glBegin,
glEnd
[New
- Windows 95, OEM Service Release 2]
The glBegin
and glEnd functions delimit the vertices of a primitive or a group of
like primitives.
void glBegin(
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GLenum mode |
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); |
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void glEnd(
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void |
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); |
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Parameters
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.
Remarks
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 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:
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:
|
Minimum
Number |
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1 |
point |
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2 |
line |
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3 |
triangle |
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4 |
quadrilateral |
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3 |
polygon |
Modes that require a certain multiple of vertices are GL_LINES (2),
GL_TRIANGLES (3), GL_QUADS (4), and GL_QUAD_STRIP (2).
Error Codes
The following
are the error codes generated and their conditions.
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Error
Code |
Condition |
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GL_INVALID_ENUM
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mode was set to an unaccepted value. |
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GL_INVALID_OPERATION
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A function
other than glVertex, glColor, glIndex, glNormal,
glTexCoord, glEvalCoord, glEvalPoint, glMaterial,
glEdgeFlag, glCallList, or glCallLists was called between glBegin
and the corresponding glEnd. |
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GL_INVALID_OPERATION
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glEnd was called before the corresponding glBegin
was called, or glBegin was called within a glBegin/glEnd
sequence. |
See Also