small.jpg");
        BufferedImage background = ImageIO.read(url);
        int w=background.getWidth(), h=background.getHeight();
        BufferedImage foreground = new BufferedImage(w, h, BufferedImage.TYPE_INT_ARGB);
        Graphics2D g = foreground.createGraphics();
        g.setColor(new Color(0,0xff,0,0x40));
        g.fillRect(0,0,w/8,h);
        g.setColor(new Color(0,0,0xff,0x40));
        g.fillRect(0,7*h/8,w,h);
        g.setColor(new Color(0xff,0,0,0x40));
        g.fillRect(7*w/8,0,w,h);
        g.setColor(new Color(0xff,0xff,0,0x40));
        g.fillRect(0,0,w,h/8);
        byte[] identity = new byte[256];
        for(int i=0; i<256; ++i)
            identity[i] = (byte) i;
        byte[] adjust = new byte[256];
        for(int i=0; i<256; ++i)
            adjust[i] = (byte) ((i<1) ? i : 255);
        ByteLookupTable table = new ByteLookupTable(0, new byte[][]{identity, identity, identity, adjust});
        LookupOp op = new LookupOp(table, null);
        op.filter(foreground, foreground);
        g.setComposite(AlphaComposite.getInstance(AlphaComposite.DST_OVER));
        g.drawRenderedImage(background, null);
        g.dispose();
        JFrame f = new JFrame("Composing");
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.getContentPane().add(new JLabel(new ImageIcon(foreground)));
        f.pack();
        f.setLocationRelativeTo(null);
        f.setVisible(true);