Gambas Documentation - /comp/gb.opengl/gl/begin

/comp/gb.opengl/gl/begin

# Gl.Begin (gb.opengl)

Static Sub Begin ( Primitive As Integer )

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

### Parameters

mode

  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.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.

### 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 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 Gl.Begin and Gl.End. The commands are Gl.Vertex, Gl.Color, Gl.SecondaryColor, Gl.Index, Gl.Normal, Gl.FogCoord, Gl.TexCoord, Gl.MultiTexCoord, Gl.VertexAttrib, Gl.EvalCoord, Gl.EvalPoint, Gl.ArrayElement, 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 executed 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.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 Gl.Begin is executed between a Gl.Begin and the corresponding execution of Gl.End.

Gl.INVALID_OPERATION is generated if Gl.End is executed without being preceded by a Gl.Begin.

Gl.INVALID_OPERATION is generated if a command other than Gl.Vertex, Gl.Color, Gl.SecondaryColor, Gl.Index, Gl.Normal, Gl.FogCoord, Gl.TexCoord, Gl.MultiTexCoord, Gl.VertexAttrib, Gl.EvalCoord, Gl.EvalPoint, Gl.ArrayElement, Gl.Material, Gl.EdgeFlag, Gl.CallList, or Gl.CallLists is executed between the execution of Gl.Begin and the corresponding execution Gl.End.

Execution of Gl.EnableClientState, Gl.DisableClientState, Gl.EdgeFlagPointer, Gl.FogCoordPointer, Gl.TexCoordPointer, Gl.ColorPointer, Gl.SecondaryColorPointer, Gl.IndexPointer, Gl.NormalPointer, Gl.VertexPointer, Gl.VertexAttribPointer, Gl.InterleavedArrays, or Gl.PixelStore is not allowed after a call to Gl.Begin and before the corresponding call to Gl.End, but an error may or may not be generated.

### See Also

Gl.ArrayElement, Gl.CallList, Gl.CallLists, Gl.Color, Gl.EdgeFlag, Gl.EvalCoord, Gl.EvalPoint, Gl.FogCoord, Gl.Index, Gl.Material, Gl.MultiTexCoord, Gl.Normal, Gl.SecondaryColor, Gl.TexCoord, Gl.Vertex, Gl.VertexAttrib

Copyright © 1991-2006 Silicon Graphics, Inc. This document is licensed under the SGI Free Software B License. For details, see http://oss.sgi.com/projects/FreeB.

➡ See original documentation