I checked the FAQ and found this very problem right at the top, with the explanation:
When starting any demo I get an AssertionError in DisplayOpenGL assert s >= 0
This is generally caused by the graphics memory allocation on the Raspberry Pi being too low (less than 32)
But that doesn't seem to be the cause of my problem. I've got the graphics memory set to 128MB right now and it fails. I've tried it at 64MB and it fails.
The machine is a Raspberry Pi 3 running the latest version of Raspbian (Raspbian Jessie with Pixel, Version: March 2017, Release date: 2017-03-02, Kernel version:4.4). I installed Pi3D vusing pip3 two days ago, and downloaded the demos directly from the github master two days ago.
Any ideas on what might be going wrong?