Graphics assignment 9

Added another answer to question 3.

graphics/ass9/source/questions.txt

@@ -40,3 +40,9 @@ Question 3:
 	  algorithm that uses already interpolated points in neigbouring tetrahedons
 	  to create a triangle strip, instead of interpolating seperately in each
 	  tetrahedon (note that there is less room for parallel processing in this case).
+	- There is an algorithm, "regularised marching tetrahedra" which combines
+	  marching tetrahedra and vertex clustering to generate iso-surfaces which
+	  are topologically consistent with the data and contain a number of triangles
+	  appropriate to the sampling resolution (typically 70% fewer than marching
+	  tetrahedra). For this algorithm, see also:
+	  http://mi.eng.cam.ac.uk/reports/svr-ftp/treece_tr333.pdf