Java Double-buffering

double-buffered Java animation

[click the applet to re-init the ball]

This Java applet demonstrates the following:

Double buffering. That is, composing the image off-screen and then drawing that final image to the screen. This technique avoids annoying flicker that results from drawing and erasing everything on-screen.

Scaling by time. Rather than moving the ball a fixed number of pixels per frame, the ball's position and velocity are stored as floating point values. Motion is calculated by moving the ball a distance scaled by the amount of time elapsed since the previous frame. This technique provides the user with what appears to be the exact same motion speed from one computer to the next, regardless of the framerate that their PC/browser is capable of. It also allows the applet to run as fast as possible, so faster computers will display smoother animation, rather than a faster-moving ball.

Timer averaging. Java's timer is notoriously erratic. In general I have found that calling System.currentTimeMillis() is accurate to roughly 50ms intervals. What that means is that if you call the function more than once within a 50ms time interval, you will not get a properly updated time. Since 50ms is good for only 20 frames per second, and since the applet will generally run at faster rates than that on fast computers, scaling by time becomes choppy and inaccurate. What this applet does is records the time intervals for the last 8 frames in order to estimate a far more accurate elapsed-time for each frame.

Threading techniques. If you come from a C, BASIC, or ASM background, Java's intrinsic multithreading may seem a little wierd. This applet should explain how to get a stable main loop up and running, and hopefully help you to avoid the pitfalls associated with fragile Java implementations prevalent in most web browsers.