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Java2D: An Introduction and Tutorial

4. Transparency in Java2D

4.1 Transparency: Overview

Java2D permits you to assign transparency (alpha) values to drawing operations so that the underlying graphics partially show through when you draw shapes or images. You set transparency by creating an AlphaComposite object then passing it to the setComposite method of the Graphics2D object. You create an AlphaComposite by calling AlphaComposite.getInstance with a mixing rule designator and a transparency (or "alpha") value.

There are 8 built-in mixing rules, but the one normally used for drawing with transparency settings is AlphaComposite.SRC_OVER. (See the AlphaComposite API for details) Alpha values range from 0.0f (completely transparent) to 1.0f (completely opaque).

4.2 Transparency: Example Code

Download the source:
TransparencyExample.java
(plus WindowUtilities.java
and ExitListener.java if you don't have them from the previous examples).

import com.sun.java.swing.*;
import java.awt.*;
import java.awt.geom.*;

/** An illustration of the use of AlphaComposite to make partially
 *  transparent drawings.
 *  1998 Marty Hall, http://www.apl.jhu.edu/~hall/java/
 */

public class TransparencyExample extends JPanel {
  private static int gap=10, width=60, offset=20,
                     deltaX=gap+width+offset;
  private Rectangle
    blueSquare = new Rectangle(gap+offset, gap+offset, width, width),
    redSquare = new Rectangle(gap, gap, width, width);

  private AlphaComposite makeComposite(float alpha) {
    int type = AlphaComposite.SRC_OVER;
    return(AlphaComposite.getInstance(type, alpha));
  }

  private void drawSquares(Graphics2D g2d, float alpha) {
    Composite originalComposite = g2d.getComposite();
    g2d.setPaint(Color.blue);
    g2d.fill(blueSquare);
    g2d.setComposite(makeComposite(alpha));
    g2d.setPaint(Color.red);
    g2d.fill(redSquare);
    g2d.setComposite(originalComposite);
  }

  public void paintComponent(Graphics g) {
    super.paintComponent(g);
    Graphics2D g2d = (Graphics2D)g;
    for(int i=0; i<11; i++) {
      drawSquares(g2d, i*0.1F);
      g2d.translate(deltaX, 0);
    }
  }

  public static void main(String[] args) {
    String title = "Transparency example: alpha of the top (red) " +
                   "square ranges from 0.0 at the left to 1.0 at " +
                   "the right. Bottom (blue) square is opaque.";
    WindowUtilities.openInJFrame(new TransparencyExample(),
                                 11*deltaX + 2*gap, deltaX + 3*gap,
                                 title, Color.lightGray);
  }
}

4.3 Transparency: Example Output

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This tutorial was prepared by Marty Hall for work in the Research and Technology Development Center of the Johns Hopkins University Applied Physics Lab, for courses in the Johns Hopkins Part-Time MS Program in Computer Science, and for various industry seminars and courses.

© 1998 Marty Hall.Java 1.2beta4 version.

This article first appeared in November, 1998 and is reprinted with permission of the author.

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