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glBegin, glEnd - delimit the vertices of a primitive or a group of
void glBegin( GLenum mode )
eqn not supported
void glEnd( void )
- Specifies the primitive or primitives
that will be created from vertices presented between glBegin and the subsequent
glEnd. Ten symbolic constants are accepted: GL_POINTS, GL_LINES, GL_LINE_STRIP,
GL_LINE_LOOP, GL_TRIANGLES, GL_TRIANGLE_STRIP, GL_TRIANGLE_FAN, GL_QUADS,
GL_QUAD_STRIP, and GL_POLYGON.
glBegin and glEnd delimit the vertices that define a primitive
or a group of like primitives. glBegin accepts a single argument that specifies
in 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:
- Treats each vertex as a single
point. Vertex $n$ defines point $n$. $N$ points are drawn.
each pair of vertices as an independent line segment. Vertices $2n^-^1$ and
$2n$ define line $n$. $N/2$ lines are drawn.
- 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.
- 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.
each triplet of vertices as an independent triangle. Vertices $3n^-^2$, $3n^-^1$,
and $3n$ define triangle $n$. $N/3$ triangles are drawn.
- Draws a connected group of triangles. One triangle is defined for each
vertex presented after the first two vertices. For 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.
- 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.
- 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.
- 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
- 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, glArrayElement, 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 executed
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.
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 GL_LINES
, GL_TRIANGLES (3)
, GL_QUADS (4)
, and GL_QUAD_STRIP (2)
is generated if mode is set to an unaccepted value.
is generated if glBegin is executed between a glBegin and the corresponding
execution of glEnd.
GL_INVALID_OPERATION is generated if glEnd is executed
without being preceded by a glBegin.
GL_INVALID_OPERATION is generated if
a command other than glVertex, glColor, glIndex, glNormal, glTexCoord,
glEvalCoord, glEvalPoint, glArrayElement, glMaterial, glEdgeFlag, glCallList,
or glCallLists is executed between the execution of glBegin and the corresponding
Execution of glEnableClientState, glDisableClientState,
glEdgeFlagPointer, glTexCoordPointer, glColorPointer, glIndexPointer, glNormalPointer,
glVertexPointer, glInterleavedArrays, or glPixelStore is not allowed after
a call to glBegin and before the corresponding call to glEnd, but an error
may or may not be generated.
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