1991-2006
Silicon Graphics, Inc.
glBegin
3G

# glBegin

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 2 &it; n - 1 and 2 &it; 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 &it; n - 2 , 3 &it; n - 1 , and 3 &it; 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 &it; n - 3 , 4 &it; n - 2 , 4 &it; n - 1 , and 4 &it; 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 &it; n - 1 , 2 &it; n , 2 &it; n + 2 , and 2 &it; 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, glSecondaryColor, glIndex, glNormal, glFogCoord, glTexCoord, glMultiTexCoord, glVertexAttrib, 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, glSecondaryColor, glIndex, glNormal, glFogCoord, glTexCoord, glMultiTexCoord, glVertexAttrib, 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, glFogCoordPointer, glTexCoordPointer, glColorPointer, glSecondaryColorPointer, glIndexPointer, glNormalPointer, glVertexPointer, glVertexAttribPointer, 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, glFogCoord, glIndex, glMaterial, glMultiTexCoord, glNormal, glSecondaryColor, glTexCoord, glVertex, glVertexAttrib

## Copyright

Copyright 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB/.