# glBegin.3G - Man Page

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 $2n\text{}-\text{}1$ 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 $n\text{\hspace{0.5em}}+\text{\hspace{0.5em}}1$ define line $n$. $N\text{}-\text{}1$ 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 $n\text{\hspace{0.5em}}+\text{\hspace{0.5em}}1$ 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 $3n\text{}-\text{}2$, $3n\text{}-\text{}1$, 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 $n$, $n\text{\hspace{0.5em}}+\text{\hspace{0.5em}}1$, and $n\text{\hspace{0.5em}}+\text{\hspace{0.5em}}2$ define triangle $n$. For even $n$, vertices $n\text{\hspace{0.5em}}+\text{\hspace{0.5em}}1$, $n$, and $n\text{\hspace{0.5em}}+\text{\hspace{0.5em}}2$ define triangle $n$. $N\text{}-\text{}2$ 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 $1$, $n\text{\hspace{0.5em}}+\text{\hspace{0.5em}}1$, and $n\text{\hspace{0.5em}}+\text{\hspace{0.5em}}2$ define triangle $n$. $N\text{}-\text{}2$ triangles are drawn.

**GL_QUADS**Treats each group of four vertices as an independent quadrilateral. Vertices $4n\text{}-\text{}3$, $4n\text{}-\text{}2$, $4n\text{}-\text{}1$, 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 $2n\text{}-\text{}1$, $2n$, $2n\text{\hspace{0.5em}}+\text{\hspace{0.5em}}2$, and $2n\text{\hspace{0.5em}}+\text{\hspace{0.5em}}1$ define quadrilateral $n$. $N/2\text{}-\text{}1$ 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(3G), glCallList(3G), glCallLists(3G), glColor(3G), glEdgeFlag(3G), glEvalCoord(3G),

glEvalPoint(3G), glIndex(3G), glMaterial(3G), glNormal(3G), glTexCoord(3G), glVertex(3G)

## Referenced By

glEdgeFlag.3G(3), glEvalCoord.3G(3), glEvalMesh.3G(3), glFeedbackBuffer.3G(3), glMap1.3G(3), glMap2.3G(3), glNormal.3G(3), glRect.3G(3), glShadeModel.3G(3), gluTessCallback.3G(3), glutStrokeCharacter(3), glVertex.3G(3).

The man page glEnd.3G(3) is an alias of glBegin.3G(3).