glEvalMesh
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:
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_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*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 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*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 <= j2; 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.
ERRORS
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.
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
SEE ALSO
glBegin,
glEvalCoord,
glEvalPoint,
glMap1,
glMap2,
glMapGrid
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© 1995 Uwe Behrens. All rights reserved.