Triangle Strip - Properties and Construction

Properties and Construction

It follows from definition that a subsequence of vertices of a triangle strip also represents a triangle strip. However, if this substrip starts at an even (with 1-based counting) vertex, then the resulting triangles will change their orientation. For example a substrip BCDEF would represent triangles: BCD,CED,DEF.

Similarly, reversal of strip's vertices will result in the same set of triangles if the strip has even number of vertices. (e.g. strip FEDCBA will represent the same triangles FED,ECD,DCB,CAB as the original strip). However, if a strip has odd number of vertices then the reversed strip will represent triangles with opposite orientation. For example, reversal of a strip ABCDE will result in strip EDCBA which represents triangles EDC, DBC, CBA).

Converting a general polygon mesh to a single long strip was until recently generally not possible. Usually the triangle strips are analogous to a set of edge loops, and poles on the model are represented by triangle fans. Tools such as Stripe or FTSG represent the model as several strips. Optimally grouping a set of triangles into sequential strips has been proven NP-complete.

Alternatively, a complete object can be described as a degenerate strip, which contains zero-area triangles that the processing software or hardware will discard. The degenerate triangles effectively introduce discontinuities or "jumps" to the strip. For example, the mesh in the diagram could also be represented as ABCDDFFEDC, which would be interpreted as triangles ABC CBD CDD DDF DFF FFE FED DEC (degenerate triangles marked with italics). Notice how this strip first builds two triangles from the left, then restarts and builds the remaining two from the right.

While discontinuities in triangle strips can always be implemented by resending vertices, APIs sometimes explicitly support this feature. IRIS GL supported Swaps (flipping two subsequent vertices in a strip), a feature relied on by early algorithms such as the SGI algorithm. Recently OpenGL/DirectX can render multiple triangle strips without degenerated triangles using Primitive Restart feature.