CroftSoft Savor is a screensaver demonstrating the early access SaverBeans Screensaver SDK version 0.2. So far I have only deployed it on Windows but since the SaverBeans SDK is Java-based it should be cross-platform. After you have a chance to download and install it, take a peek at the source code.
The first implementation I created draws colored ovals on the screen. On Windows, download croftsoft_savor2d_setup.exe and double-click to install. After installation, right-click on your desktop and select "Properties" to bring up the "Display Properties" window. Click on the "Screen Saver" tab and change the screensaver on the pulldown menu to "croftsoft_savor2d". You can configure the screensaver to change the oval size by clicking on the "Settings" button on the same Windows dialog where you choose the desktop screensaver.