Java fade in and out of images

Basically, what this does is use the same alpha value, fading in from 0-1 and then using the same alpha, going from 1-0, allowing the two images to cross fade over each other...

The magic basically, happens in the paintComponent, where the image coming in using the alpha value and the outgoing image uses 1f - alpha.

Switching between the two images is actually a the same process, expect the inImage is swapped for the outImage

The timing is little different. Instead of a straight move from 0-1 using a standard delta (ie 0.01 for example), this uses a time based algorithm.

That is, I use a timer which ticks every 40 milliseconds or so, it then does a calculation based on the amount of time the timer has being running and calculates the alpha value accordingly...

This allows you to change the amount of time the animation will take, but also provides a slightly better algorithm that takes into account the passive nature of Swings rendering engine...

import java.awt.AlphaComposite;
import java.awt.Dimension;
import java.awt.EventQueue;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.Timer;
import javax.swing.UIManager;
import javax.swing.UnsupportedLookAndFeelException;

public class FadeImage {

    public static void main(String[] args) {
        new FadeImage();
    }

    public FadeImage() {
        EventQueue.invokeLater(new Runnable() {
            @Override
            public void run() {
                try {
                    UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
                } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | UnsupportedLookAndFeelException ex) {
                    ex.printStackTrace();
                }

                JFrame frame = new JFrame("Testing");
                frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
                frame.add(new TestPane());
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    public static class TestPane extends JPanel {

        public static final long RUNNING_TIME = 2000;

        private BufferedImage inImage;
        private BufferedImage outImage;

        private float alpha = 0f;
        private long startTime = -1;

        public TestPane() {
            try {
                inImage = ImageIO.read(new File("/path/to/inImage"));
                outImage = ImageIO.read(new File("/path/to/outImage"));
            } catch (IOException exp) {
                exp.printStackTrace();
            }

            final Timer timer = new Timer(40, new ActionListener() {
                @Override
                public void actionPerformed(ActionEvent e) {
                    if (startTime < 0) {
                        startTime = System.currentTimeMillis();
                    } else {

                        long time = System.currentTimeMillis();
                        long duration = time - startTime;
                        if (duration >= RUNNING_TIME) {
                            startTime = -1;
                            ((Timer) e.getSource()).stop();
                            alpha = 0f;
                        } else {
                            alpha = 1f - ((float) duration / (float) RUNNING_TIME);
                        }
                        repaint();
                    }
                }
            });
            addMouseListener(new MouseAdapter() {

                @Override
                public void mouseClicked(MouseEvent e) {
                    alpha = 0f;
                    BufferedImage tmp = inImage;
                    inImage = outImage;
                    outImage = tmp;
                    timer.start();
                }

            });
        }

        @Override
        public Dimension getPreferredSize() {
            return new Dimension(
                            Math.max(inImage.getWidth(), outImage.getWidth()), 
                            Math.max(inImage.getHeight(), outImage.getHeight()));
        }

        @Override
        protected void paintComponent(Graphics g) {
            super.paintComponent(g);
            Graphics2D g2d = (Graphics2D) g.create();
            g2d.setComposite(AlphaComposite.SrcOver.derive(alpha));
            int x = (getWidth() - inImage.getWidth()) / 2;
            int y = (getHeight() - inImage.getHeight()) / 2;
            g2d.drawImage(inImage, x, y, this);

            g2d.setComposite(AlphaComposite.SrcOver.derive(1f - alpha));
            x = (getWidth() - outImage.getWidth()) / 2;
            y = (getHeight() - outImage.getHeight()) / 2;
            g2d.drawImage(outImage, x, y, this);
            g2d.dispose();
        }

    }

}