glBegin, glEnd  delimit the vertices of a primitive or a
group of like primglBeginitives
void glBegin(
GLenum mode ); 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. Treats each pair of vertices
as an independent line segment. Vertices 2n1 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. N1 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. Treats each triplet
of vertices as an independent triangle. Vertices 3n2,
3n1, 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. N2 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. N2 triangles are drawn. Treats
each group of four vertices as an independent quadrilateral.
Vertices 4n3, 4n2, 4n1, 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
2n1, 2n, 2n+2, and 2n+1 define quadrilateral n.
N/21 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. 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 option 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).
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.
glArrayElement(3), glCallList(3), glCallLists(),
glColor(3), glEdgeFlag(3), glEvalCoord(3), glEvalPoint(3),
glIndex(3), glMaterial(3), glNormal(3), glTexCoord(3),
glVertex(3)
glBegin(3G)
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