Graphics - Java Examples in a Nutshell - page 307

Example 11•17: Hypnosis.java (continued)
* Swing calls this method to ask the component to redraw itself.
* This method uses double-buffering to make the animation smoother.
* Swing does double-buffering automatically, so this may not actually
* make much difference, but it is important to understand the technique.
**/
public void paintComponent(Graphics g) {
// Clear the background of the off-screen image
osg.setColor(getBackground());
osg.fillRect(0, 0, buffer.getWidth(), buffer.getHeight());
// Now draw a black spiral into the off-screen image
osg.setColor(Color.black);
osg.draw(new Spiral(r2+linewidth/2, r2+linewidth/2, r1, a1, r2, a2));
// Now copy that off-screen image onto the screen
g.drawImage(buffer, (int)(x-r2), (int)(y-r2), this);
}
/**
* This method implements the ActionListener interface. Our Timer object
* calls this method periodically. It updates the position and angles
* of the spiral and requests a redraw. Instead of redrawing the entire
* component, however, this method requests a redraw only for the
* area that has changed.
**/
public void actionPerformed(ActionEvent e) {
// Ask to have the old bounding box of the spiral redrawn.
// Nothing else has anything drawn in it, so it doesn't need a redraw
repaint((int)(x-r2-linewidth), (int)(y-r2-linewidth),
(int)(2*(r2+linewidth)), (int)(2*(r2+linewidth)));
// Now animate: update the position and angles of the spiral
// Bounce if we've hit an edge
Rectangle bounds = getBounds();
if ((x - r2 + deltaX < 0) || (x + r2 + deltaX > bounds.width))
deltaX = -deltaX;
if ((y - r2 + deltaY < 0) || (y + r2 + deltaY > bounds.height))
deltaY = -deltaY;
// Move the center of the spiral
x += deltaX;
y += deltaY;
// Increment the start and end angles;
a1 += deltaA;
a2 += deltaA;
if (a1 > 2*Math.PI) { // Don't let them get too big
a1 -= 2*Math.PI;
a2 -= 2*Math.PI;
}
// Now ask to have the new bounding box of the spiral redrawn. This
// rectangle will be intersected with the redraw rectangle requested
// above, and only the combined region will be redrawn
repaint((int)(x-r2-linewidth), (int)(y-r2-linewidth),
(int)(2*(r2+linewidth)), (int)(2*(r2+linewidth)));
}