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Exploring Java

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14. Working With Images

Contents:
Image Processing
Working with Audio

14.1 Image Processing

Up to this point, we've confined ourselves to working with the high-level drawing commands of the Graphics class and using images in a hands-off mode. In this section, we'll clear up some of the mystery surrounding images and see how they are produced and used. The classes in the java.awt.image package handle image processing; Figure 14.1 shows the classes in this package.

First, we'll return to our discussion about image observers and see how we can get more control over image data as it's processed asynchronously by AWT components. Then we'll open the hood and have a look at image production. If you're interested in creating sophisticated graphics, such as rendered images or video streams, this will teach you about the foundations of image construction in Java.[1]

[1] You will also want to pay attention to the forthcoming Java Media API. Java Media will support plug-n-play streaming media.

Objects that work with image data fall into one of three categories: image-data producers, image-data consumers, and image-status observers. Image producers implement the ImageProducer interface. They create pixel data and distribute it to one or more consumers. Image consumers implement a corresponding ImageConsumer interface. They eat the pixel data and do something useful with it, such as display it on screen or analyze its contents. Image observers, as I mentioned earlier, implement the ImageObserver interface. They are effectively nosy neighbors of image consumers that watch as the image data arrives.

Image producers generate the information that defines each pixel of an image. A pixel has both a color and a transparency; the transparency specifies how pixels underneath the image show through. Image producers maintain a list of registered consumers for the image and send them this pixel data in one or more passes, as the pixels are generated. Image producers give the consumers other kinds of information as well, such as the image's dimensions. The producer also notifies the consumer when it has reached a boundary of the image. For a static image, such as GIF or JPEG data, the producer signals when the entire image is complete, and production is finished. For a video source or animation, the image producer could generate a continuous stream of pixel data and mark the end of each frame.

An image producer delivers pixel data and other image-attribute information by invoking methods in its consumers, as shown in Figure 14.2. This diagram illustrates an image producer sending pixel data to three consumers by invoking their setPixels() methods.

Each consumer represents a view of the image. A given consumer might prepare the image for display on a particular medium, or it might simply serve as a filter and pass the image data to another consumer down the line.

Figure 14.2 also shows an image observer, watching the status of one of the consumers. The observer is notified as new portions of the image and new attributes are ready. Its job is to track this information and let another part of the application know its status. As I discussed earlier, the image observer is essentially a callback that is notified asynchronously as the image is built. The default Component class image observer that we used in our previous examples called repaint() for us each time a new section of the image was available, so that the screen was updated more or less continuously as the data arrived. A different kind of image observer might wait for the entire image before telling the application to display it; yet another observer might update a loading meter showing how far the image loading had progressed.

Implementing an ImageObserver

To be an image observer, you have to implement the single method, imageUpdate(), defined by the java.awt.image.ImageObserver interface:

public boolean imageUpdate(Image image, int flags, int x, int y, 
                           int width, int height)

imageUpdate() is called by the consumer, as needed, to pass the observer information about the construction of its view of the image. Essentially, any time the image changes, the consumer tells the observer so that the observer can perform any necessary actions, like repainting. image holds a reference to the Image object the consumer is processing. flags is an integer whose bits specify what information about the image is now available. The values of the flags are defined as static identifiers in the ImageObserver interface, as shown in Table 14.1.

Table 14.1: ImageObserver Information Flags
Flag Description
HEIGHT

The height of the image is ready

WIDTH

The width of the image is ready

FRAMEBITS

A frame is complete

SOMEBITS

An arbitrary number of pixels have arrived

ALLBITS

The image is complete

ABORT

The image loading has been aborted

ERROR

An error occurred during image processing; attempts to display the image will fail

The flags determine which of the other parameters, x, y, width, and height, hold valid data and what they mean. To test whether a particular flag in the flags integer is set, we have to resort to some binary shenanigans. The following class, MyObserver, implements the ImageObserver interface and prints its information as it's called:

import java.awt.*; 
import java.awt.image.*; 
 
class MyObserver implements ImageObserver { 
 
    public boolean imageUpdate( Image image, int flags, int x, int y, 
                                int width, int height) { 
 
        if ( (flags & HEIGHT) !=0 ) 
            System.out.println("Image height = " + height ); 
 
        if ( (flags & WIDTH ) !=0 ) 
            System.out.println("Image width = " + width ); 
         
        if ( (flags & FRAMEBITS) != 0 ) 
            System.out.println("Another frame finished."); 
 
        if ( (flags & SOMEBITS) != 0 ) 
            System.out.println("Image section :" 
                         + new Rectangle( x, y, width, height ) ); 
 
        if ( (flags & ALLBITS) != 0 ) { 
            System.out.println("Image finished!"); 
            return false;  
        } 
 
        if ( (flags & ABORT) != 0 )  { 
            System.out.println("Image load aborted..."); 
            return false;  
        } 
 
        return true;  
    } 
} 

The imageUpdate() method of MyObserver is called by the consumer periodically, and prints simple status messages about the construction of the image. Notice that width and height play a dual role. If SOMEBITS is set, they represent the size of the chunk of the image that has just been delivered. If HEIGHT or WIDTH is set, however, they represent the overall image dimensions. Just for amusement, we have used the java.awt.Rectangle class to help us print the bounds of rectangular region.

imageUpdate() returns a boolean value indicating whether or not it's interested in future updates. If the image is finished or aborted, imageUpdate() returns false to indicate it isn't interested in further updates. Otherwise, it returns true.

The following example uses MyObserver to print information about an image as AWT loads it:

import java.awt.*; 
 
public class ObserveImage extends java.applet.Applet {  
    Image img; 
    public void init() { 
        img = getImage( getClass().getResource(getParameter("img")) ); 
        MyObserver mo = new MyObserver(); 
        img.getWidth( mo ); 
        img.getHeight( mo ); 
        prepareImage( img, mo ); 
    } 
} 

After requesting the Image object with getImage(), we perform three operations on it to kick-start the loading process. getWidth() and getHeight() ask for the image's width and height. If the image hasn't been loaded yet, or its size can't be determined until loading is finished, our observer will be called when the data is ready. prepareImage() asks that the image be readied for display on the component. It's a general mechanism for getting AWT started loading, converting, and possibly scaling the image. If the image hasn't been otherwise prepared or displayed, this happens asynchronously, and our image observer will be notified as the data is constructed.

You may be wondering where the image consumer is, since we never see a call to imageUpdate(). That's a good question, but for now I'd like you to take it on faith that the consumer exists. As you'll see later, image consumers are rather mysterious objects that tend to hide beneath the surface of image-processing applications. In this case, the consumer is hiding deep inside the implementation of Applet.

You should be able to see how we could implement all sorts of sophisticated image loading and tracking schemes. The two most obvious strategies, however, are to draw an image progressively, as it's constructed, or to wait until it's complete and draw it in its entirety. We have already seen that the Component class implements the first scheme. Another class, java.awt.MediaTracker, is a general utility that tracks the loading of a number of images or other media types for us. We'll look at it next.

Using a MediaTracker

java.awt.MediaTracker is a utility class that simplifies life if we have to wait for one or more images to be loaded before they're displayed. A MediaTracker monitors the preparation of an image or a group of images and lets us check on them periodically, or wait until they are completed. MediaTracker uses the ImageObserver interface internally to receive image updates.

The following applet, LoadMe, uses a MediaTracker to wait while an image is prepared. It shows a "Loading..." message while it's waiting. (If you are retrieving the image from a local disk or very fast network, this might go by quickly, so pay attention.)

import java.awt.*;
public class TrackImage extends java.applet.Applet implements Runnable { 
    Image img;
    final int MAIN_IMAGE = 0;
    MediaTracker tracker;
    boolean show = false;
    Thread runme;
    String message = "Loading...";
    public void init() {
        img = getImage( getClass().getResource(getParameter("img")) );
        tracker = new MediaTracker(this);
        tracker.addImage( img, MAIN_IMAGE );
    }
    public void start() {
        if ( !tracker.checkID( MAIN_IMAGE ) ) {
            runme = new Thread( this );
            runme.start();
        }
    }
    public void stop() {
        runme.stop();
        runme = null;
    }
    public void run() {
        repaint();
        try {
            tracker.waitForID( MAIN_IMAGE );
        } catch( InterruptedException e) { }
        if ( tracker.isErrorID( MAIN_IMAGE ) )
            message= "Error";
        else
            show = true;
        repaint();
    }
    public void paint( Graphics g ) {
        if ( show )
            g.drawImage( img, 0, 0, this );
        else {
            g.drawRect( 0, 0, getSize().width-1, getSize().height-1);
            g.drawString( message, 20, 20 );
        }
    }
}

From its init() method, LoadMe requests its image and creates a MediaTracker to manage it. Later, after the applet is started, LoadMe fires up a thread to wait while the image is loaded. Note that we don't do this in init() because it would be rude to do anything time-consuming there; it would take up time in an AWT thread that we don't own. In this case, waiting in init() would be especially bad because paint() would never get called and our "loading" message wouldn't be displayed; the applet would just hang until the image loaded. It's often better to create a new thread for initialization and display a startup message in the interim.

When we construct a MediaTracker, we give it a reference to our component (this). After creating a MediaTracker, we assign it images to manage. Each image is associated with an integer identifier we'll use later for checking on its status. Multiple images can be associated with the same identifier, letting us manage them as a group. The value of the identifier is also used to prioritize loading when waiting on multiple sets of images; lower IDs have higher priority. In this case, we want to manage only a single image, so we created one identifier called MAIN_IMAGE and passed it as the ID for our image in the call to addImage().

In our applet's start() method, we call the MediaTracker's checkID() routine with the ID of the image to see if it's already been loaded. If it hasn't, the applet fires up a new thread to fetch it. The thread executes the run() method, which simply calls the MediaTracker waitforID() routine and blocks on the image, waiting for it to finish loading. The show flag tells paint() whether to display our status message or the actual image. We do a repaint() immediately upon entering run() to display the "Loading..." status, and again upon exiting to change the display. We test for errors during image preparation with isErrorID() and change the status message if we find one.

This may seem like a lot of work to go through, just to put up a status message while loading a single image. MediaTracker is more valuable when we are working with many images that have to be available before we can begin parts of our application. It saves us from implementing a custom ImageObserver for every application. In the future, MediaTracker should also be able to track the status of audio clips and other kinds of media (as its name suggests).

Producing Image Data

What if we want to make our own image data? To be an image producer, we have to implement the five methods defined in the ImageProducer interface:

  • addConsumer()

  • startProduction()

  • isConsumer()

  • removeConsumer()

  • requestTopDownLeftRightResend()

Four methods of ImageProducer simply deal with the process of registering consumers. addConsumer() takes an ImageConsumer as an argument and adds it to the list of consumers. Our producer can then start sending image data to the consumer whenever it's ready. startProduction() is identical to addConsumer(), except that it asks the producer to start sending data as soon as possible. The difference might be that a given producer would send the current frame of data or initiate construction of a frame immediately, rather than waiting until its next cycle. isConsumer() tests whether a particular consumer is already registered, and removeConsumer() removes a consumer from the list. We'll see shortly that we can perform these kinds of operations easily with a Vector; see Chapter 7, Basic Utility Classes for a complete discussion of Vector objects.

An ImageProducer also needs to know how to use the ImageConsumer interface of its clients. The final method of the ImageProducer interface, requestTopDownLeftRightResend(), asks that the image data be resent to the consumer, in order, from beginning to end. In general, a producer can generate pixel data and send it to the consumer in any order that it likes. The setPixels() method of the ImageConsumer interface takes parameters telling the consumer what part of the image is being delivered on each call. A call to requestTopDownLeftRightResend() asks the producer to send the pixel data again, in order. A consumer might do this so that it can use a higher quality conversion algorithm that relies on receiving the pixel data in sequence. It's important to note that the producer is allowed to ignore this request; it doesn't have to be able to send the data in sequence.

Color models

Everybody wants to work with color in their application, but using color raises problems. The most important problem is simply how to represent a color. There are many different ways to encode color information: red, green, blue (RGB) values; hue, saturation, value (HSV); hue, lightness, saturation (HLS); and more. In addition, you can provide full color information for each pixel, or you can just specify an index into a color table (palette) for each pixel. The way you represent a color is called a color model. AWT provides tools for two broad groups of color models: direct and indexed.

As you might expect, you need to specify a color model in order to generate pixel data; the abstract class java.awt.image.ColorModel represents a color model. A ColorModel is one of the arguments to the setPixels() method an image producer calls to deliver pixels to a consumer. What you probably wouldn't expect is that you can use a different color model every time you call setPixels(). Exactly why you'd do this is another matter. Most of the time, you'll want to work with a single color model; that model will probably be the default direct color model. But the additional flexibility is there if you need it.

By default, the core AWT components use a direct color model called ARGB. The A stands for "alpha,", which is the historical name for transparency. RGB refers to the red, green, and blue color components that are combined to produce a single, composite color. In the default ARGB model, each pixel is represented by a 32-bit integer that is interpreted as four 8-bit fields; in order, the fields represent the transparency (A), red, green, and blue components of the color, as shown in Figure 14.3.

To create an instance of the default ARGB model, call the static getRGBdefault() method in ColorModel. This method returns a DirectColorModel object; DirectColorModel is a subclass of ColorModel. You can also create other direct color models by calling a DirectColorModel constructor, but you shouldn't need to unless you have a fairly exotic application.

In an indexed color model, each pixel is represented by a smaller amount of information: an index into a table of real color values. For some applications, generating data with an indexed model may be more convenient. If you have an 8-bit display or smaller, using an indexed model may be more efficient, since your hardware is internally using an indexed color model of some form.

While AWT provides IndexedColorModel objects, we won't cover them in this book. It's sufficient to work with the DirectColorModel. Even if you have an 8-bit display, the Java implementation on your platform should accommodate the hardware you have and, if necessary, dither colors to fit your display. Java also produces transparency on systems that don't natively support it by dithering colors.

Creating an image

Let's take a look at producing some image data. A picture may be worth a thousand words, but fortunately, we can generate a picture in significantly fewer than a thousand words of Java. If we're interested in producing a static image with just one frame, we can use a utility class that acts as an ImageProducer for us.

java.awt.image.MemoryImageSource is a simple utility class that implements the ImageProducer interface; we give it pixel data in an array and it sends that data to an image consumer. A MemoryImageSource can be constructed for a given color model, with various options to specify the type and positioning of its data. We'll use the simplest form, which assumes an ARGB color model.

The following applet, ColorPan, creates an image from an array of integers holding ARGB pixel values:


import java.awt.*;
import java.awt.image.*;
public class ColorPan extends java.applet.Applet { 
    Image img;
    int width, height;
    int [] pixData;
    public void init() {
        width = getSize().width;
        height = getSize().height;
        pixData = new int [width * height];
        int i=0;
        for (int y = 0; y < height; y++) {
            int red = (y * 255) / (height - 1);
            for (int x = 0; x < width; x++) {
                int green = (x * 255) / (width - 1);
                int blue = 128;
                int alpha = 255;
                pixData[i++] = (alpha << 24) | (red << 16) | 
                               (green << 8 ) | blue;
            }
        }
    }
    public void paint( Graphics g ) {
        if ( img == null )
            img = createImage( new MemoryImageSource(width, height, 
                                          pixData, 0, width));
        g.drawImage( img, 0, 0, this );
    }
}

Give it a try. The size of the image is determined by the size of the applet when it starts up. What you should get is a very colorful box that pans from deep blue at the upper left-hand corner to bright yellow at the bottom right with green and red at the other extremes. (You'll have to imagine something more colorful than the black and white image above.)

We create the pixel data for our image in the init()method, and then use MemoryImageSource to create and display the image in paint(). The variable pixData is a one-dimensional array of integers that holds 32-bit ARGB pixel values. In init() we loop over every pixel in the image and assign it an ARGB value. The alpha (transparency) component is always 255, which means the image is opaque. The blue component is always 128, half its maximum intensity. The red component varies from 0 to 255 along the y axis; likewise, the green component varies from 0 to 255 along the x axis. The line below combines these components into an ARGB value:

pixData[i++] = (alpha << 24) | (red << 16) | (green << 8 ) | blue; 

The bitwise left-shift operator (<<) should be familiar to C programmers. It simply shoves the bits over by the specified number of positions. The alpha value takes the top byte of the integer, followed by the red, green, and blue values.

When we construct the MemoryImageSource as a producer for this data, we give it five parameters: the width and height of the image to construct (in pixels), the pixData array, an offset into that array, and the width of each scan line (in pixels). We'll start with the first element (offset 0) of pixData; the width of each scan line and the width of the image are the same. The array pixData has width * height elements, which means it has one element for each pixel.

We create the actual image once, in paint(), using the createImage() method that our applet inherits from Component. In the double-buffering and off-screen drawing examples, we used createImage() to give us an empty off-screen image buffer. Here we use createImage() to generate an image from a specified ImageProducer. createImage() creates the Image object and receives pixel data from the producer to construct the image. Note that there's nothing particularly special about MemoryImageSource; we could use any object that implements the image-producer interface inside of createImage(), including one we wrote ourselves. Once we have the image, we can draw it on the display with the familiar drawImage() method.

You can use MemoryImageSource to produce complex, pixel-by-pixel graphics or render images from arbitrary sources. It produces static images, though; when it reaches the end of its pixel data, its job is done. To generate a stream of image data or update pixel values, we need a more persistent image producer.

Updating a MemoryImageSource

MemoryImageSource can also be used to generate a sequence of images, or to update an image dynamically. In Java 1.1, this is probably the easiest way to build your own low-level animation software. This example simulates the static on a television screen. It generates successive frames of random black and white pixels and displays each frame when it is complete. The figure below shows one frame of random static, followed by the code:

[Graphic: Figure from the text]

import java.awt.*;
import java.awt.image.*;
public class StaticGenerator 
        extends java.applet.Applet 
        implements Runnable {
    int arrayLength, pixels[];
    MemoryImageSource source;
    Image image;
    int width, height;
    public void init() {
        width = getSize().width; height = getSize().height;
        arrayLength = width * height;
        pixels = new int [arrayLength];
        source = new MemoryImageSource(width, height, pixels, 0, width);
        source.setAnimated(true);
        image = createImage(source);
        new Thread(this).start();
    }
    public void run() {
        while (true) {
            try {
                Thread.sleep(1000/24);
            } catch( InterruptedException e ) { /* die */ }
            for (int x = 0; x < width; x++) 
                for (int y = 0; y < height; y++)  {
                    boolean rand = Math.random() > 0.5;
                    pixels[y*width+x] = 
                        rand ? Color.black.getRGB() : Color.white.getRGB();
                }
            // Push out the new data
            source.newPixels(0, 0, width, height);
        }
    }
    public void paint( Graphics g ) {
        g.drawImage(image, 0, 0, this);
    }
}

The init() method sets up the MemoryImageSource that produces the sequence of images. It first computes the size of the array needed to hold the image data. It then creates a MemoryImageSource object that produces images the width and height of the display, using the default color model (the constructor we use assumes that we want the default). We start taking pixels from the beginning of the pixel array, and scan lines in the array have the same width as the image. Once we have created the MemoryImageSource, we call its setAnimated() method to tell it that we will be generating an image sequence. Then we use the source to create an Image that will display our sequence.

We next start a thread that generates the pixel data. For every element in the array, we get a random number, and set the pixel to black if the random number is greater than 0.5. Because pixels is an int array, we can't assign Color objects to it directly; we use getRGB() to extract the color components from the black and white Color constants. When we have filled the entire array with data, we call the newPixels() method, which delivers the new data to the image.

That's about all there is. We provide a very uninteresting paint() method, that just calls drawImage() to put the current state of the image on the screen. Whenever paint() is called, we see the latest collection of static. The image observer, which is the Applet itself, schedules a call to paint() whenever anything interesting has happened to the image. It's worth noting how simple it is to create this animation. Once we have the MemoryImageSource, we use it to create an image that we treat like any other image. The code that generates the image sequence can be arbitrarily complex--certainly in any reasonable example, it would be more complex than our (admittedly cheesy) static. But that complexity never infects the simple task of getting the image on the screen and updating it.

Image Producers and Consumers

In this section we'll create an image producer that generates a stream of image frames rather than just a static image. Unfortunately, it would take too many lines of code to generate anything really interesting, so we'll stick with a simple modification of our ColorPan example. After all, figuring out what to display is your job; I'm primarily concerned with giving you the necessary tools. After this, you should have the needed tools to implement more interesting applications.

A word of advice: if you find yourself writing image producers, you're probably making your life excessively difficult. Most situations can be handled by the dynamic MemoryImageSource technique that we just demonstrated. Before going to the trouble of writing an image producer, convince yourself that there isn't a simpler solution. Even if you never write an image producer yourself, it's good (like Motherhood and Apple Pie) to understand how Java's image rendering tools work.

Image consumers

First, we have to know a little more about the image consumers we'll be feeding. An image consumer implements the seven methods that are defined in the ImageConsumer interface. Two of these methods are overloaded versions of the setPixels() method that accept the actual pixel data for a region of the image. They are identical except that one takes the pixel data as an array of integers, and the other uses an array of bytes. (An array of bytes is natural when you're using an indexed color model because each pixel is specified by an index into a color array.) A call to setPixels() looks something like the following:

setPixels(x, y, width, height, colorModel, pixels, offset, scanLength); 

pixels is the one-dimensional array of bytes or integers that holds the pixel data. Often, you deliver only part of the image with each call to setPixels(). The x, y, width, and height values define the rectangle of the image for which pixels are being delivered. x and y specify the upper left-hand corner of the chunk you're delivering, relative to the upper left-hand corner of the image as a whole. width specifies the width in pixels of the chunk; height specifies the number of scan lines in the chunk. offset specifies the point in pixels at which the data being delivered in this call to setPixels() starts. Finally, scanLength indicates the width of the entire image, which is not necessarily the same as width. The pixels array must be large enough to accommodate width*length+offset elements; if it's larger, any leftover data is ignored.

We haven't said anything yet about the colorModel argument to setPixels(). In our previous example, we drew our image using the default ARGB color model for pixel values; the version of the MemoryImageSource constructor that we used supplied the default color model for us. In this example, we also stick with the default model, but this time we have to specify it explicitly. The remaining five methods of the ImageConsumer interface accept general attributes and framing information about the image:

  • setHints()

  • setDimensions()

  • setProperties()

  • setColorModel()

  • imageComplete()

Before delivering any data to a consumer, the producer should call the consumer's setHints() method to pass it information about how pixels will be delivered. Hints are specified in the form of flags defined in the ImageConsumer interface. The flags are described in Table 14.2. The consumer uses these hints to optimize the way it builds the image; it's also free to ignore them.

Table 14.2: ImageConsumer setHints() Flags
Flag Description
RANDOMPIXELORDER The pixels are delivered in random order
TOPDOWNLEFTRIGHT The pixels are delivered from top to bottom, left to right
COMPLETESCANLINES

Each call to setPixels() delivers one or more complete scan lines

SINGLEPASS Each pixel is delivered only once
SINGLEFRAME The pixels define a single, static image

setDimensions() is called to pass the width and height of the image when they are known.

setProperties() is used to pass a hashtable of image properties, stored by name. This method isn't particularly useful without some prior agreement between the producer and consumer about what properties are meaningful. For example, image formats such as GIF and TIFF can include additional information about the image. These image attributes could be delivered to the consumer in the hashtable.

setColorModel() is called to tell the consumer which color model will be used to process most of the pixel data. However, remember that each call to setPixels() also specifies a ColorModel for its group of pixels. The color model specified in setColorModel() is really only a hint that the consumer can use for optimization. You're not required to use this color model to deliver all (or for that matter, any) of the pixels in the image.

The producer calls the consumer's imageComplete() method when it has completely delivered the image or a frame of an image sequence. If the consumer doesn't wish to receive further frames of the image, it should unregister itself from the producer at this point. The producer passes a status flag formed from the flags shown in Table 14.3.

Table 14.3: ImageConsumer imageComplete() Flags
Flag Description
STATICIMAGEDONE

A single static image is complete

SINGLEFRAMEDONE

One frame of an image sequence is complete

IMAGEERROR

An error occurred while generating the image

As you can see, the ImageProducer and ImageConsumer interfaces provide a very flexible mechanism for distributing image data. Now let's look at a simple producer.

A sequence of images

The following class, ImageSequence, shows how to implement an ImageProducer that generates a sequence of images. The images are a lot like the ColorPan image we generated a few pages back, except that the blue component of each pixel changes with every frame. This image producer doesn't do anything you couldn't do with a MemoryImageSource. It reads ARGB data from an array, and consults the object that creates the array to give it an opportunity to update the data between each frame.

This is a complex example, so before diving into the code, let's take a broad look at the pieces. The ImageSequence class is an image producer; it generates data and sends it to image consumers to be displayed. To make our design more modular, we define an interface called FrameARGBData that describes how our rendering code provides each frame of ARGB pixel data to our producer. To do the computation and provide the raw bits, we create a class called ColorPanCycle that implements FrameARGBData. This means that ImageSequence doesn't care specifically where the data comes from; if we wanted to draw different images, we could just drop in another class, provided that the new class implements FrameARGBData. Finally, we create an applet called UpdatingImage that includes two image consumers to display the data.

Here's the ImageSequence class:


import java.awt.image.*;
import java.util.*;
public class ImageSequence extends Thread implements ImageProducer {
    int width, height, delay;
    ColorModel model = ColorModel.getRGBdefault();
    FrameARGBData frameData;
    private Vector consumers = new Vector();
    public void run() {
        while ( frameData != null ) {
            frameData.nextFrame();
            sendFrame();
            try {
                sleep( delay );
            } catch ( InterruptedException e ) {}
        }
    }
    public ImageSequence(FrameARGBData src, int maxFPS ) {
        frameData = src;
        width = frameData.size().width;
        height = frameData.size().height;
        delay = 1000/maxFPS;
        setPriority( MIN_PRIORITY + 1 );
    }
    public synchronized void addConsumer(ImageConsumer c) {
        if ( isConsumer( c ) ) 
            return;
        consumers.addElement( c );
        c.setHints(ImageConsumer.TOPDOWNLEFTRIGHT |
                 ImageConsumer.SINGLEPASS );
        c.setDimensions( width, height );
        c.setProperties( new Hashtable() );
        c.setColorModel( model );
    }
    public synchronized boolean isConsumer(ImageConsumer c) {
        return ( consumers.contains( c ) );
    }
    public synchronized void removeConsumer(ImageConsumer c) {
        consumers.removeElement( c );
    }
    public void startProduction(ImageConsumer ic) {
        addConsumer(ic);
    }
    public void requestTopDownLeftRightResend(ImageConsumer ic) { }
    private void sendFrame() {
        for ( Enumeration e = consumers.elements(); e.hasMoreElements();  ) {
            ImageConsumer c = (ImageConsumer)e.nextElement();
            c.setPixels(0, 0, width, height, model, frameData.getPixels(), 0, width);
            c.imageComplete(ImageConsumer.SINGLEFRAMEDONE);
        }
    }
}

The bulk of the code in ImageSequence creates the skeleton we need for implementing the ImageProducer interface. ImageSequence is actually a simple subclass of Thread whose run() method loops, generating and sending a frame of data on each iteration. The ImageSequence constructor takes two items: a FrameARGBData object that updates the array of pixel data for each frame, and an integer that specifies the maximum number of frames per second to generate. We give the thread a low priority (MIN_PRIORITY+1) so that it can't run away with all of our CPU time.

Our FrameARGBData object implements the following interface:

interface FrameARGBData { 
    java.awt.Dimension size(); 
    int [] getPixels(); 
    void nextFrame(); 
} 

In ImageSequence's run() method, we call nextFrame() to compute the array of pixels for each frame. After computing the pixels, we call our own sendFrame() method to deliver the data to the consumers. sendFrame() calls getPixels() to retrieve the updated array of pixel data from the FrameARGBData object. sendFrame() then sends the new data to all of the consumers by invoking each of their setPixels() methods and signaling the end of the frame with imageComplete(). Note that sendFrame() can handle multiple consumers; it iterates through a Vector of image consumers. In a more realistic implementation, we would also check for errors and notify the consumers if any occurred.

The business of managing the Vector of consumers is handled by addConsumer() and the other methods in the ImageProducer interface. addConsumer() adds an item to consumers. A Vector is a perfect tool for this task, since it's an automatically extendable array, with methods for finding out how many elements it has, whether or not a given element is already a member, and so on.

addConsumer() also gives the consumer hints about how the data will be delivered by calling setHints(). This image provider always works from top to bottom and left to right, and makes only one pass through the data. addConsumer() next gives the consumer an empty hashtable of image properties. Finally, it reports that most of the pixels will use the default ARGB color model (we initialized the variable model to ColorModel.getRGBDefault()). In this example, we always start sending image data on the next frame, so startProduction() simply calls addConsumer().

We've discussed the mechanism for communications between the consumer and producer, but I haven't yet told you where the data comes from. We have a FrameARGBData interface that defines how to retrieve the data, but we don't yet have an object that implements the interface. The following class, ColorPanCycle, implements FrameARGBData; we'll use it to generate our pixels:

import java.awt.*;
class ColorPanCycle implements FrameARGBData {
    int frame = 0, width, height;
    private int [] pixels;
    ColorPanCycle ( int w, int h ) {
        width = w;
        height = h;
        pixels = new int [ width * height ];
        nextFrame();
    }
    public synchronized int [] getPixels() {
        return pixels;
    }
    public synchronized void nextFrame() {
        int index = 0;
        for (int y = 0; y < height; y++) {
            for (int x = 0; x < width; x++) {
                int red = (y * 255) / (height - 1);
                int green = (x * 255) / (width - 1);
                int blue = (frame * 10) & 0xff;
                pixels[index++] = 
                    (255 << 24) | (red << 16) | (green << 8) | blue;
            }
        }
        frame++;
    }
    public Dimension size() {
        return new Dimension ( width, height );
    }
}

ColorPanCycle is like our previous ColorPan example, except that it adjusts each pixel's blue component each time nextFrame() is called. This should produce a color cycling effect; as time goes on, the image becomes more blue.

Now let's put the pieces together by writing an applet that displays a sequence of changing images: UpdatingImage. In fact, we'll do better than displaying one sequence. To prove that ImageSequence really can deal with multiple consumers, UpdatingImage creates two components that display different views of the image. Once the mechanism has been set up, it's surprising how little code you need to add additional displays.

import java.awt.*;
import java.awt.image.*;
public class UpdatingImage extends java.applet.Applet { 
    ImageSequence seq;
    public void init() {
        seq = new ImageSequence( new ColorPanCycle(100, 100), 10);
        setLayout( null );
        add( new ImageCanvas( seq, 50, 50 ) );
        add( new ImageCanvas( seq, 100, 100 ) );
        seq.start();
    }
    public void stop() {
        if ( seq != null ) {
            seq.stop();
            seq = null;
        }
    }
}
class ImageCanvas extends Canvas { 
    Image img;
    ImageProducer source;
    ImageCanvas ( ImageProducer p, int w, int h ) {
        source = p;
        setSize( w, h );
    }
    public void update( Graphics g ) {
        paint(g);
    }
    public void paint( Graphics g ) {
        if ( img == null )
            img = createImage( source );
        g.drawImage( img, 0, 0, getSize().width, getSize().height, this );
    }
}

UpdatingImage constructs a new ImageSequence producer with an instance of our ColorPanCycle object as its frame source. It then creates two ImageCanvas components that create and display the two views of our animation. ImageCanvas is a subclass of Canvas; it takes an ImageProducer and a width and height in its constructor and creates and displays an appropriately scaled version of the image in its paint() method. UpdatingImage places the smaller view on top of the larger one for a sort of "picture in picture" effect.

If you've followed the example to this point, you're probably wondering where in the heck is the image consumer. After all, we spent a lot of time writing methods in ImageSequence for the consumer to call. If you look back at the code, you'll see that an ImageSequence object gets passed to the ImageCanvas constructor, and that this object is used as an argument to createImage(). But nobody appears to call addConsumer(). And the image producer calls setPixels() and other consumer methods; but it always digs a consumer out of its Vector of registered consumers, so we never see where these consumers come from.

In UpdatingImage, the image consumer is behind the scenes, hidden deep inside the Canvas--in fact, inside the Canvas' peer. The call to createImage() tells its component (i.e., our canvas) to become an image consumer. Something deep inside the component is calling addConsumer() behind our backs and registering a mysterious consumer, and that consumer is the one the producer uses in calls to setPixels() and other methods. We haven't implemented any ImageConsumer objects in this book because, as you might imagine, most image consumers are implemented in native code, since they need to display things on the screen. There are others though; the java.awt.image.PixelGrabber class is a consumer that returns the pixel data as a byte array. You might use it to save an image. You can make your own consumer do anything you like with pixel data from a producer. But in reality, you rarely need to write an image consumer yourself. Let them stay hidden; take it on faith that they exist.

Now for the next question: How does the screen get updated? Even though we are updating the consumer with new data, the new image will not appear on the display unless the applet repaints it periodically. By now, this part of the machinery should be familiar: what we need is an image observer. Remember that all components are image observers (i.e., the class Component implements ImageObserver). The call to drawImage() specifies our ImageCanvas as its image observer. The default Component class-image-observer functionality then repaints our image whenever new pixel data arrives.

In this example, we haven't bothered to stop and start our applet properly; it continues running and wasting CPU time even when it's invisible. There are two strategies for stopping and restarting our thread. We can destroy the thread and create a new one, which would require recreating our ImageCanvas objects, or we could suspend and resume the active thread. As discussed in Chapter 6, Threads, neither option is particularly difficult.

Filtering Image Data

As I said above, you rarely need to write an image consumer. However, there is one kind of image consumer that's worth knowing about. In this final section on images, we'll build a simple image filter. An image filter is simply a class that performs some work on image data before passing the data to another consumer.

The ColorSep applet acquires an image; uses an image filter to separate the image into red, green, and blue components; and displays the three resulting images. With this applet and a few million dollars, you could build your own color separation plant.

import java.awt.*;
import java.awt.image.*;
public class ColorSep extends java.applet.Applet { 
    Image img, redImg, greenImg, blueImg;
    public void init() {
        img = getImage( getClass().getResource( getParameter("img")) );
        redImg = createImage(new FilteredImageSource(img.getSource(), 
                                        new ColorMaskFilter( Color.red )));
        greenImg = createImage(new FilteredImageSource(img.getSource(), 
                                        new ColorMaskFilter( Color.green )));
        blueImg = createImage(new FilteredImageSource(img.getSource(), 
                                        new ColorMaskFilter( Color.blue )));
    }
    public void paint( Graphics g ) {
        int width = getSize().width, height = getSize().height;
        g.drawImage( redImg, 0, 0, width/3, height, this );
        g.drawImage( greenImg, width/3, 0, width/3, height, this );
        g.drawImage( blueImg, 2*width/3, 0, width/3, height, this );
    }
}
class ColorMaskFilter extends RGBImageFilter {
    Color color;
    ColorMaskFilter( Color mask ) {
        color = mask;
        canFilterIndexColorModel = true;
    }
    public int filterRGB(int x, int y, int pixel ) {
        return 
            255 << 24 |
            (((pixel & 0xff0000) >> 16) * color.getRed()/255) << 16 |
            (((pixel & 0xff00) >> 8) * color.getGreen()/255) << 8 |
            (pixel & 0xff) * color.getBlue()/255 ;
    }
}

The FilteredImageSource and RGBImageFilter classes form the basis for building and using image filters. A FilteredImageSource is an image producer (like MemoryImageSource) that is constructed from an image and an ImageFilter object. It fetches pixel data from the image and feeds it through the image filter before passing the data along. Because FilteredImageSource is an image producer, we can use it in our calls to createImage().

But where's the consumer? FilteredImageSource obviously consumes image data as well as producing it. The image consumer is still mostly hidden, but is peeking out from under its rock. Our class ColorMaskFilter extends RGBImageFilter, which in turn extends ImageFilter. And ImageFilter is (finally!) an image consumer. Of course, we still don't see the calls to addConsumer(), and we don't see an implementation of setPixels(); they're hidden in the ImageFilter sources and inherited by ColorMaskFilter.

So what does ColorMaskFilter actually do? Not much. ColorMaskFilter is a simple subclass of RGBImageFilter that implements one method, filterRGB(), through which all of the pixel data are fed. Its constructor saves a mask value we use for filtering. The filterRGB() method accepts a pixel value, along with its x and y coordinates, and returns the filtered version of the pixel. In ColorMaskFilter, we simply multiply the color components by the mask color to get the proper effect. A more complex filter, however, might use the coordinates to change its behavior based on the pixel's position.

One final detail: the constructor for ColorMaskFilter sets the flag canFilterIndexColorModel. This flag is inherited from RGBImageFilter. It means our filter doesn't depend on the pixel's position. In turn, this means it can filter the colors in a color table. If we were using an indexed color model, filtering the color table would be much faster than filtering the individual pixels.


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