Animating a Sequence of Images

Computer animation at its most basic consists of drawing an image at a specific place, moving the location of the image, and telling the computer to redraw the image at its new location. Many animations on web pages are a series of image files, usually GIF files that are displayed in the same place in a certain order. You can do this to simulate motion or to create some other effect. The first program you will be writing today uses a series of image files to create an animated picture of the Anastasia Island Lighthouse in St. Augustine, Florida. Several details about the animation will be customizable with parameters, so you can replace any images of your own for those provided for this example. Create a new file in your word processor called Enter Listing 24.1 into the file, and remember to save the file when you're done entering the text.

Listing 24.1. The Full Text of
 1: import java.awt.*;
 3: public class Animate extends javax.swing.JApplet
 4: implements Runnable {
 6: Image[] picture = new Image[6];
 7: int totalPictures = 0;
 8: int current = 0;
 9: Thread runner;
10: int pause = 500;
12: public void init() {
13: for (int i = 0; i < 6; i++) {
14: String imageText = null;
15: imageText = getParameter("image"+i);
16: if (imageText != null) {
17: totalPictures++;
18: picture[i] = getImage(getCodeBase(), imageText);
19: } else {
20: break;
21: }
22: }
23: String pauseText = null;
24: pauseText = getParameter("pause");
25: if (pauseText != null) {
26: pause = Integer.parseInt(pauseText);
27: }
28: }
30: public void paint(Graphics screen) {
31: super.paint(screen);
32: Graphics2D screen2D = (Graphics2D) screen;
33: if (picture[current] != null) {
34: screen2D.drawImage(picture[current], 0, 0, this);
35: }
36: }
38: public void start() {
39: if (runner == null) {
40: runner = new Thread(this);
41: runner.start();
42: }
43: }
45: public void run() {
46: Thread thisThread = Thread.currentThread();
47: while (runner == thisThread) {
48: repaint();
49: current++;
50: if (current >= totalPictures) {
51: current = 0;
52: }
53: try {
54: Thread.sleep(pause);
55: } catch (InterruptedException e) {
56: // do nothing
57: }
58: }
59: }
61: public void stop() {
62: if (runner != null) {
63: runner = null;
64: }
65: }
66: }

Because animation is usually a process that continues over a period of time, the portion of the program that manipulates and animates images should be designed to run in its own thread. This becomes especially important in a Swing program that must be able to respond to user input while an animation is taking place. Without threads, animation often takes up so much of the Java interpreter's time that the rest of a program's graphical user interface is sluggish to respond. The Animate program uses the same threaded applet structure that you used during Hour 19, "Creating a Threaded Program." Threads also are useful in animation coding because they give you the ability to control the timing of the animation. The THRead.sleep() method is an effective way to determine how long each image should be displayed before the next image is shown. The Animate applet retrieves images as parameters on a web page. The parameters should have names starting at image0 and ending at the last image of the animation, such as image3 in this hour's example. The maximum number of images that can be displayed by this applet is six, but you could raise this number by making changes to Lines 6 and 13. The totalPicture integer variable determines how many different images will be displayed in an animation. If less than six image files have been specified by parameters, the Animate applet will determine this during the init() method when imageText equals null after Line 15. The speed of the animation is specified by a "pause" parameter. Because all parameters from a web page are received as strings, the Integer.parseInt() method is needed to convert the text into an integer. The pause variable keeps track of the number of milliseconds to pause after displaying each image in an animation.

Loading and Displaying Images

As with most threaded programs, the run() method contains the main part of the program. A while (runner == thisThread) statement in Line 47 causes Lines 48–57 to loop until something causes these two Thread objects to have different values. The first thing that happens in the while loop is a call to the applet's repaint() method. This statement requests that the applet's paint() method be called so that the screen can be updated. Use repaint() any time you know something has changed and the display needs to be changed to bring it up to date. In this case, every time the Animate loop goes around once, a different image should be shown.

By the Way

In Java, you can never be sure that calling repaint() will result in the component or applet window being repainted. The interpreter will ignore calls to repaint() if it can't process them as quickly as they are being called, or if some other task takes up most of its time.

The paint() method in Lines 30–36 contains the following statements:

Graphics2D screen2D = (Graphics2D) screen;
if (picture[current] != null)
 screen2D.drawImage(picture[current], 0, 0, this);

First, a Graphics2D object is cast so that it can be used when drawing to the applet window. Next, an if statement determines whether the Image object stored in picture[current] has a null value. When it does not equal null, this indicates that an image is ready to be displayed. The drawImage() method of the screen2D object displays the current Image object at the (x,y) position specified.

By the Way

The paint() method of this applet does not call the paint() method of its superclass, unlike some of the other graphical programs in the tutorial, because it makes the animated sequence look terrible. The applet's paint() method clears the window each time it is called, which is okay when you're drawing a graphical user interface or some other graphics that don't change. However, clearing it again and again in a short time causes an animation to flicker.

The this statement sent as the fourth argument to drawImage() enables the program to use a class called ImageObserver. This class tracks when an image is being loaded and when it is finished. The JApplet class contains behavior that works behind the scenes to take care of this process, so all you have to do is specify this as an argument to drawImage() and some other methods related to image display. The rest is taken care of for you. An Image object must be created and loaded with a valid image before you can use the drawImage() method. The way to load an image in an applet is to use the getImage() method. This method takes two arguments: the web address or folder that contains the image file and the filename of the image. The first argument is taken care of with the getCodeBase() method, which is part of the JApplet class. This method returns the location of the applet itself, so if you put your images in the same folder as the applet's class file, you can use getCodeBase(). The second argument should be a GIF file or JPEG file to load. In the following example, a turtlePicture object is created and an image file called Mertle.gif is loaded into it:

Image turtlePicture = getImage(getCodeBase(), "Mertle.gif");

By the Way

As you look over the source code to the Animate applet, you might wonder why the test for a null value in Line 33 is necessary. This check is required because the paint() method may be called before an image file has been fully loaded into a picture[] element. Calling getImage() begins the process of loading an image. To prevent a slowdown, the Java interpreter continues to run the rest of the program while images are being loaded.

Storing a Group of Related Images

In the Animate applet, images are loaded into an array of Image objects called pictures. The pictures array is set up to handle six elements in Line 6 of the program, so you can have Image objects ranging from picture[0] to picture[5]. The following statement in the applet's paint() method displays the current image:

screen.drawImage(picture[current], 0, 0, this);

The current variable is used in the applet to keep track of which image to display in the paint() method. It has an initial value of 0, so the first image to be displayed is the one stored in picture[0]. After each call to the repaint() statement in Line 48 of the run() method, the current variable is incremented by one in Line 49. The totalPictures variable is an integer that keeps track of how many images should be displayed. It is set when images are loaded from parameters off the web page. When current equals totalPictures, it is set back to 0. As a result, current cycles through each image of the animation, and then begins again at the first image.