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 spec-
ifies 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 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 ver-
tices. 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 ver-
tices. 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, 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 primi-
tive 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 quadrilat-
eral, 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 exe-
cution 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)