A triangle strip is a series of connected triangles, sharing vertices, allowing for faster rendering and more efficient memory usage for computer graphics. They are optimized on most graphics cards, making them the most efficient way of describing an object. There are two primary reasons to use triangle strips:
For example, the four triangles in the diagram, without using triangle strips, would have to be stored and interpreted as four separate triangles: ABC, CBD, CDE, and EDF. However, using a triangle strip, they can be stored simply as a sequence of vertices ABCDEF. This sequence would be decoded as a set of triangles ABC, BCD, CDE and DEF then every even-numbered (with counting starting from one) triangle would be reversed resulting in the original triangles.
Contents |
OpenGL has innate support for triangle strips using the glBegin(), glVertex*(), and glEnd() functions. To draw a triangle strip, glBegin() must be passed the argument GL_TRIANGLE_STRIP, which notifies OpenGL a triangle strip is about to be drawn. The glVertex*() family of functions specify the coordinates for each vertex in the triangle strip. For more information, consult The OpenGL Redbook.[1]
To draw the triangle strip in the diagram, the code is as follows:
//Vertices below are in Clockwise orientation //Default setting for glFrontFace is Counter-clockwise glFrontFace(GL_CW); glBegin(GL_TRIANGLE_STRIP); glVertex3f( 0.0f, 0.0f, 0.0f ); //vertex 1 glVertex3f( 0.0f, 1.0f, 0.0f ); //vertex 2 glVertex3f( 1.0f, 0.0f, 0.0f ); //vertex 3 glVertex3f( 1.5f, 1.0f, 0.0f ); //vertex 4 glEnd();
Note that only one additional vertex is needed to draw the second triangle. In OpenGL, the order in which the vertices are specified is important so that surface normals are consistent.
Quoted directly from the OpenGL Programming Guide:
GL_TRIANGLE_STRIP Draws a series of triangles (three-sided polygons) using vertices v0, v1, v2, then v2, v1, v3 (note the order), then v2, v3, v4, and so on. The ordering is to ensure that the triangles are all drawn with the same orientation so that the strip can correctly form part of a surface.
The above code sample and diagram demonstrate triangles drawn in a clockwise orientation. For those to be considered front-facing, a preceding call to glFrontFace(GL_CW) is necessary, which otherwise has an initial value of GL_CCW (meaning that triangles drawn counter-clockwise are front-facing by default).[2] This is significant if glEnable(GL_CULL_FACE) and glCullFace(GL_BACK) are already active (GL_BACK by default[3]), because back-facing triangles will be culled, so will not be drawn and will not appear on-screen at all.[4]
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[5] or FTSG[6] represent the model as several strips. Optimally grouping a set of triangles into sequential strips has been proven NP-complete.[7]
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.