glEvalMesh(3G) OpenGL Reference glEvalMesh(3G)
NAME
glEvalMesh1, glEvalMesh2 - compute a one- or two-dimensional grid of
points or lines
C SPECIFICATION
void glEvalMesh1( GLenum mode,
GLint i1,
GLint i2 )
PARAMETERS
mode In glEvalMesh1, specifies whether to compute a one-dimensional mesh
of points or lines. Symbolic constants GLPOINT and GLLINE are
accepted.
i1, i2
Specify the first and last integer values for grid domain variable
i.
C SPECIFICATION
void glEvalMesh2( GLenum mode,
GLint i1,
GLint i2,
GLint j1,
GLint j2 )
PARAMETERS
mode In glEvalMesh2, specifies whether to compute a two-dimensional mesh
of points, lines, or polygons. Symbolic constants GLPOINT,
GLLINE, and GLFILL are accepted.
i1, i2
Specify the first and last integer values for grid domain variable
i.
j1, j2
Specify the first and last integer values for grid domain variable
j.
DESCRIPTION
glMapGrid and glEvalMesh are used in tandem to efficiently generate and
evaluate a series of evenly spaced map domain values. glEvalMesh steps
through the integer domain of a one- or two-dimensional grid, whose range
is the domain of the evaluation maps specified by glMap1 and glMap2.
mode determines whether the resulting vertices are connected as points,
lines, or filled polygons.
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glEvalMesh(3G) OpenGL Reference glEvalMesh(3G)
In the one-dimensional case, glEvalMesh1, the mesh is generated as if the
following code fragment were executed:
glBegin(type);
for (i = i1; i <= i2; i += 1)
glEvalCoord1(i*du + u )
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glEnd();
where
du = (u -u )/n
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and n, u , and u are the arguments to the most recent glMapGrid1
1 2
command. type is GLPOINTS if mode is GLPOINT, or GLLINES if mode is
GLLINE. The one absolute numeric requirement is that if i = n, then the
value computed from i*du + u is exactly u .
1 2
In the two-dimensional case, glEvalMesh2, let
du=(u -u )/n
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dv=(v -v )/m,
2 1
where n, u , u , m, v , and v are the arguments to the most recent
1 2 1 2
glMapGrid2 command. Then, if mode is GLFILL, the glEvalMesh2 command is
equivalent to:
for (j = j1; j < j2; j += 1) {
glBegin(GLQUADSTRIP);
for (i = i1; i <= i2; i += 1) {
glEvalCoord2(i*du + u , j*dv + v );
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glEvalCoord2(i*du + u , (j+1)*dv + v );
1 1
}
glEnd();
}
If mode is GLLINE, then a call to glEvalMesh2 is equivalent to:
for (j = j1; j <= j2; j += 1) {
glBegin(GLLINESTRIP);
for (i = i1; i <= i2; i += 1)
glEvalCoord2(i*du + u , j*dv + v );
1 1
glEnd();
}
for (i = i1; i <= i2; i += 1) {
glBegin(GLLINESTRIP);
for (j = j1; j <= j1; j += 1)
glEvalCoord2(i*du + u , j*dv + v );
1 1
glEnd();
}
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glEvalMesh(3G) OpenGL Reference glEvalMesh(3G)
And finally, if mode is GLPOINT, then a call to glEvalMesh2 is
equivalent to:
glBegin(GLPOINTS);
for (j = j1; j <= j2; j += 1) {
for (i = i1; i <= i2; i += 1) {
glEvalCoord2(i*du + u , j*dv + v );
1 1
}
}
glEnd();
In all three cases, the only absolute numeric requirements are that if
i = n, then the value computed from i*du + u is exactly u , and if
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j = m, then the value computed from j*dv + v is exactly v .
1 2
ERRORS
GLINVALIDENUM is generated if mode is not an accepted value.
GLINVALIDOPERATION is generated if glEvalMesh is executed between the
execution of glBegin and the corresponding execution of glEnd.
ASSOCIATED GETS
glGet with argument GLMAP1GRIDDOMAIN
glGet with argument GLMAP2GRIDDOMAIN
glGet with argument GLMAP1GRIDSEGMENTS
glGet with argument GLMAP2GRIDSEGMENTS
MACHINE DEPENDENCIES
RealityEngine, RealityEngine2, and VTX systems do not handle 1D maps for
colors correctly.
SEE ALSO
glBegin, glEvalCoord, glEvalPoint, glMap1, glMap2, glMapGrid
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