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.
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
2
n
-
1
and
2
n
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
3
n
-
2
,
3
n
-
1
, and
3
n
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
4
n
-
3
,
4
n
-
2
,
4
n
-
1
, and
4
n
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
2
n
-
1
,
2
n
,
2
n
+
2
, and
2
n
+
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, 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 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 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, glCallList, glCallLists, glColor, glEdgeFlag, glEvalCoord, glEvalPoint, glIndex, glMaterial, glNormal, glTexCoord, glVertex