glBegin or glEnd Subroutine

OpenGL 1.1 for AIX: Reference Manual

glBegin or glEnd Subroutine

Purpose

Delimits the vertices of a primitive or group of like primitives.

Library

OpenGL C bindings library: libGL.a

C Syntax

void glBegin(GLenum mode)

void glEnd(void)

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

The glBegin and glEnd subroutines delimit the vertices that define a primitive or group of like primitives. The glBegin subroutine accepts a single argument that specifies which of 10 ways the vertices will be interpreted. Taking n as an integer count starting at 1 (one), and N as the total number of vertices specified, the interpretations are:

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 subroutines can be used between the glBegin and glEnd subroutines. The subroutines are: glVertex, glColor, glIndex, glNormal, glTexCoord, glEvalCoord, glEvalPoint, glMaterial, and glEdgeFlag. Also, it is acceptable to use glCallList or glCallLists to execute display lists that include only the preceding subroutines. If any other GL subroutine is called between the glBegin and glEnd subroutines, the error flag is set and the subroutine is ignored.

Regardless of the value chosen for mode, there is no limit to the number of vertices that can be defined between the glBegin and glEnd subroutines. 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

INVALID_ENUM	Indicates that mode is set to an unaccepted value.
GL_INVALID_OPERATION	Indicates that a subroutine other than glVertex, glColor, glIndex, glNormal, glTexCoord, glEvalCoord, glEvalPoint, glMaterial, glEdgeFlag, glCallList, or glCallLists subroutine is called between glBegin and the corresponding glEnd.
GL_INVALID_OPERATION	Indicates that glEnd is called before the corresponding glBegin is called.

Files

/usr/include/GL/gl.h

	Contains C language constants, variable type definitions,
and
ANSI function prototypes for OpenGL.

Related Information

The glArrayElement subroutine, glArrayElementEXT subroutine, glColor subroutine, glCallList subroutine, glCallLists subroutine, glEdgeFlag subroutine, glEvalCoord subroutine, glEvalPoint subroutine, glIndex subroutine, glMaterial subroutine, glNormal subroutine, glTexCoord subroutine, glVertex subroutine.

OpenGL Overview.