glMap2f
GLMAP2(3G) GLMAP2(3G)
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 )
PARAMETERS
target Specifies the kind of values that are generated by the evalu-
ator. Symbolic constants GL_MAP2_VERTEX_3, GL_MAP2_VERTEX_4,
GL_MAP2_INDEX, GL_MAP2_COLOR_4, GL_MAP2_NORMAL,
GL_MAP2_TEXTURE_COORD_1, GL_MAP2_TEXTURE_COORD_2,
GL_MAP2_TEXTURE_COORD_3, and GL_MAP2_TEXTURE_COORD_4 are
accepted.
u1, u2 Specify a linear mapping of u, as presented to glEvalCoord2,
to u, 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 Rij and the beginning of control point
R(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, 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 Rij and the beginning of control point
Ri(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 sur-
faces, and so on.
Evaluators define surfaces based on bivariate Bernstein polynomials.
Define p(u,v) as
p(u,v)=iΣ0jΣ0Bi(u)Bj(v)Rij
where Rij is a control point, Bi(u) is the ith Bernstein polynomial of
degree
n (uorder = n+1)
n
Bi(u)=( )ui(1−u)n−i
i
and Bj(v) is the jth Bernstein polynomial of degree m (vorder = m+1)
m
Bj(v)=( )vj(1−v)m−j
j
Recall that n
00≡1 and ( )≡1
0
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
calling glEnable and glDisable with the map name, one of the nine pre-
defined values for target, described below. When glEvalCoord2
presents values u and v, the bivariate Bernstein polynomials are eval-
uated using u and v, where
u=_____
v=_____
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. Internal
glVertex3 commands are generated when the map
is evaluated.
GL_MAP2_VERTEX_4 Each control point is four floating-point
values 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 com-
mands.
GL_MAP2_COLOR_4 Each control point is four floating-point
values representing red, green, blue, and
alpha. Internal glColor4 commands are gener-
ated when the map is evaluated but the cur-
rent 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 compo-
nents 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
coordinates 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 coordi-
nates. Internal
glTexCoord2 commands 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_3 Each control point is three floating-point
values representing the s, t, and r texture
coordinates. Internal glTexCoord3 commands
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
values representing the s, t, r, and q tex-
ture coordinates. Internal
glTexCoord4 commands are generated when the
map is evaluated but the current texture
coordinates 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 contigu-
ous memory locations, depending on which map is being defined. There
are uorder×vorder control points in the array. ustride specifies how
many float or double locations are skipped to advance the internal
memory pointer from control point Rij to control point R(i+1)j.
vstride specifies how many float or double locations are skipped to
advance the internal memory pointer from control point Rij to control
point Ri(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.
When the GL_ARB_multitexture extension is supported,
GL_INVALID_OPERATION is generated if glMap2 is called and the value of
GL_ACTIVE_TEXTURE_ARB is not GL_TEXTURE0_ARB.
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(3G), glColor(3G), glEnable(3G), glEvalCoord(3G),
glEvalMesh(3G), glEvalPoint(3G), glMap1(3G), glMapGrid(3G),
glNormal(3G), glTexCoord(3G), glVertex(3G)
GLMAP2(3G)
