glBegin
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 )
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 ver-
tices 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 indepen-
dent 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 tri-
angle is defined for each vertex presented
after the first two vertices. For odd n, ver-
tices n, n+1, and n+2 define triangle n. For
even n, vertices n+1, n, and n+2 define trian-
gle n. N−2 triangles are drawn.
GL_TRIANGLE_FAN Draws a connected group of triangles. One tri-
angle 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 inde-
pendent 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 ver-
tices 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 con-
struct a quadrilateral from strip data is dif-
ferent 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 num-
ber of vertices that can be defined between glBegin and glEnd. Lines,
triangles, quadrilaterals, and polygons that are incompletely speci-
fied 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 primi-
tive is ignored; the rest are drawn.
The minimum specification of vertices for each primitive is as fol-
lows: 1 for a point, 2 for a line, 3 for a triangle, 4 for a quadri-
lateral, 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 execu-
tion 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 ALSO
glArrayElement(3G), glCallList(3G), glCallLists(3G), glColor(3G),
glEdgeFlag(3G), glEvalCoord(3G),
glEvalPoint(3G), glIndex(3G), glMaterial(3G), glNormal(3G),
glTexCoord(3G), glVertex(3G)
GLBEGIN(3G)
