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Learning Java, 4th Edition by Patrick Niemeyer, Daniel Leuck

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Displaying Images

So far, we’ve worked with methods for drawing simple shapes and displaying text. For more complex graphics, we’ll be working with images. In a typical Swing application, the simplest way to display an image in your application is to use an ImageIcon with a JLabel component. Here, we are talking about working with image data at a lower level, for painting. The 2D API has a powerful set of tools for generating and displaying image data. We’ll start with the basics of the java.awt.Image class and see how to load an image into an application and draw it where you want it. The Java AWT toolkit will handle most of the details for us. In the next chapter, we’ll go further to discuss how to manage image loading manually as well as how to create and manipulate raw pixel data, allowing you to create any kind of graphics you can dream up.

The core AWT supports images encoded in JPEG, PNG, and GIF. (This includes GIF89a animations so that you can work with simple animations as easily as static images.) If you need to work with other types of images, you can turn to the Java Advanced Imaging javax.imageio framework. We’ll mention it briefly here and again in the next chapter when we discuss the BufferedImage class.

In many ways, the ImageIO framework supercedes and replaces the older image handling functionality of the core AWT just as Swing extends and replaces the old AWT components. The ImageIO framework is easily extensible for new image types through plug-ins. However, out of the box, all that it adds in terms of image type support is the ability to read bitmap (BMP) and wireless bitmap (WBMP) images. Since most Java code can and does use the original AWT functionality, that is where we’ll focus.

The Image Class

The java.awt.Image class represents a view of an image. The view is created from an image source that produces pixel data. Images can be from a static source, such as a JPEG file, or a dynamic one, such as a video stream or a graphics engine.

AWT Images are created with the getImage() and createImage() methods of the java.awt.Toolkit class. There are two forms of each method, which accept a URL or plain filename, respectively. createImage() can also accept a byte array of image data directly.

When bundling images with your application, you should use the Class class’s getResource() method (discussed in Chapter 1) to construct a URL reference to the file from the application classpath. getResource() allows you to bundle images along with your application, inside JAR files or anywhere else in the classpath. The following code fragment shows some examples of loading images with the getImage() method:

    Toolkit toolkit = Toolkit.getDefaultToolkit();

    // Application resource URL - Best method
    URL daffyURL = getClass().getResource("/cartoons/images/daffy.gif");
    Image daffyDuckImage = toolkit.getImage( daffyURL );

    // Absolute URL -
    URL monaURL = new URL( "http://myserver/images/mona_lisa.png");
    Image monaImage = toolkit.getImage( monaURL );

    // Local file -
    Image elvisImage = toolkit.getImage("c:/elvis/lateryears/fatelvis1.jpg" );

The createImage() method looks just like getImage(); the difference is that getImage() “interns” images and shares them when it receives multiple requests for the same data. The createImage() method does not do this (it creates a new Image object every time) and relies on you to cache and share the image. getImage() is convenient in an application that uses a limited number of images for the life of the application, but it may not ever release the image data. You should use createImage() and cache the Image objects yourself when it’s an issue.

The javax.imageio.ImageIO class similarly provides several static read() methods that can load images from a File, URL, or InputStream:

    URL daffyURL = getClass().getResource("/cartoons/images/daffy.gif");
    Image daffyDuckImage = ImageIO.read( daffyURL );

We’ll discuss image loading with AWT and the ImageIO framework in more detail in Chapter 21.

Once we have an Image object, we can draw it into a graphics context with the drawImage() method of the Graphics class. The simplest form of the drawImage() method takes four parameters: the Image object, the x, y coordinates at which to draw it, and a reference to a special image observer object. We’ll show an example involving drawImage() soon, but first let’s say a word about image observers.

Image Observers

Images are processed asynchronously, which means that Java performs image operations, such as loading and scaling in the background (allowing the user code to continue). In a typical client application, this might not be important; images may be small for things like buttons, and are probably bundled with the application for almost instant retrieval. However, Java was designed to work with image data over the Web as well as locally, and you will see this expressed in the APIs for working with image data.

For example, the getImage() method always returns immediately, even if the image data has to be retrieved over the network from Mars and isn’t available yet. In fact, if it’s a new image, Java won’t even begin to fetch the data until we try to display or manipulate it. The advantage of this technique is that Java can do the work of a powerful, multithreaded image processing environment for us. However, it also introduces several problems. If Java is loading an image for us, how do we know when it’s completely loaded? What if we want to work with the image as it arrives? What if we need to know properties of the image (like its dimensions) before we can start working with it? What if there’s an error in loading the image?

These issues are handled by image observers, objects that implement the ImageObserver interface. All operations that draw or examine Image objects are asynchronous and take an image observer object as a parameter. The ImageObserver monitors the image operation’s status and can make that information available to the rest of the application. When image data is loaded from its source by the graphics system, your image observer is notified of its progress, including when new pixels are available, when a complete frame of the image is ready, and if there is an error during loading. The image observer also receives attribute information about the image, such as its dimensions and properties, as soon as they are known.

The drawImage() method, like other image operations, takes a reference to an ImageObserver object as a parameter. drawImage() returns a boolean value specifying whether or not the image was painted in its entirety. If the image data has not yet been loaded or is only partially available, drawImage() paints whatever fraction of the image it can and returns. In the background, the graphics system starts (or continues) loading the image data. The image observer object is registered as interested in information about the image. The observer is then called repeatedly as more pixel information is available and again when the entire image is complete. The image observer can do whatever it wants with this information. Most often the information is used to call repaint() to prompt the application to draw the image again with the updated data. In this way, an application or applet can draw the image as it arrives for a progressive loading effect. Alternatively, it could wait until the entire image is loaded before displaying it.

Image observers are covered in Chapter 21. For now, let’s avoid the issue by using a prefabricated image observer. The Component class implements the ImageObserver interface and provides some simple repainting behavior, which means every component can serve as its own default image observer. We can simply pass a reference to whatever component is doing the painting as the image observer parameter of a drawImage() call:

    public void paint( Graphics g ) {
        g.drawImage( monaImage, x, y, this );
        ...

Our component serves as the image observer and calls repaint() for us to redraw the image as necessary. If the image arrives slowly, our component is notified repeatedly as new chunks become available. As a result, the image appears gradually as it’s loaded.[44]

Preloading images

We’ll discuss image loading in more detail in the next chapter when we look at the MediaTracker utility, which monitors the load progress of one or more images. However, we’ll skip ahead a bit here and show you the easy shortcut for loading a single image and making sure it’s complete and ready to draw. You can use the javax.swing.ImageIcon class to do the dirty work for you:

    ImageIcon icon = new ImageIcon("myimage.jpg");
    Image image = icon.getImage();

Images loaded by the ImageIO read() methods are returned fully loaded. ImageIO provides its own API for monitoring image loading progress. That API follows a more standard event source/listener pattern, but we won’t get into it here.

Scaling and Size

Another version of drawImage() renders a scaled version of the image:

    g.drawImage( monaImage, x, y, x2, y2, this );

This draws the entire image within the rectangle formed by the points x, y and x2, y2, scaling as necessary. drawImage() behaves the same as before; the image is processed by the component as it arrives, and the image observer is notified as more pixel data and the completed image are available. Several other overloaded versions of drawImage() provide more complex options: you can scale, crop, and perform some simple transpositions.

Normally, however, for performance you want to make a scaled copy of an image (as opposed to simply painting one at draw time); you can use getScaledInstance() for this purpose. Here’s how:

    Image scaledDaffy =
      daffyImage.getScaledInstance( 100, 200, Image.SCALE_AREA_AVERAGING );

This method scales the original image to the given size—in this case, 100 by 200 pixels. It returns a new Image that you can draw like any other image. SCALE_AREA_AVERAGING is a constant that tells getScaledImage() what scaling algorithm to use. The algorithm used here tries to do a decent job of scaling at the expense of time. Some alternatives that take less time are SCALE_REPLICATE, which scales by replicating scan lines and columns (which is fast, but probably not pretty). You can also specify either SCALE_FAST or SCALE_SMOOTH and let the implementation choose an appropriate algorithm that optimizes for time or quality. If you don’t have specific requirements, you should use SCALE_DEFAULT, which ideally would be set by a preference in the user’s environment.

If you are going to draw the image more than once (which you almost always will), creating a scaled copy of the image can improve performance dramatically. Otherwise, repeated calls to drawImage() with scaling requirements cause the image to be scaled every time, which wastes processing time.

The Image getHeight() and getWidth() methods retrieve the dimensions of an image. Because this information may not be available until the image data is completely loaded, both methods also take an ImageObserver object as a parameter. If the dimensions aren’t yet available, they return values of -1 and notify the observer when the actual value is known. We’ll see how to deal with these and other problems a bit later. For now, we’ll continue to use our Component as the image observer and move on to some general painting techniques.



[44] The awt.image.incrementaldraw and awt.image.redrawrate system properties control this behavior. redrawrate limits how often repaint() is called; the default value is every 100 milliseconds. incrementaldraw’s default value, true, enables this behavior. Setting it to false delays drawing until the entire image has arrived.

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