I very much like vtk’s data flow pipeline approach. It is similar to work I have done on animation software systems (e.g. Omnibus PRISMS, improved and sold by SideFX http://www.sidefx.com/). The availability of full source code to vtk without royalties is a big plus. (Note: there are some algorithms in the vtk baseline, such as ‘marching cubes’, that are patented by GE. A license is required to use these in a commercial product.) There is a UK company, Principa Mathmatica, that is developing a visual (iconic) scripting front end to vtk ( http://www.principiamathematica.com/).