Name
glEvalMesh1, glEvalMesh2 — compute a one- or two-dimensional grid of points or lines
C Specification
void glEvalMesh1 | ( | GLenum mode, GLint i1, GLint i2); |
Python Specification
glEvalMesh1 | ( | mode, i1, i2) → None |
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*delta_u + u1 );
glEnd();
where
delta_u
=
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
·
Δ
u
+
u
1
is exactly
u
2
.
In the two-dimensional case, glEvalMesh2, let
Δ
u
=
(
u
2
-
u
1
)
/
n
Δ
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
·
Δ
u
+
u
1
, j
·
Δ
v
+
v
1
); glEvalCoord2( i
·
Δ
u
+
u
1
, (j+1)
·
Δ
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( i
·
Δ
u
+
u
1
, j
·
Δ
v
+
v
1
); glEnd(); } for ( i = i1; i <= i2; i += 1 ) {
glBegin( GL_LINE_STRIP ); for ( j = j1; j <= j1; j += 1 )
glEvalCoord2( i
·
Δ
u
+
u
1
, j
·
Δ
v
+
v
1
); 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
·
Δ
u
+
u
1
, j
·
Δ
v
+
v
1
); glEnd();
In all three cases, the only absolute numeric requirements are that if
i
=
n
, then the value computed from
i
·
Δ
u
+
u
1
is exactly
u
2
, and if
j
=
m
, then the value computed from
j
·
Δ
v
+
v
1
is exactly
v
2
.
Errors
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.
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