Blending Colors

Question

Hey guys,
I'm experimenting with composites, trying to wrap my head around opacity, alpha values, and color blending. I'm trying to display several rectangles, each with a different color. Where the rectangles overlap, their colors should mix together (so if a red and a blue rectangle overlap, the overlap should be purple). I have something that almost does what I want, except that where the rectangles do not overlap, they appear too light (in other words, their alpha value is too low).
I understand why this is happening: since the rectangles all have an alpha level of 1/2, the white background is showing through where there is no overlap. Is there a way to blend the colors together in a way that the alpha level of any given pixel will be 100%, but still have color blending where there is any overlap of colors? In the SSCCE below, the goal would be to have a (fully opaque) red rectangle on top, a purple rectangle in the middle where the red and blue overlap, and the bottom of the screen should show a mixture of red, blue, and green.
The SSCCE:
import javax.swing.*;
import java.awt.*;
public class ColorBlendTest extends JPanel{
   Color[] colors = {Color.RED, Color.BLUE, Color.GREEN};
    public ColorBlendTest() {
         super();
         JFrame frame = new JFrame("Color Graph");
         frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
         frame.add(this);
         frame.setSize(500,500);
         frame.setVisible(true);
    public void paintComponent(Graphics g){
         super.paintComponent(g);
         Graphics2D g2d = (Graphics2D)g;
         g2d.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER, .5f));
         for(int i = 1; i <= colors.length; i ++){
              int x = 0;
              int y = getHeight() - getHeight()/i;
              int width = getWidth();
              int height = getHeight();
              g2d.setColor(colors[i-1]);
              g2d.fillRect(x, y, width, height);
    public static void main(String[] args) {
        new ColorBlendTest();
}I appreciate any help/suggestions/pointers you can offer.
Thanks,
Kevin

Answer 1

I finally figured something out, using an Image to act as a middle step between painting and adjusting the alpha value. It's probably a bit of a hack, and there are a ton of ways to optimize it, but here's what I came up with:
import javax.swing.*;
import java.awt.*;
import java.awt.image.*;
public class ColorBlendTest extends JPanel{
   Color[] colors = {Color.RED, Color.BLUE, Color.GREEN};
    public ColorBlendTest() {
         super();
         JFrame frame = new JFrame("Color Graph");
         frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
         frame.add(this);
         frame.setSize(500,500);
         frame.setVisible(true);
    public void paintComponent(Graphics g){
         super.paintComponent(g);
         BufferedImage image = getGraphicsConfiguration().createCompatibleImage(getWidth(), getHeight(), Transparency.TRANSLUCENT);
         Graphics2D imageGraphics = (Graphics2D) image.getGraphics();
         imageGraphics.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER, 1f/colors.length));
         for(int i = 1; i <= colors.length; i ++){
              int x = 0;
              int y = getHeight() - getHeight()/i;
              int width = getWidth();
              int height = getHeight();
              imageGraphics.setColor(colors[i-1]);
              imageGraphics.fillRect(x, y, width, height);
       for(int w = 0; w < image.getWidth(); w++){
             for(int h = 0; h < image.getHeight(); h++){
                  int rgb = image.getRGB(w, h);
                  int a = (rgb >>> 24) & 0xFF;
               int r = (rgb >>> 16) & 0xFF;
               int gr = (rgb >>> 8) & 0xFF;
               int b = (rgb >>> 0) & 0xFF;
               a = 255;
               rgb = (a << 24) | (r << 16) | (gr << 8) | b;
               image.setRGB(w, h, rgb);
         g.drawImage(image, 0, 0, null);     
    public static void main(String[] args) {
        new ColorBlendTest();
}I'm still extremely open to suggestions though, so I'd definitely love some feedback, if you have any.