fglBegin(3G) OpenGL Reference fglBegin(3G)NAME
fglBegin, fglEnd - delimit the vertices of a primitive or a group of like
primitives
FORTRAN SPECIFICATION
SUBROUTINE fglBegin( INTEGER*4 mode )
PARAMETERS
mode Specifies the primitive or primitives that will be created from
vertices presented between fglBegin and the subsequent fglEnd. 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.
FORTRAN SPECIFICATION
SUBROUTINE fglEnd( )
DESCRIPTION
fglBegin and fglEnd delimit the vertices that define a primitive or a
group of like primitives. fglBegin 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 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
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fglBegin(3G) OpenGL Reference fglBegin(3G)
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/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 fglBegin and fglEnd.
The commands are fglVertex, fglColor, fglIndex, fglNormal, fglTexCoord,
fglEvalCoord, fglEvalPoint, fglArrayElement, fglMaterial, and
fglEdgeFlag. Also, it is acceptable to use fglCallList or fglCallLists
to execute display lists that include only the preceding commands. If
any other GL command is executed between fglBegin and fglEnd, 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 fglBegin and fglEnd. 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.
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fglBegin(3G) OpenGL Reference fglBegin(3G)
GL_INVALID_OPERATION is generated if fglBegin is executed between a
fglBegin and the corresponding execution of fglEnd.
GL_INVALID_OPERATION is generated if fglEnd is executed without being
preceded by a fglBegin.
GL_INVALID_OPERATION is generated if a command other than fglVertex,
fglColor, fglIndex, fglNormal, fglTexCoord, fglEvalCoord, fglEvalPoint,
fglArrayElement, fglMaterial, fglEdgeFlag, fglCallList, or fglCallLists
is executed between the execution of fglBegin and the corresponding
execution fglEnd.
Execution of fglEnableClientState, fglDisableClientState,
fglEdgeFlagPointer, fglTexCoordPointer, fglColorPointer, fglIndexPointer,
fglNormalPointer, fglVertexPointer, fglInterleavedArrays, or
fglPixelStore is not allowed after a call to fglBegin and before the
corresponding call to fglEnd, but an error may or may not be generated.
SEE ALSO
fglArrayElement, fglCallList, fglCallLists, fglColor, fglEdgeFlag,
fglEvalCoord, fglEvalPoint, fglIndex, fglMaterial, fglNormal,
fglTexCoord, fglVertex
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