Normally I try to make the X-Plane scenery and modeling system as opaque as possible — I want to make sure that nobody ever actually uses the rendering features that I spend weeks and weeks developing.
But the other night I had a little bit too much to drink, got distracted, and posted these:
In all seriousness, I have been trying to find time to put more documentation up on the Wiki. For these features, you will find an explanation of how the planes work, as well as a link to the planes (with plugins) to download, and a link to the plugin source code (on the SDK site, with sample makefiles for 3 operating systems).
Plugins? Do not panic! While plugins are necessary for some of the features demonstrated here, others can be created without additional programming.
BTW, if the existing documentation uses a concept that is not explained anywhere, please email me. I sometimes leave holes in the documentation by accident.