GLBEGIN(3G)GLBEGIN(3G)NAME
glBegin, glEnd - delimit the vertices of a primitive or a group of like
primitives
C SPECIFICATION
void glBegin( GLenum mode )
delim $$
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: 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.
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 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:
GL_POINTS Treats each vertex as a single point. Vertex
$n$ defines point $n$. $N$ points are drawn.
GL_LINES Treats each pair of vertices as an independent
line segment. Vertices $2n^-^1$ and $2n$ define
line $n$. $N/2$ lines are drawn.
GL_LINE_STRIP 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.
GL_LINE_LOOP 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.
GL_TRIANGLES Treats each triplet of vertices as an
independent triangle. Vertices $3n^-^2$,
$3n^-^1$, and $3n$ define triangle $n$. $N/3$
triangles are drawn.
GL_TRIANGLE_STRIP 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.
GL_TRIANGLE_FAN 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.
GL_QUADS 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.
GL_QUAD_STRIP 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.
GL_POLYGON 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.
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 GL_LINES (2), GL_TRIANGLES (3), GL_QUADS (4), and GL_QUAD_STRIP
(2).
ERRORS
GL_INVALID_ENUM is generated if mode is set to an unaccepted value.
GL_INVALID_OPERATION 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 glEnd.
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.
SEE ALSOglArrayElement(3G), glCallList(3G), glCallLists(3G), glColor(3G),
glEdgeFlag(3G), glEvalCoord(3G),
glEvalPoint(3G), glIndex(3G), glMaterial(3G), glNormal(3G),
glTexCoord(3G), glVertex(3G)
March 1, 2011
