GLBEGIN(3G) UNIX Programmer's Manual 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 primi-
tive 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 seg-
ments 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 seg-
ments from the first vertex to the
last, then back to the first. Ver-
tices 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
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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 ver-
tex 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 trian-
gles are drawn.
GL_TRIANGLE_FAN Draws a connected group of triangles.
One triangle is defined for each ver-
tex presented after the first two
vertices. Vertices 1, n + 1, and
n + 2 define triangle n. N-2 trian-
gles are drawn.
GL_QUADS Treats each group of four vertices as
an independent quadrilateral. Ver-
tices 4n-3, 4n-2, 4n-1, and 4n define
quadrilateral n. N/4 quadrilaterals
are drawn.
GL_QUAD_STRIP Draws a connected group of quadrila-
terals. One quadrilateral is defined
for each pair of vertices presented
after the first pair. Vertices 2n-1,
2n, 2n + 2, and 2n + 1 define quadri-
lateral n. N/2-1 quadrilaterals are
drawn. Note that the order in which
vertices are used to construct a qua-
drilateral from strip data is dif-
ferent from that used with indepen-
dent data.
GL_POLYGON Draws a single, convex polygon. Ver-
tices 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.
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GLBEGIN(3G) UNIX Programmer's Manual GLBEGIN(3G)
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 ver-
tices 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 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)
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