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 GL_POINT and GL_LINE 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 GL_POINT,
GL_LINE, and GL_FILL 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.
In the one-dimensional case, glEvalMesh1, the mesh is generated as if the
following code fragment were executed:
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glEvalMesh(3G) OpenGL Reference glEvalMesh(3G)
glBegin( type );
for ( i = i1; i <= i2; i += 1 )
.
glEvalCoord1( i Wu + u );
1
glEnd();
where
Wu = (u - u )/n
2 1
and n, u , and u are the arguments to the most recent glMapGrid1
1 2
command. type is GL_POINTS if mode is GL_POINT, or GL_LINES if mode is
GL_LINE.
The one absolute numeric requirement is that if i = n, then the value
.
computed from i Wu + u is exactly u .
1 2
In the two-dimensional case, glEvalMesh2, let
Wu = (u - u )/n
2 1
Wv = (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 GL_FILL, the glEvalMesh2 command is
equivalent to:
for ( j = j1; j < j2; j += 1 ) {
glBegin( GL_QUAD_STRIP );
for ( i = i1; i <= i2; i += 1 ) {
. .
glEvalCoord2( i Wu + u , j Wv + v );
1 1
. .
glEvalCoord2( i Wu + u , (j+1) Wv + v );
1 1
}
glEnd();
}
If mode is GL_LINE, then a call to glEvalMesh2 is equivalent to:
for ( j = j1; j <= j2; j += 1 ) {
glBegin( GL_LINE_STRIP );
for ( i = i1; i <= i2; i += 1 )
. .
glEvalCoord2( i Wu + u , j Wv + v );
1 1
glEnd();
}
for ( i = i1; i <= i2; i += 1 ) {
glBegin( GL_LINE_STRIP );
for ( j = j1; j <= j1; j += 1 )
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glEvalMesh(3G) OpenGL Reference glEvalMesh(3G)
. .
glEvalCoord2( i Wu + u , j Wv + v );
1 1
glEnd();
}
And finally, if mode is GL_POINT, then a call to glEvalMesh2 is
equivalent to:
glBegin( GL_POINTS );
for ( j = j1; j <= j2; j += 1 )
for ( i = i1; i <= i2; i += 1 )
. .
glEvalCoord2( i Wu + u , j Wv + v );
1 1
glEnd();
In all three cases, the only absolute numeric requirements are that if
.
i = n, then the value computed from i Wu + u is exactly u , and if
1 2
.
j = m, then the value computed from j Wv + v is exactly v .
1 2
ERRORS
GL_INVALID_ENUM is generated if mode is not an accepted value.
GL_INVALID_OPERATION is generated if glEvalMesh is executed between the
execution of glBegin and the corresponding execution of glEnd.
ASSOCIATED GETS
glGet with argument GL_MAP1_GRID_DOMAIN
glGet with argument GL_MAP2_GRID_DOMAIN
glGet with argument GL_MAP1_GRID_SEGMENTS
glGet with argument GL_MAP2_GRID_SEGMENTS
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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