# Problems with Subdivision Surfaces

It sounds like Subdivision Surfaces are better than NURBS but not as good as T-splines. I would like to know some of the disadvantages of Subdivision Surfaces, what they can't do or what they do inefficiently.

I am considering what type of system to use. Whether it be B-splines, NURBS, subdivision surfaces, or T-splines, or something else. Here I am mainly wondering how good subdivision surfaces are.

• Please describe what your intentions are, what are you going to use it for - it might affect your answer. Jan 17, 2019 at 7:46
• I would like a general answer if possible. Jan 19, 2019 at 1:06

Both T-splines and subdivision surfaces are capable of handling an arbitrary topology input mesh, whereas NURBS can only handle meshes with regular topology.

Complex NURBS objects are therefore made out of multiple regular meshes. These meshes are trimmed and fit together to form a single complex object. However, along the trimming lines of the mesh discontinuities can occur. This is because the boundary curve of a trimmed NURBS patch is not in general a NURBS curve. This complicates the joining of several NURBS patches, potentially sacrificing water tightness.

Subdivision surfaces are able to handle meshes with arbitrary topology. This means your mesh can contain extraordinary faces (faces with more or less than four sides) and extraordinary vertices (vertices with a valency other than 4). The generated surfaces are in general curvature continuous in regular regions and tangent plane continuous at extraordinary vertices. The evaluation of a subdivision surface is an iterative process. Although regular regions can be converted to B-spline patches problems occur when rendering the irregular regions as an infinite number of B-spline patches will be generated. The OpenSubDiv library by Pixar is a good start for speedy subdivision as it calculates many things on the GPU.

Most subdivision surfaces are based on uniform subdivision and some work has been done to handle non-uniform and rational subdivision, but it is quite extensive and has some limitations.

T-splines can do everything NURBS can do as well. The most striking feature of T-splines is the ability to handle T-junctions, as well as arbitrary topology, without sacrificing in surface continuity. This allows the designer to insert more detail in some places without it effecting the mesh in other regions. In this way it is possible for a very organic design process. Take for instance this video. A downside of T-splines is that it is a patented technology.

• So subdivision surfaces sound best. Wondering if you have a reading recommendation to learn the depths, you seem to know a lot about the details which I haven't heard about. Will check out OpenSubDiv. Jan 18, 2019 at 1:53
• Wondering if you could elaborate on this problem you mentioned with subdivision surfaces: "Subdivision curves do not suffer from the problem that subdivision surfaces have around extraordinary points and therefore all subdivision" Jan 18, 2019 at 22:55
• Around extraordinary vertices subdivision surfaces will generate an infinite number of patches and have only tangent plane continuity at the extraordinary vertex. Jan 21, 2019 at 9:37