In higher-end rendering, a quad that can be non-planar is called a bilinear patch. Yes, it can curve (cross sections tend to give parabolas), and while you won

In higher-end rendering, a quad that can be non-planar is called a bilinear patch. Yes, it can curve (cross sections tend to give parabolas), and while you won't get infinitely sharp creases you will get saddle-shapes. See, e.g.: http://www.sci.utah.edu/~kpotter/publications/ramsey-2004-RBPI.pdf http://www.sci.utah.edu/~kpotter/publications/ramsey-2004-RB...