glBegin, glEnd
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 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,
glIndex,
glTexCoord,
glEvalPoint,
glArrayElement,
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
glColor,
glNormal,
glEvalCoord,
glEvalPoint,
glArrayElement,
glMaterial,
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,
glCallList,
glCallLists,
glColor,
glEdgeFlag,
glEvalCoord,
glEvalPoint,
glIndex,
glMaterial,
glNormal,
glTexCoord,
glVertex