glBegin

NAME

glBegin, glEnd -- 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 verties that define a primitive or a group of like primitives. glBegin accepts a single argument that specifies which of ten ways the vertices are interpreted. Taking n as the 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-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 quadriliteral. Vertices 4n-3, 4n-2, 4n-1, and 4n define quadriliteral n. N/4 quadriliterals are drawn.

GL_QUAD_STRIP

Draws a connected group of quadriliterals. One quadriliteral is defined for each pair of vertices presented after the first pair. Vertices 2n-1, 2n, 2n+2, and 2n+1 define quadriliteral n. N/2-1 quadriliterals are drawn. Note that the order in which vertices are used to construct a quadriliteral 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 betwen glBegin and glEnd. The commands 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 preceeding commands. If any other GL command is called 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 betwen glBegin and glEnd. Lines, triangles, quadriliterals, 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 quadriliteral, 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 a command other than glVertex, glColor, glIndex, glNormal, glTexCoord, glEvalCoord, glEvalPoint, glMaterial, glEdgeFlag, glCallList or glCallLists is called between glBegin and the corresponding glEnd.

GL_INVALID_OPERATION is generated if glEnd is called before the corresponding glBegin is called.