glBegin function

Article
03/09/2021

The glBegin and glend functions delimit the vertices of a primitive or a group of like primitives.

Syntax

void WINAPI glBegin(
   GLenum mode
);

Parameters

mode

The primitive or primitives that will be created from vertices presented between glBegin and the subsequent glend. The following are accepted symbolic constants and their meanings:

Value	Meaning
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, 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.

Return value

This function does not return a value.

Error codes

The following error codes can be retrieved by the glGetError function.

Name	Meaning
GL_INVALID_ENUM	mode was set to an unaccepted value.
GL_INVALID_OPERATION	A function other than glVertex, glColor, glIndex, glNormal, glTexCoord, glEvalCoord, glEvalPoint, glMaterial, glEdgeFlag, glCallList, or glCallLists was called between glBegin and the corresponding glend. The function glend was called before the corresponding glBegin was called, or glBegin was called within a glBegin/glend sequence.

Remarks

The glBegin and glend functions delimit the vertices that define a primitive or a group of like primitives. The glBegin function accepts a single argument that specifies which of ten primitives the vertices compose. Taking n as an integer count starting at one, and N as the total number of vertices specified, the interpretations are as follows:

You can use only a subset of OpenGL functions between glBegin and glend. The functions you can use are:

glVertex

glColor

glIndex

glNormal

glTexCoord

glEvalCoord

glEvalPoint

glMaterial

glEdgeFlag

You can also use glCallList or glCallLists to execute display lists that include only the preceding functions. If any other OpenGL function is called between glBegin and glend, the error flag is set and the function is ignored.
Regardless of the value chosen for mode in glBegin, there is no limit to the number of vertices you can define 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 complete primitives are drawn.
The minimum specification of vertices for each primitive is:

Minimum number of vertices Type of primitive

1 point

2 line

3 triangle

4 quadrilateral

3 polygon
Modes that require a certain multiple of vertices are GL_LINES (2), GL_TRIANGLES (3), GL_QUADS (4), and GL_QUAD_STRIP (2).

Minimum number of vertices	Type of primitive
1	point
2	line
3	triangle
4	quadrilateral
3	polygon

Requirements

Requirement	Value
Minimum supported client	Windows 2000 Professional [desktop apps only]
Minimum supported server	Windows 2000 Server [desktop apps only]
Header	Gl.h
Library	Opengl32.lib
DLL	Opengl32.dll