The Common Desktop Environment (CDE) has many screensavers for use, you can select which you like the most from the Style Manager Screen menu. There is the screen saver action definition, which describes each screen saver for the desktop environment, it is a metadata which contains the short name of the screensaver and the command line with the parameters to start it. When the desktop is idle for some minutes, and it reaches the treshold, it starts the screensaver with the specified command line. There is a variable called DTSCREENSAVERLIST which describes which screensaver actions are available for a specific user. If you want to add a custom screensaver to the drop-down list of Style Manager, you should modify this variable e.g. from your ~/.dtprofile .
When a screensaver gets started, it gets information about the opened windows trough the DtSaverGetWindows() API, and draws on top of the windows. If you want to code your own screensaver, there is a sample screensaver which can be used as a starting point, you can find it in /usr/dt/examples/dtscreen/dt/appconfig/types/C/screensaver.dt , the name of the action is SampleScreenSaver.
Stay tuned and I will show you how to get your own custom-built screensaver into the Style Manager Screen menu!