GLMAP2()GLMAP2()NAME
glMap2d, glMap2f - define a two-dimensional evaluator
C SPECIFICATION
void glMap2d( GLenum target,
GLdouble u1,
GLdouble u2,
GLint ustride,
GLint uorder,
GLdouble v1,
GLdouble v2,
GLint vstride,
GLint vorder,
const GLdouble *points )
void glMap2f( GLenum target,
GLfloat u1,
GLfloat u2,
GLint ustride,
GLint uorder,
GLfloat v1,
GLfloat v2,
GLint vstride,
GLint vorder,
const GLfloat *points )
delim $$
PARAMETERS
target Specifies the kind of values that are generated by the evalua‐
tor. Symbolic constants GL_MAP2_VERTEX_3, GL_MAP2_VERTEX_4,
GL_MAP2_INDEX, GL_MAP2_COLOR_4, GL_MAP2_NORMAL, GL_MAP2_TEX‐
TURE_COORD_1, GL_MAP2_TEXTURE_COORD_2, GL_MAP2_TEX‐
TURE_COORD_3, and GL_MAP2_TEXTURE_COORD_4 are accepted.
u1, u2 Specify a linear mapping of $u$, as presented to glEvalCoord2,
to $u hat$, one of the two variables that are evaluated by the
equations specified by this command. Initially, u1 is 0 and u2
is 1.
ustride Specifies the number of floats or doubles between the begin‐
ning of control point $R sub ij$ and the beginning of control
point $R sub { (i+1) j }$, where $i$ and $j$ are the $u$ and
$v$ control point indices, respectively. This allows control
points to be embedded in arbitrary data structures. The only
constraint is that the values for a particular control point
must occupy contiguous memory locations. The initial value of
ustride is 0.
uorder Specifies the dimension of the control point array in the $u$
axis. Must be positive. The initial value is 1.
v1, v2 Specify a linear mapping of $v$, as presented to glEvalCoord2,
to $v hat$, one of the two variables that are evaluated by the
equations specified by this command. Initially, v1 is 0 and v2
is 1.
vstride Specifies the number of floats or doubles between the begin‐
ning of control point $R sub ij$ and the beginning of control
point $R sub { i (j+1) }$, where $i$ and $j$ are the $u$ and
$v$ control point indices, respectively. This allows control
points to be embedded in arbitrary data structures. The only
constraint is that the values for a particular control point
must occupy contiguous memory locations. The initial value of
vstride is 0.
vorder Specifies the dimension of the control point array in the $v$
axis. Must be positive. The initial value is 1.
points Specifies a pointer to the array of control points.
DESCRIPTION
Evaluators provide a way to use polynomial or rational polynomial map‐
ping to produce vertices, normals, texture coordinates, and colors.
The values produced by an evaluator are sent on to further stages of GL
processing just as if they had been presented using glVertex, glNormal,
glTexCoord, and glColor commands, except that the generated values do
not update the current normal, texture coordinates, or color.
All polynomial or rational polynomial splines of any degree (up to the
maximum degree supported by the GL implementation) can be described
using evaluators. These include almost all surfaces used in computer
graphics, including B-spline surfaces, NURBS surfaces, Bezier surfaces,
and so on.
Evaluators define surfaces based on bivariate Bernstein polynomials.
Define $p ( u hat , v hat ) $ as
p ( u hat , v hat ^) ~~=~~ up 10 {sum from i=0 to n sum from j=0 to m}
B sub i sup n ( u hat ^) B sub j sup m ( v hat ^) R sub ij
where $R sub ij$ is a control point, $B sub i sup n ( u hat ^)$ is the
$i$th Bernstein polynomial of degree
$n$ (uorder = $n + 1$)
B sub i sup n ( u hat ^) ~~=~~ left ( down 20 {cpile { n above i }} ~~
right ) u hat sup i ( 1 - u hat ^) sup { n - i }
and $B sub j sup m ( v hat ^)$ is the $j$th Bernstein polynomial of
degree $m$ (vorder = $m + 1$)
B sub j sup m ( v hat ^) ~~=~~ left ( down 20 {cpile { m above j }} ~~
right ) v hat sup j ( 1 - v hat ^) sup { m - j }
Recall that
$0 sup 0 ~==~ 1 $ and $ left ( down 20 {cpile { n above ~0 }} ~~ right
) ~~==~~ 1 $
glMap2 is used to define the basis and to specify what kind of values
are produced. Once defined, a map can be enabled and disabled by call‐
ing glEnable and glDisable with the map name, one of the nine prede‐
fined values for target, described below. When glEvalCoord2 presents
values $u$ and $v$, the bivariate Bernstein polynomials are evaluated
using $u hat$ and $v hat$, where
$u hat ~~=~~ {u ~-~ "u1"} over {"u2" ~-~ "u1"}$
$v hat ~~=~~ {v ~-~ "v1"} over {"v2" ~-~ "v1"}$
target is a symbolic constant that indicates what kind of control
points are provided in points, and what output is generated when the
map is evaluated. It can assume one of nine predefined values:
GL_MAP2_VERTEX_3 Each control point is three floating-point
values representing $x$, $y$, and $z$. Inter‐
nal glVertex3 commands are generated when the
map is evaluated.
GL_MAP2_VERTEX_4 Each control point is four floating-point val‐
ues representing $x$, $y$, $z$, and $w$.
Internal glVertex4 commands are generated when
the map is evaluated.
GL_MAP2_INDEX Each control point is a single floating-point
value representing a color index. Internal
glIndex commands are generated when the map is
evaluated but the current index is not updated
with the value of these glIndex commands.
GL_MAP2_COLOR_4 Each control point is four floating-point val‐
ues representing red, green, blue, and alpha.
Internal glColor4 commands are generated when
the map is evaluated but the current color is
not updated with the value of these glColor4
commands.
GL_MAP2_NORMAL Each control point is three floating-point
values representing the $x$, $y$, and $z$ com‐
ponents of a normal vector. Internal glNormal
commands are generated when the map is evalu‐
ated but the current normal is not updated
with the value of these glNormal commands.
GL_MAP2_TEXTURE_COORD_1 Each control point is a single floating-point
value representing the $s$ texture coordinate.
Internal
glTexCoord1 commands are generated when the
map is evaluated but the current texture coor‐
dinates are not updated with the value of
these glTexCoord commands.
GL_MAP2_TEXTURE_COORD_2 Each control point is two floating-point val‐
ues representing the $s$ and $t$ texture coor‐
dinates. Internal
glTexCoord2 commands are generated when the
map is evaluated but the current texture coor‐
dinates are not updated with the value of
these glTexCoord commands.
GL_MAP2_TEXTURE_COORD_3 Each control point is three floating-point
values representing the $s$, $t$, and $r$ tex‐
ture coordinates. Internal glTexCoord3 com‐
mands are generated when the map is evaluated
but the current texture coordinates are not
updated with the value of these glTexCoord
commands.
GL_MAP2_TEXTURE_COORD_4 Each control point is four floating-point val‐
ues representing the $s$, $t$, $r$, and $q$
texture coordinates. Internal
glTexCoord4 commands are generated when the
map is evaluated but the current texture coor‐
dinates are not updated with the value of
these glTexCoord commands.
ustride, uorder, vstride, vorder, and points define the array address‐
ing for accessing the control points. points is the location of the
first control point, which occupies one, two, three, or four contiguous
memory locations, depending on which map is being defined. There are $
"uorder" times "vorder" $ control points in the array. ustride speci‐
fies how many float or double locations are skipped to advance the
internal memory pointer from control point $R sub {i j} $ to control
point $R sub {(i+1) j} $. vstride specifies how many float or double
locations are skipped to advance the internal memory pointer from con‐
trol point $R sub {i j} $ to control point $R sub {i (j+1) } $.
NOTES
As is the case with all GL commands that accept pointers to data, it is
as if the contents of points were copied by glMap2 before glMap2
returns. Changes to the contents of points have no effect after glMap2
is called.
Initially, GL_AUTO_NORMAL is enabled. If GL_AUTO_NORMAL is enabled,
normal vectors are generated when either GL_MAP2_VERTEX_3 or
GL_MAP2_VERTEX_4 is used to generate vertices.
ERRORS
GL_INVALID_ENUM is generated if target is not an accepted value.
GL_INVALID_VALUE is generated if u1 is equal to u2, or if v1 is equal
to v2.
GL_INVALID_VALUE is generated if either ustride or vstride is less than
the number of values in a control point.
GL_INVALID_VALUE is generated if either uorder or vorder is less than 1
or greater than the return value of GL_MAX_EVAL_ORDER.
GL_INVALID_OPERATION is generated if glMap2 is executed between the
execution of glBegin and the corresponding execution of glEnd.
ASSOCIATED GETS
glGetMap
glGet with argument GL_MAX_EVAL_ORDER
glIsEnabled with argument GL_MAP2_VERTEX_3
glIsEnabled with argument GL_MAP2_VERTEX_4
glIsEnabled with argument GL_MAP2_INDEX
glIsEnabled with argument GL_MAP2_COLOR_4
glIsEnabled with argument GL_MAP2_NORMAL
glIsEnabled with argument GL_MAP2_TEXTURE_COORD_1
glIsEnabled with argument GL_MAP2_TEXTURE_COORD_2
glIsEnabled with argument GL_MAP2_TEXTURE_COORD_3
glIsEnabled with argument GL_MAP2_TEXTURE_COORD_4
SEE ALSO
glBegin, glColor, glEnable, glEvalCoord, glEvalMesh, glEvalPoint,
glMap1, glMapGrid, glNormal, glTexCoord, glVertex
GLMAP2()
