glBegin
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## Name

glBegin, glEnd — delimit the vertices of a primitive or a group of like primitives

## C Specification

`void `glBegin`` | `(` | `GLenum `mode`);`

## Python Specification

``glBegin`` | `(` | ``mode`) → `None``

## 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`` | `(` | `);`

## Python Specification

``glEnd`` | `(` | `) → `None``

## 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 n - 1 and 2 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 n - 2 , 3 n - 1 , and 3 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 n - 3 , 4 n - 2 , 4 n - 1 , and 4 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 n - 1 , 2 n , 2 n + 2 , and 2 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`, `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, glCallList, glCallLists, glColor, glEdgeFlag, glEvalCoord, glEvalPoint, glIndex, glMaterial, glNormal, glTexCoord, glVertex

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