https://babel.hathitrust.org/cgi/ls?field1=ocr;q1=shadow;a=srchls;lmt=ft
https://www.redblobgames.com/articles/visibility/
http://teaching.csse.uwa.edu.au/units/CITS4241/Handouts/
http://www.design-tech.org/casting-shadows/
The Dipole Repeller
https://www.youtube.com/watch?v=NpV0GQo3P0c
http://home.cern/about/updates/2017/06/lhcs-objective-maximum-intensity