glBegin, glEnd

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

void glBegin(
  GLenum mode);void glEnd( void);

• 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:
  • 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 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 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.

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:

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

The following are the error codes generated and their conditions.

**  Windows NT/2000:** Requires Windows NT 3.5 or later.
**  Windows 95/98:** Requires Windows 95 or later. Available as a redistributable for Windows 95.
**  Header:** Declared in Gl.h.
**  Library:** Use Opengl32.lib.

glCallList, glCallLists, glColor, glEdgeFlag, glEvalCoord, glEvalPoint, glIndex, glMaterial, glNormal, glTexCoord, glVertex