- delimit the vertices of a primitive or a group of like
primitives
C SPECIFICATION
PARAMETERS
_param1 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
DESCRIPTION
glBegin and glEnd delimit the vertices that define a prim-
itive 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 trian-
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 quadri-
laterals. 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 quadrilat-
eral 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 _param1(1), there is no
limit to the number of vertices that can be defined
between glBegin and glEnd. Lines, triangles, quadrilater-
als, and polygons that are incompletely specified are not
drawn. Incomplete specification results when either too
few vertices are provided to specify even a single primi-
tive or when an incorrect multiple of vertices is speci-
fied. 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 trian-
gle, 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 _param1(1) is set to an
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 corre-
sponding call to glEnd, but an error may or may not be
generated.
SEE ALSO
glArrayElement, glCallList, glCallLists, glColor,
glEdgeFlag, glEvalCoord,
glEvalPoint, glIndex, glMaterial, glNormal, glTexCoord,
glVertex
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