glEvalMesh, glEvalMesh2 - compute a one- or two-dimensional
grid of points or lines
void glEvalMesh1(
GLenum mode,
GLint i1,
GLint i2 );
In glEvalMesh1(), specifies whether to compute a onedimensional
mesh of points or lines. Symbolic constants
GL_POINT and GL_LINE are accepted. Specify the first and
last integer values for grid domain variable i.
void glEvalMesh2(
GLenum mode,
GLint i1,
GLint i2,
GLint j1,
GLint j2 );
In glEvalMesh2(), specifies whether to compute a twodimensional
mesh of points, lines, or polygons. Symbolic
constants GL_POINT, GL_LINE, and GL_FILL are accepted.
Specify the first and last integer values for grid domain
variable i. Specify the first and last integer values for
grid domain variable j.
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:
glBegin( type ); for ( i = i1; i <= i2; i += 1 )
glEvalCoord1( i . DELTA u + u[1] ); glEnd();
where DELTA u = (u[ 2] - u[1] ) / n
and n, u[ 1], and u[2] are the arguments to the most
recent glMapGrid1() 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 . DELTA u + u[1] is
exactly u[2].
In the two-dimensional case, glEvalMesh2(), let DELTA u =
( u[2] - u[1] ) / n DELTA v = ( v[2] - v[1] ) / m,
where n, u[1], u[2], m, v[1], and v[2] are the arguments
to the most recent 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.DELTA u + u[1], j.DELTA v + v[1] );
glEvalCoord2( i.DELTA u + u[1], (j+1)cdotDELTA v +
v[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( icdotDELTA u + u[1], jcdotDELTA v +
v[1] );
}
glEnd(); }
for ( i = i1; i <= i2; i += 1 ) {
glBegin( GL_LINE_STRIP );
for ( j = j1; j <= i2; j += 1 ) {
glEvalCoord2( icdotDELTA u + u[1], jcdotDELTA v +
v[1] );
}
glEnd(); }
And finally, if mode is GL_POINT, then a call toglEvalMesh2()
is equivalent to:
glBegin( GL_POINTS ); for ( j = j1; j <= j2; j += 1 )
for ( i = i1; i <= i2; i += 1 )
glEvalCoord2( i^cdot^DELTA u + u[1], j^cdot^DELTA v
+ v[1] ); glEnd();
In all three cases, the only absolute numeric requirements
are that if i=n, then the value computed from icdotDELTA u
+ u[1] is exactly u[2], and if j=m, then the value computed
from j cdot DELTA v + v[1] is exactly v[2].
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().
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
glBegin(3), glEvalCoord(3), glEvalPoint(3), glMap1(3),
glMap2(3), glMapGrid(3)
glEvalMesh(3G)
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