glEvalMesh1, glEvalMesh2 - compute a one- or two-dimensional grid of points or lines
void glEvalMesh1( GLenum mode, GLint i1, GLint i2 )
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.
void glEvalMesh2( GLenum mode, GLint i1, GLint i2, GLint j1, GLint j2 )
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.
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 · du + u1)
glEnd();
where
du = (u2-u1)/n
and n, u1, and u2 are the arguments to the most recent glMapGrid1 command.
type is GL_POINT 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·du + u1 is exactly u2.
In the two-dimensional case, glEvalMesh2, let
du = (u2-u1)/n
dv = (v2-v1)/m
where n, u1, u2, m, v1, and v2 are the arguments to the most recent
glMapGrid2 comand. 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·du + u1, j·dv + v1);
glEvalCoord2(i·du + u1, (j+1)·dv + v1);
}
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·du + u1, j·dv + v1);
glEnd();
}
for (i = i1; i <= i2; i += 1) {
glBegin(GL_LINE_STRIP);
for (j = j1; j <= j1; j += 1)
glEvalCoord2(i·du + u1, j·dv + v1);
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·du + u1, j·dv + v1);
}
}
glEnd();
In all three cases, the only absolute numeric requirements are that if
i = n, then the value computed from i·du + u1 is exactly u2, and if j = m,
then the value computed from j·dv + v1 is exactly v2.
GL_INVALID_ENUM is generated if mode is not an accepted value. GL_INVALID_OPERATION is generated if glEvalMesh is called between a call to glBegin and the corresponding call to 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, glEvalCoord, glEvalPoint, glMap1, glMap2, glMapGrid
Introduction | Alphabetic | Specification
Last Edited: Mon, May 22, 1995