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 specifies in which of ten ways the vertices are interpreted.
Taking as an integer count starting at one, and 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 tri-angle triangle
angle n. n-2 triangles are drawn.
GL_QUADS Treats each group of four vertices as an independent quadrilateral. Vertices
4n-3, 4n-2, 4n-3, 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 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 incom-pletely incompletely
pletely 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 exe-cution execution
cution 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)
GLBEGIN(3G)
|