GLEVALMESH() MachTen Programmer’s Manual 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 )

delim $$

PARAMETERS
mode In glEvalMesh1, specifies whether to compute a one-
dimensional mesh of points or lines. Symbolic con-
stants 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 gen-
erated as if the following code fragment were executed:

glBegin (type);
for (i = i1; i <= i2; i += 1)
glEvalCoord1(i . DELTA u + u sub 1)
glEnd();

where

DELTA u = (u - u ) / 1
2 1

and n, u, and u are the arguments to the most recent
1 2
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

is exactly u.

In the two-dimensional case, glEvalMesh2, let

DELTA u = (u - u )/n
2 1

DELTA v = (v - v )/m,
2 1

where n, u , u , m, v , and v
1 2 1 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 , j . DELTA v + v );
1 1
glEvalCoord2(i . DELTA u + u , (j+1) . DELTA v + v );
1 1
}
glEnd();
}

If mode is GL_LINE, then a call to glEvalMesh2 is equiva-
lent to:

for (j = j1; j <= j2; j += 1) {
glBegin(GL_LINE_STRIP);
for (i = i1; i <= i2; i += 1)
glEvalCoord2(i . DELTA u + u , j . DELTA v + v );
1 1
glEnd();
}
for (i = i1; i <= i2; i += 1) {
glBegin(GL_LINE_STRIP);
for (j = j1; j <= j1; j += 1)
glEvalCoord2)(i . DELTA u + u , j . DELTA v + v );
1 1
glEnd();
}

And finally, if mode is GL_POINT, then a call to glE-
valMesh2 is equivalent to:

glBegin (GL_POINTS);
for (j = j1; j <= j2; j += 1) {
for (i = i1; i <= i2; i += 1) {
glEvalCoord2(i . DELTA u + u , j . DELTA v + v );
1 1
}
}
glEnd();

In all three cases, the only absolute numeric requirements
are that if $i~=~n$, then the value computed from
i . DELTA u + u is exactly u ,
1 2
and if $j~=~m$,
then the value computed from
j . DELTA v + 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 exe-
cuted between the execution of glBegin and the correspond-
ing 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

SEE ALSO
glBegin, glEvalCoord, glEvalPoint, glMap1, glMap2, glMap-
Grid

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