Beginning Java 2- JDK 1.3 Version
Images and Animation
Displaying the Image
The image is displayed by the ImagePanel object, so we need to
implement the paint() method for this class to display the image
normally when imageY is zero or negative, and deal with squashing
the image when imageY is positive. This is going to be easy since
we can use the drawImage() method we used in the previous example
to draw the image normally, and use an overloaded version that is specifically
intended for scaling an image on the fly to fit a particular area. The
overloaded method has the following arguments:
When you call this method, the image is scaled on the fly to fit the destination
area specified by the arguments. Since you specify the coordinates of the top-
left of the image, as well as its width and height, it is possible to use this
method to display part of an image, and fit it to the destination space that you
specify. Like all the drawImage() methods, this method can draw
part of an image when loading of the image is not complete. In this case the
method returns false. The ImageObserver that is passed as the last
argument – usually the applet itself – is notified when more of the image
becomes available, with the result that the image is repainted. When the entire
image has been drawn, the drawImage() method returns true.
It would also be useful to color the background with a color that contrasts with
the image. With this in mind we can implement the paint() method
for the ImagePanel class as:
public void paint(Graphics g)
{
Graphics2D g2D = (Graphics2D)g;
g2D.setPaint(Color.lightGray); // Set a background color
g2D.fillRect(0, 0, imageWidth, imageHeight); // paint background
if(imageY<=0)
g2D.drawImage(image, imageX, imageY, this); // Draw normally
else // or scaled...
g2D.drawImage(image, // The image
imageX, imageY, imageWidth, imageHeight, // Destination
0, 0, imageWidth, imageHeight, // Image area
this); // Image observer
}
The fillRect() method fills the entire area of the applet with the
color that we set in the setPaint() call,
Color.lightGray. When imageY is greater than 0, we use
the version of drawImage() that scales the image on the fly to fit
the area available, from the imageY position to the bottom of the
applet at the y coordinate, imageHeight.
It is worth noting that a JPanel object is double-buffered by
default. This means that all rendering for a new image that is to be displayed
is done in a buffer in memory, and only when image is complete are the pixels
for the entire picture written to the screen. Since the existing image that is
displayed is not altered while the new image is being created, this eliminates
the flicker and flashing that can occur if your display buffer is updated
incrementally while it is displayed.
If you have put together all the bits of code we have discussed, you should have
a working applet.
Join us in one week for the next section!
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