glEvalMesh1, glEvalMesh2 − compute a one or twodimensional grid of points or lines
void glEvalMesh1( GLenum mode,
GLint i1, 

GLint i2 ) 
mode 
In glEvalMesh1, specifies whether to compute a onedimensional 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 .
void glEvalMesh2( GLenum mode,
GLint i1,
GLint i2, 

GLint j1, 

GLint j2 ) 
mode 
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. 

i1, i2 
Specify the first and last integer values for grid domain variable . 

j1, j2 
Specify the first and last integer values for grid domain variable . 
glMapGrid and glEvalMesh are used in tandem to efficiently generate and evaluate a series of evenlyspaced map domain values. glEvalMesh steps through the integer domain of a one or twodimensional 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 onedimensional 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 );
glEnd();
where
and , , and 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 , then the value computed from is exactly .
In the twodimensional case, glEvalMesh2, let
,
where , , , , , and 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 , j );
glEvalCoord2( i , (j+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 , j );
glEnd();
}
for ( i = i1; i <= i2; i += 1 ) {
glBegin( GL_LINE_STRIP );
for ( j = j1; j <= j1; j += 1 )
glEvalCoord2( i , j );
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 , j ); glEnd();
In all three cases, the only absolute numeric requirements are that if , then the value computed from is exactly , and if , then the value computed from is exactly .
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(3G), glEvalCoord(3G), glEvalPoint(3G), glMap1(3G), glMap2(3G), glMapGrid(3G)