The OpenGL Graphics System: A Specification

2.6. BEGIN/END PARADIGM 16
The required state consists of the processed vertex produced from the last vertex
that was sent (so that a line segment can be generated from it to the current
vertex), and a boolean flag indicating if the current vertex is the first vertex.
Line Loops. Line loops, specified with the LINE LOOP argument value to
Begin, are the same as line strips except that a final segment is added from the final
specified vertex to the first vertex. The additional state consists of the processed
first vertex.
Separate Lines. Individual line segments, each specified by a pair of vertices,
are generated by surrounding vertex pairs with Begin and End when the value
of the argument to Begin is LINES. In this case, 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 lines but it is used differently: a
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. Depending on the current state of the
GL, a polygon may be rendered in one of several ways such as outlining its border
or filling its interior. A polygon described with fewer than three vertices does not
generate a primitive.
Only convex polygons are guaranteed to be drawn correctly by the GL. If a
specified polygon is nonconvex when projected onto the window, then the rendered
polygon need only lie within the convex hull of the projected vertices defining its
boundary.
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. The order of the vertices is significant in
lighting and polygon rasterization (see sections 2.14.1 and 3.5.1).
Triangle strips. A triangle strip is a series of triangles connected along shared
edges. A triangle strip is specified by giving a series of defining vertices between
a Begin/End pair when Begin is called with 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. A Begin/End pair enclosing fewer
than three vertices, when TRIANGLE STRIP has been supplied to Begin, produces
no primitive. See figure 2.4.
The state required to support triangle strips consists of a flag indicating if the
first triangle has been completed, two stored processed vertices, (called vertex A
Version 2.0 - October 22, 2004