OpenGL 4.1 (Compatibility Profile) - July 25, 2010

2.6. BEGIN/END PARADIGM 23
first vertex. The required state consists of the processed first vertex, in addition to
the state required for line strips.
Separate Lines
Individual line segments, each specified by a pair of vertices, may be specified
with mode LINES. The first two vertices between a Begin and End pair define the
first segment, with subsequent pairs of vertices each defining one more segment.
If the number of specified vertices is odd, then the last one is ignored. The state
required is the same as for line strips but it is used differently: a processed vertex
holding the first vertex of the current segment, and a boolean flag indicating
whether the current vertex is odd or even (a segment start or end).
Polygons
A polygon is described by specifying its boundary as a series of line segments.
When Begin is called with POLYGON, the bounding line segments are specified in
the same way as line loops. A polygon described with fewer than three vertices
does not generate a primitive.
The state required to support polygons consists of at least two processed vertices
(more than two are never required, although an implementation may use
more); this is because a convex polygon can be rasterized as its vertices arrive,
before all of them have been specified.
Triangle Strips
A triangle strip is a series of triangles connected along shared edges, and may
be specified with mode TRIANGLE_STRIP. In this case, the first three vertices
define the first triangle (and their order is significant, just as for polygons). Each
subsequent vertex defines a new triangle using that point along with two vertices
from the previous triangle. If fewer than three vertices are specified, no primitive
is produced. See figure 2.4.
The required state consists of a flag indicating if the first triangle has been
completed, two stored processed vertices, (called vertex A and vertex B), and a
one bit pointer indicating which stored vertex will be replaced with the next vertex.
After a Begin(TRIANGLE_STRIP), the pointer is initialized to point to vertex A.
Each successive vertex toggles the pointer. Therefore, the first vertex is stored as
vertex A, the second stored as vertex B, the third stored as vertex A, and so on.
Any vertex after the second one sent forms a triangle from vertex A, vertex B, and
the current vertex (in that order).
Triangle Fans
A triangle fan is the same as a triangle strip with one exception: each vertex
OpenGL 4.1 (Compatibility Profile) - July 25, 2010