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Java in a Nutshell

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21. The java.awt.image Package

The java.awt.image package is, by any standard, a confusing one. The purpose of the package is to support image processing, and the classes in the package provide a powerful infrastructure for that purpose. (see Figure 21.1.) Most of the classes are part of the infrastructure, however, and are not normally used by ordinary applications that have only simple image manipulation requirements.

To understand this package, it is first important to note that the Image class itself is part of the java.awt package, not the java.awt.image package. Furthermore, the java.awt.image classes are not the source of images; they simply serve to manipulate images that come from somewhere else. The Applet.getImage() method is perhaps the most common method for obtaining an image in Java--it downloads the image from a specified URL. In a stand-alone application, the URL.getContent() method can be used to obtain an ImageProducer object, which can then be passed to the Component.createImage() method to obtain an Image object.

The ImageProducer interface is one you'll encounter frequently in java.awt.image. It represents an image source. If you've created an Image object with Applet.getImage(), you can obtain the ImageProducer for that Image (which has not been downloaded yet) with Image.getSource(). Conversely, given an ImageProducer object, you can create an Image from it with the createImage() method of any Component (such as an Applet). Once you have an ImageProducer object, you can manipulate it with the other java.awt.image classes.

FilteredImageSource is the most important class for image manipulation. It is itself a type of ImageProducer that, when created, applies a specified ImageFilter object to some other specified ImageProducer object. The FilteredImageSource thus configured can be used as an ImageProducer to display a filtered image. CropImageFilter is a predefined type of ImageFilter that you can use to extract a specified rectangle out of a larger image.

RGBImageFilter is another subclass of ImageFilter that makes it easy to filter the colors of an image. To do so, you must subclass RGBImageFilter and provide the definition of a single simple method that manipulates the image colors. In order to manipulate image colors, you will probably need to be familiar with the ColorModel class and its two subclasses, DirectColorModel and IndexColorModel. An instance of ColorModel or of one of its subclasses converts a pixel value to the red, green, and blue components of the color it represents.

Finally, two other classes in the java.awt.image package are worth noting. MemoryImageSource is a type of ImageProducer that generates an image from an array of bytes or integers in memory. PixelGrabber does the reverse--it captures pixels from an ImageProducer and stores them into an array. You can use these classes separately or together to perform your own custom image manipulation.

21.1 java.awt.image.AreaAveragingScaleFilter (JDK 1.1)

This class implements an ImageFilter that scales an image to a specified pixel size. It uses a scaling algorithm that averages adjacent pixel values when shrinking an image, which produces relatively smooth scaled images. Its superclass, ReplicateScaleFilter, implements a faster, less smooth scaling algorithm.

The methods of this class are ImageConsumer methods intended for communication between the image filter and the FilteredImageSource that uses it. Applications do not usually call these methods directly.

The easiest way to use this filter is to call Image.getScaledInstance(), specifying an appropriate hint constant.

public class AreaAveragingScaleFilter extends ReplicateScaleFilter {
    // Public Constructor
            public AreaAveragingScaleFilter(int width, int height);
    // Public Instance Methods
            public void setHints(int hints);  // Overrides ImageFilter
            public void setPixels(int x, int y, int w, int h, ColorModel model, byte[] pixels, int off, int scansize);
            public void setPixels'u'// Overrides ReplicateScaleFilter
            public void setPixels(int x, int y, int w, int h, ColorModel model, int[] pixels, int off, int scansize);
            public void setPixels'u'// Overrides ReplicateScaleFilter
}

Hierarchy:

Object->ImageFilter(ImageConsumer, Cloneable)-> ReplicateScaleFilter->AreaAveragingScaleFilter


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