Three-dimensional curvature Curvature in 2 directions at any point on the surface of the fabric. Simpler form is a sharp 4-post structure with 2 diagonally opposite points high and 2 diagonally opposite points low, resulting in a “saddle” or “twisted” form, this is properly known as a Hypar. The quality of the 3D shape in a structure is related to its lifetime. Flat 2 dimensional structures (regardless of the type of fabric) will end up loosening with the stroke, lose their tension and eventually fail. Structures with a good amount of 3D shapes will fare better in strong winds, maintain their tension better, drain water more efficiently and result in surviving for many, many years.