##
table of contents

GLMAP2(3G) | GLMAP2(3G) |

# NAME¶

**glMap2d, glMap2f** - define a two-dimensional evaluator

# C SPECIFICATION¶

void **glMap2d**( GLenum *target*,

GLdoubleu1, GLdoubleu2, GLintustride, GLintuorder, GLdoublev1, GLdoublev2, GLintvstride, GLintvorder, const GLdouble*points)

void **glMap2f**( GLenum *target*,

GLfloatu1, GLfloatu2, GLintustride, GLintuorder, GLfloatv1, GLfloatv2, GLintvstride, GLintvorder, const GLfloat*points)

# PARAMETERS¶

*target*- Specifies the kind of values that are generated by the evaluator. 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 $\hat{u}$ , 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 beginning of control
point
${R}_{\mathrm{ij}}$
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 $\hat{v}$ , 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 beginning of control
point
${R}_{\mathrm{ij}}$
and the beginning of control point
${R}_{i(j+1)}$
, 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
mapping 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 $p(\hat{u},\hat{v})$ as

$p(\hat{u},\hat{v})=\sum _{i=0}^{n}\sum _{j=0}^{m}{B}_{i}^{n}\left(\hat{u}\right){B}_{j}^{m}\left(\hat{v}\right){R}_{\mathrm{ij}}$

where
${R}_{\mathrm{ij}}$
is a control point,
${B}_{i}^{n}\left(\hat{u}\right)$
is the
$i$
th Bernstein polynomial of degree

$n$
(*uorder* =
$n+1$
)

${B}_{i}^{n}\left(\hat{u}\right)=\left(\begin{array}{c}n\\ i\end{array}\right){\hat{u}}^{i}(1-\hat{u}{)}^{n-i}$

and
${B}_{j}^{m}\left(\hat{v}\right)$
is the
$j$
th Bernstein polynomial of degree
$m$
(*vorder* =
$m+1$
)

${B}_{j}^{m}\left(\hat{v}\right)=\left(\begin{array}{c}m\\ j\end{array}\right){\hat{v}}^{j}(1-\hat{v}{)}^{m-j}$

Recall that

${0}^{0}\equiv 1$
and
$\left(\begin{array}{c}n\\ 0\end{array}\right)\equiv 1$

**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
calling **glEnable** and **glDisable** with the map name, one of the
nine predefined values for *target*, described below. When
**glEvalCoord2** presents values
$u$
and
$v$
, the bivariate Bernstein polynomials are evaluated using
$\hat{u}$
and
$\hat{v}$
, where

$\hat{u}=\frac{u-\mathrm{u1}}{\mathrm{u2}-\mathrm{u1}}$

$\hat{v}=\frac{v-\mathrm{v1}}{\mathrm{v2}-\mathrm{v1}}$

*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
$z$
. 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**commands. **GL_MAP2_COLOR_4**- Each control point is four floating-point values 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
$z$
components of a normal vector. Internal
**glNormal**commands are generated when the map is evaluated 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 values representing the
$s$
and
$t$
texture coordinates. 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
$q$
texture 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 addressing 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
$\mathrm{uorder}\times \mathrm{vorder}$
control points in the array. *ustride* specifies how many float or
double locations are skipped to advance the internal memory pointer from
control point
${R}_{ij}$
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
${R}_{ij}$
to control point
${R}_{i(j+1)}$
.

# 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**.

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)**