Beginning Java 2- JDK 1.3 Version
Images and Animation
Creating an Image
A sprite is a small graphical image that you can draw over a static image
to create an animation. To create the animation effect, you just draw the
sprite in different positions and orientations over time, and of course
transformations of the coordinate system can be a great help in making this
easier. Games often use sprites – they can make the animation take much
less processor time because you only need to draw the sprite against a
static background. Our interest in using BufferedImage objects
means we won't get into the best techniques for minimizing processor time.
We will instead concentrate on understanding how we can create and use
images internally in a program.
Our BufferedImage object is going to look as shown below.
The image is a square with sides of length spriteSize. Dimensions of other
parts of the image are relative to this. There are really only two
geometric entities here, a line and a circle, each repeated in different
positions and orientations, so if we create a Line2D.Double object for the
line, and an Ellipse2D.Double object for the circle, we should be able to
draw the whole thing by moving the user coordinate system around and
drawing one or other of these two objects.
A true object-oriented approach would define a class representing a sprite,
possibly as a subclass of BufferedImage, but since we are
exploring the mechanics of using a BufferedImage object, it
will suit our purpose better to develop a method,
createSprite(), that draws the sprite on a
BufferedImage object. The method will just be a member of our
applet class so we will add data members to the applet to store any data
required. You can plug the data members we will be using into the applet
class outline now:
double totalAngle; // Current angular position of sprite
double spriteAngle; // Rotation angle of sprite about its center
ImagePanel imagePanel; // Panel to display animation
BufferedImage sprite; // Stores reference to the sprite
int spriteSize = 100; // Diameter of the sprite
Ellipse2D.Double circle; // A circle - part of the sprite
Line2D.Double line; // A line - part of the sprite
// Colors used in sprite
Color[] colors = {Color.red , Color.yellow, Color.green , Color.blue,
Color.cyan, Color.pink , Color.magenta, Color.orange};
java.util.Timer timer; // Timer for the animation
long interval = 50; // Time interval msec between repaints
The general use of these members should be clear from the comments. We will
see how they are used as we develop the code.
The first thing the createSprite() method needs to do is to
create the BufferedImage object, sprite, and we will need a
Graphics2D object to use to draw on the sprite image. The code
to do this is as follows:
BufferedImage createSprite(int spriteSize)
{
// Create image with RGB and alpha channel
BufferedImage sprite = new BufferedImage(spriteSize, spriteSize,
BufferedImage.TYPE_INT_ARGB);
Graphics2D g2D = sprite.createGraphics(); // Context for buffered image
// plus the rest of the method...
}
The sprite object has a width and height of spriteSize, and
the image is of the type TYPE_INT_ARGB, so the alpha and color
components for each pixel will be stored as a single int value, and the
color will be stored as 8 bit red, green and blue components. This means
that our sprite image will occupy 40,000 bytes, which is a small indication
of how browsing a web page can gobble up memory. This doesn't affect the
download time for the page – this memory is allocated in the local machine
when the applet is executed. Apart from the contents of the HTML file that
is the page, the download time is affected by the size of the .class file
for the applet, plus any image or other files that it downloads when it
executes.
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