/ comp / gb.opengl / gl / begin
Gl.Begin (gb.opengl)
Syntax

    STATIC SUB Begin ( Primitive AS Integer )

### Delimit the vertices of a primitive or a group of like primitives

#### Parameters

- Primitive specifies the primitive or primitives that will be created from vertices presented between Gl.Begin and the subsequent Gl.End. Ten symbolic constants are accepted: Gl.GL_POINTS, Gl.GL_LINES, Gl.GL_LINE_STRIP, Gl.GL_LINE_LOOP, Gl.GL_TRIANGLES, Gl.GL_TRIANGLE_STRIP, Gl.GL_TRIANGLE_FAN, Gl.GL_QUADS, Gl.GL_QUAD_STRIP, and Gl.GL_POLYGON.

#### Description

Gl.Begin and Gl.End delimit the vertices that define a Primitive or a group of like primitives. Gl.Begin accepts a single argument that specifies 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.GL_POINTS treats each vertex as a single point. Vertex n defines point n. N points are drawn.

- Gl.GL_LINES treates each pair of vertices as an independent line segment. Vertices 2n-1 and 2n define line n. N/2 lines are drawn.

- Gl.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 drawn.

- Gl.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.GL_TRIANGLES treates each triplet of vertices as an independent triangle. Vertices 3n-2, 3n-1, and 3n define triangle n. N/3 triangles are drawn.

- Gl.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.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.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.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. N2-1 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.GL_POLYGON draws a single, convex polygon. Vertices 1 through N define this polygon.

### Examples

    ' Draw two points, depending on viewport setting
    Gl.Begin(Gl.GL_POINTS)
      Gl.Vertex2i(100, 200)
      Gl.Vertex2i(300, 400)
    Gl.End()

Only a subset of GL commands can be used between Gl.Begin and Gl.End. The commands are Gl.Vertex, Gl.Color, Gl.Index, Gl.Normal, Gl.TexCoord, Gl.EvalCoord, Gl.EvalPoint, Gl.Material, and Gl.EdgeFlag. Also, it is acceptable to use Gl.CallList or Gl.CallLists to execute display lists that include only the preceding commands. If any other GL command is called between Gl.Begin and Gl.End, 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 Gl.Begin and Gl.End. 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.GL_LINES, Gl.GL_TRIANGLES, Gl.GL_QUADS, and Gl.GL_QUAD_STRIP.

#### Errors

Gl.GL_INVALID_ENUM is generated if mode is set to an unaccepted value.
Gl.GL_INVALID_OPERATION is generated if a command other than Gl.Vertex, Gl.Color, Gl.Index, Gl.Normal, Gl.TexCoord, Gl.EvalCoord, Gl.EvalPoint, Gl.Material, Gl.EdgeFlag, Gl.CallList, or Gl.CallLists is called between Gl.Begin and the corresponding Gl.End.
Gl.GL_INVALID_OPERATION is generated if Gl.End is called before the corresponding Gl.Begin is called, or if Gl.Begin is called within a Gl.Begin / Gl.End sequence.

### See also

Gl.CallList , Gl.CallLists , Gl.Color, Gl.EdgeFlag, Gl.EvalCoord, Gl.EvalPoint, Gl.Index, Gl.Material, Gl.Normal, Gl.TexCoord, Gl.Vertex