glBegin(3G) OpenGL Reference glBegin(3G)
NAME
glBegin, glEnd - delimit the vertices of a primitive or a group of like
primitives
C SPECIFICATION
void glBegin( GLenum mode )
PARAMETERS
mode Specifies the primitive or primitives that will be created from
vertices presented between glBegin and the subsequent glEnd. Ten
symbolic constants are accepted: GLPOINTS, GLLINES,
GLLINESTRIP, GLLINELOOP, GLTRIANGLES, GLTRIANGLESTRIP,
GLTRIANGLEFAN, GLQUADS, GLQUADSTRIP, and GLPOLYGON.
C SPECIFICATION
void glEnd( void )
DESCRIPTION
glBegin and glEnd delimit the vertices that define a primitive or a group
of like primitives. glBegin accepts a single argument that specifies
which of ten ways the vertices are interpreted. Taking n as an integer
count starting at one, and N as the total number of vertices specified,
the interpretations are as follows:
GLPOINTS Treats each vertex as a single point. Vertex n defines point
n. N points are drawn.
GLLINES Treates each pair of vertices as an independent line segment.
Vertices 2n-1 and 2n define line n. N/2 lines are drawn.
GLLINESTRIP
Draws a connected group of line segments from the first vertex
to the last. Vertices n and n+1 define line n. N-1 lines are
drawn.
GLLINELOOP
Draws a connected group of line segments from the first vertex
to the last, then back to the first. Vertices n and n+1 define
line n. The last line, however, is defined by vertices N and
1. N lines are drawn.
GLTRIANGLES
Treates each triplet of vertices as an independent triangle.
Vertices 3n-2, 3n-1, and 3n define triangle n. N/3 triangles
are drawn.
GLTRIANGLESTRIP
Draws a connected group of triangles. One triangle is defined
for each vertex presented after the first two vertices. For
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glBegin(3G) OpenGL Reference glBegin(3G)
odd n, vertices n, n+1, and n+2 define triangle n. For even n,
vertices n+1, n, and n+2 define triangle n. N-2 triangles are
drawn.
GLTRIANGLEFAN
Draws a connected group of triangles. One triangle is defined
for each vertex presented after the first two vertices.
Vertices 1, n+1, and n+2 define triangle n. N-2 triangles are
drawn.
GLQUADS Treats each group of four vertices as an independent
quadrilateral. Vertices 4n-3, 4n-2, 4n-1, and 4n define
quadrilateral n. N/4 quadrilaterals are drawn.
GLQUADSTRIP
Draws a connected group of quadrilaterals. One quadrilateral
is defined for each pair of vertices presented after the first
pair. Vertices 2n-1, 2n, 2n+2, and 2n+1 define quadrilateral
n. N/2-1 quadrilaterals are drawn. Note that the order in
which vertices are used to construct a quadrilateral from strip
data is different from that used with independent data.
GLPOLYGON
Draws a single, convex polygon. Vertices 1 through N define
this polygon.
Only a subset of GL commands can be used between glBegin and glEnd. The
commands are glVertex, glColor, glIndex, glNormal, glTexCoord,
glEvalCoord, glEvalPoint, glMaterial, and glEdgeFlag. Also, it is
acceptable to use glCallList or glCallLists to execute display lists that
include only the preceding commands. If any other GL command is called
between glBegin and glEnd, the error flag is set and the command is
ignored.
Regardless of the value chosen for mode, there is no limit to the number
of vertices that can be defined between glBegin and glEnd. Lines,
triangles, quadrilaterals, and polygons that are incompletely specified
are not drawn. Incomplete specification results when either too few
vertices are provided to specify even a single primitive or when an
incorrect multiple of vertices is specified. The incomplete primitive is
ignored; the rest are drawn.
The minimum specification of vertices for each primitive is as follows:
1 for a point, 2 for a line, 3 for a triangle, 4 for a quadrilateral, and
3 for a polygon. Modes that require a certain multiple of vertices are
GLLINES (2), GLTRIANGLES (3), GLQUADS (4), and GLQUADSTRIP (2).
ERRORS
GLINVALIDENUM is generated if mode is set to an unaccepted value.
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glBegin(3G) OpenGL Reference glBegin(3G)
GLINVALIDOPERATION is generated if a command other than glVertex,
glColor, glIndex, glNormal, glTexCoord, glEvalCoord, glEvalPoint,
glMaterial, glEdgeFlag, glCallList, or glCallLists is executed between
the execution of glBegin and the corresponding execution of glEnd.
GLINVALIDOPERATION is generated if glEnd is executed before the
corresponding glBegin is executed, or if glBegin is executed within a
glBegin/glEnd sequence.
SEE ALSO
glCallList, glCallLists, glColor, glEdgeFlag, glEvalCoord, glEvalPoint,
glIndex, glMaterial, glNormal, glTexCoord, glVertex
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