Pixel graphics - An Introduction to Graphical User Interfaces with Java Swing - page 154

34. {
35.

WebImageFrame wifr = new WebImageFrame();

36.

wifr.showIt("WebImageFrame: loading Image from the net");

37. }
38. }

■ 15.4 Manipulating images

You may want to resize, rotate or reflect your image, or display only part of it,
rather than view it as it is stored on the hard disk. These operations are supported
by classes from the AWT library. We shall now introduce some techniques to solve
the aforementioned image manipulations. Class Image from the AWT library plays
a central role for image manipulations.

15.4.1 Loading images asynchronously

As mentioned above, loading a picture might take some time, especially if the
image has to be transferred over a network. To avoid blocking the main application
while the image is fetched, the loading process can be run asynchronously. Class
Toolkit from the AWT library supplies methods for this purpose. We shall use
the getImage -method of the default toolkit to load images. One can specify the
name of the image file or a web location (URL) where the image is stored. Method
getImage returns an object of class Image :

Image picture = Toolkit.getDefaultToolkit.getImage(String filename);
Image picture = Toolkit.getDefaultToolkit.getImage(String url);
Once the picture is loaded into an object of class Image ,itcan be manipulated
by methods of this class. As a first application we show how a number of images
can be arranged. We derive class ImagePanel from JPanel .Animage panel will
automatically arrange pictures and display them. The background colour is set to
be yellow to emphasize the contrast to the pictures.
The images are displayed in method paintComponent of the panel. We use the
following method of class Graphics . The arguments are explained afterwards.

drawImage(Image picture, int left, int top, ImageObserver imObs)

The first argument ( picture ) contains the image to display. The two integer
arguments left and top specify the pixel position of the upper left corner of the
image inside the panel. The last argument is of type ImageObserver . This is an
interface from the AWT library which is of great help when loading or displaying
images. Fortunately, we do not have to implement this interface ourselves. Every
graphical component, regardless whether it is AWT or Swing, implements this
interface. We therefore can use this as image observer.