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Blending Slides

Although Blender1 and Blender2 let you experience image blending via both techniques, neither offers the best venue for demonstrating this transition effect's usefulness. For this reason, I've created a SlideShow application that employs blending to transition from one slide to the next. Check out Listing 4 for this application's source code.

Listing 4 SlideShow.java.

// SlideShow.java

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import java.awt.image.*;

import java.io.*;

import java.util.ArrayList;

import javax.imageio.*;

import javax.swing.*;

 * This class describes and contains the entry point to the slideshow
 * application.

public class SlideShow
  static Projector projector;

  * Create SlideShow's GUI and start the projector.

  SlideShow ()
   JFrame frame = new JFrame ("SlideShow 1.0");
   frame.addWindowListener (new WindowAdapter ()
                  public void windowClosing (WindowEvent we)
                   projector.stop ();
                   System.exit (0);

   Screen screen = new Screen (projector.getSlideWidth (),
                 projector.getSlideHeight ());
   frame.setContentPane (screen);

   frame.pack ();
   frame.setResizable (false);
   frame.setLocationRelativeTo (null); // center frame on screen
   frame.setVisible (true);

   projector.start (screen);

  * Application entry point.
  * @param args array of command-line arguments

  public static void main (String [] args) throws IOException
   if (args.length != 1)
     System.err.println ("usage: java SlideShow imagesPath");

   projector = new Projector (args [0]);

   Runnable r = new Runnable ()
            public void run ()
             new SlideShow ();
   EventQueue.invokeLater (r);

 * This class describes the projector that runs the slideshow.

class Projector implements ActionListener
  * Create the projector, loading images from the specified path into
  * slides.
  * @param imagesPath path to images (GIFs and/or JPEGS, and/or other
  * formats)

  Projector (String imagesPath) throws IOException
   load (imagesPath);

   slideCount = slides.size ();

   show_timer = new Timer (SLIDE_DELAY, this);

  * Callback for show_timer -- invoked approximately every SLIDE_DELAY
  * milliseconds.
  * @param ae action event

  public void actionPerformed (ActionEvent ae)
   ActionListener fader;
   fader = new ActionListener ()
         int weight = 100;

         public void actionPerformed (ActionEvent ae)
           // When the projector's stop() method is invoked, it
           // stops the show_timer. However, the fade_timer will
           // continue to run until 100 steps have been completed.
           // The following if statement ensures that the fade timer
           // also stops as quickly as possible.

           if (!show_timer.isRunning ())
             fade_timer.stop ();

           // Because slideIndex is initialized to 1, slideIndex-1
           // results in the introductory slide being blended with
           // the first real slide of the slideshow.

           BufferedImage biSlide;
           biSlide = blend (slides.get (slideIndex-1),
                   slides.get (slideIndex), weight/100.0);

           // Project the slide onto the screen.

           screen.showSlide (biSlide);

           // Countdown until weight drops below 0. At that point,
           // the fade_timer must stop. Also, the slideshow must
           // advance to the next slide. If there are no more
           // slides, the show_timer must stop.

           if (--weight < 0)
             fade_timer.stop ();

             if (++slideIndex == slideCount)
               show_timer.stop ();

   // The following timer must finish before the next invocation of this
   // method.

   fade_timer = new Timer (FADE_DELAY, fader);
   fade_timer.start ();

  * Return a slide's height -- each slide must have the same height.

  int getSlideHeight ()
   return slideHeight;

  * Return a slide's width -- each slide must have the same width.

  int getSlideWidth ()
   return slideWidth;

  * Start the projector.
  * @param screen the display surface on which slides are projected

  void start (Screen screen)
   this.screen = screen;

   // Obtain introductory slide from the screen and replace null
   // placeholder with this special slide.

   slides.set (0, screen.getIntroSlide ());

   show_timer.start ();

  * Stop the projector.

  void stop ()
   show_timer.stop ();

  * Blend the contents of two BufferedImages according to a specified
  * weight.
  * @param bi1 first BufferedImage
  * @param bi2 second BufferedImage
  * @param weight the fractional percentage of the first image to keep
  * @return new BufferedImage containing blended contents of BufferedImage
  * arguments

  private BufferedImage blend (BufferedImage bi1, BufferedImage bi2,
                double weight)
   BufferedImage bi3 = new BufferedImage (slideWidth, slideHeight,
   Graphics2D g2d = bi3.createGraphics ();
   g2d.drawImage (bi1, null, 0, 0);
   g2d.setComposite (AlphaComposite.getInstance (AlphaComposite.SRC_OVER,
                          (float) (1.0-weight)));
   g2d.drawImage (bi2, null, 0, 0);
   g2d.dispose ();

   return bi3;

  * Load all images from a specific directory. Each image is converted into
  * a slide.
  * @param imagesPath path to images directory
  * @throws IOException imagesPath represents a file, an image cannot be
  * read, an image's width doesn't match the slide width (or the height
  * doesn't match the slide height), or fewer than two images are loaded

  private void load (String imagesPath) throws IOException
   File imageFilesPath = new File (imagesPath);
   if (!imageFilesPath.isDirectory ())
     throw new IOException (imagesPath+" identifies a file");

   slides = new ArrayList<BufferedImage> ();

   // To simplify the previous action listener, a null placeholder for the
   // screen's introductory slide is added to the slides array list via the
   // following method call. This placeholder is replaced with the actual
   // introductory slide during the projector's start() method call.

   slides.add (null);

   File [] imageFiles = imageFilesPath.listFiles ();
   for (File imageFile: imageFiles)
      // Skip subdirectories.

      if (imageFile.isDirectory ())

      // Read next image file. Skip non-image files.

      BufferedImage slide = ImageIO.read (imageFile);
      if (slide == null)

      // Capture initial slide width and height as standard slide width
      // and height.

      if (slideWidth == 0)
        slideWidth = slide.getWidth ();
        slideHeight = slide.getHeight ();

      // All slides must have the same width and height.

      if (slide.getWidth () != slideWidth ||
        slide.getHeight () != slideHeight)
        throw new IOException ("slide width/height does not match "+
                   "widths/heights of previous slides");

      slides.add (slide);

   if (slides.size () < 2)
     throw new IOException ("at least one image must be loaded");

  final static int FADE_DELAY = 10;

  final static int SLIDE_DELAY = 4500;

  private ArrayList<BufferedImage> slides;

  private int slideHeight, slideCount, slideIndex = 1, slideWidth;

  private Screen screen;

  private Timer fade_timer, show_timer;

 * This class creates a panel that displays each slide.

class Screen extends JPanel
  * Create the screen with a specified size. Also create an introductory
  * slide, which the projector later acquires at startup.
  * @param width screen's preferred width
  * @param height screen's preferred height

  Screen (int width, int height)
   size = new Dimension (width, height);

   biIntroSlide = new BufferedImage (width, height,
   Graphics2D g2d = biIntroSlide.createGraphics ();
   g2d.setColor (Color.white);
   g2d.fillRect (0, 0, width, height);
   g2d.setRenderingHint (RenderingHints.KEY_ANTIALIASING,

   g2d.setFont (new Font ("Arial", Font.BOLD | Font.ITALIC, 36));
   FontMetrics fm = g2d.getFontMetrics ();
   Rectangle2D r2d = fm.getStringBounds (TITLE, g2d);
   int text_width = (int)((Rectangle2D.Float) r2d).width;
   int text_height = fm.getHeight ();

   g2d.setColor (Color.black);
   g2d.drawString (TITLE, (width-text_width)/2+SHADOW_OFFSET,

   g2d.setColor (Color.blue);
   g2d.drawString (TITLE, (width-text_width)/2, height/2);
   g2d.dispose ();

  * Return the introductory slide.

  BufferedImage getIntroSlide ()
   return biIntroSlide;

  * Make the screen's preferred size available to the windowing toolkit for
  * layout.

  public Dimension getPreferredSize ()
   return size;

  * Paint the screen.
  * @param g graphics context used to performing painting

  public void paintComponent (Graphics g)
   Graphics2D g2d = (Graphics2D) g;

   // The screen will appear before the projector starts running. Because
   // there is no regular slide at this point, the contents of the
   // introductory slide will be displayed. The introductory slide will
   // eventually fade out and the first projector slide will fade in.

   if (biSlide != null)
     g2d.drawImage (biSlide, null, 0, 0);
     g2d.drawImage (biIntroSlide, null, 0, 0);

  * Display the next slide.
  * @param biSlide next slide to be presented on the screen

  void showSlide (BufferedImage biSlide)
   this.biSlide = biSlide;
   repaint ();

  final static int SHADOW_OFFSET = 3;

  final static String TITLE = "SlideShow 1.0";

  private BufferedImage biSlide, biIntroSlide;

  private Dimension size;

Listing 4 presents a SlideShow class that models a slideshow in terms of Projector and Screen classes—a projector loads images from a specific directory, and then projects them successively onto the screen. Furthermore, this listing refers to images that will be projected (and that have the same size) as slides.

SlideShow first verifies at least one command-line argument (identifying a directory that contains image files), and then creates a projector. In turn, the project loads these images and stores them in an array of BufferedImages. The application then creates the GUI, consisting of a Screen component, and finally starts and identifies the screen to the projector.

The projector-starting method obtains the first slide from the screen and makes sure that it's the first slide to be displayed—the screen's constructor draws an introductory message onto a BufferedImage and also displays this image at startup. This way, the first blending transition will blend the first loaded image with the introductory message, completely replacing this message by the end of the transition.

Moving on, Projector's start() method creates and starts a timer to begin the slideshow. Each timer tick causes the timer's action listener to be invoked, which creates and starts a separate timer, to manage a blending transition from a given slide being completely displayed to the next slide being completely displayed. Both timers stop after the last slide has been completely displayed.

  • It's not necessary (and a potential performance problem) for Projector's blend() method to create a new BufferedImage to hold the blended image each time blend() is invoked. Instead, you could create the BufferedImage in Projector's constructor, and reference this image from within blend().
  • Projector's load() method throws an exception when it encounters an image whose width or height differs from the standard slide width and height (which are set to the dimensions of the first loaded image). You might consider automatically resizing a loaded image to the slide width and height, instead of throwing this exception.
  • Apart from unnecessary BufferedImage creation in the blend() method, SlideShow is a memory hog: It loads all images into memory. If there's not enough virtual machine heap memory to store these images, the virtual machine will indicate an out-of-memory error. You could overcome this problem by loading images on demand, instead of all at once.

After compiling SlideShow.java, run this application with a single command-line argument that identifies a directory containing at least one image file. For example, java SlideShow images assumes that the current directory contains an images subdirectory. In Figure 2, SlideShow's GUI displays its introductory slide.

Figure 2

Figure 2 The introductory slide presents a startup message.

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