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non-uniform national b-splines
Chapter 13. Evaluators and NURBS > http://www.parallab.uib.no/SGI_bookshelves/SGI_Developer/books/OpenGL_PG/sgi_html/ch13.html#id5524141
Chapter 13. Evaluators and NURBS > http://www.parallab.uib.no/SGI_bookshelves/SGI_Developer/books/OpenGL_PG/sgi_html/ch13.html#id5524141