glMap2

Name

glMap2 — define a two-dimensional evaluator

C Specification

 void glMap2f( GLenum target, GLfloat u1, GLfloat u2, GLint ustride, GLint uorder, GLfloat v1, GLfloat v2, GLint vstride, GLint vorder, const GLfloat * points);
 void glMap2d( GLenum target, GLdouble u1, GLdouble u2, GLint ustride, GLint uorder, GLdouble v1, GLdouble v2, GLint vstride, GLint vorder, const GLdouble * 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 $u^$, 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 $R ij$ and the beginning of control point , 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 beginning of control point $R ij$ and the beginning of control point , 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 $p ⁡ u^ v^$ as

where $R ij$ is a control point, is the $i$th Bernstein polynomial of degree $n$ (uorder = $n + 1$)

and is the $j$th Bernstein polynomial of degree $m$ (vorder = $m + 1$)

Recall that $0 0 == 1$ and

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 $u^$ and $v^$, where

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 $uorder × vorder$ control points in the array. ustride specifies how many float or double locations are skipped to advance the internal memory pointer from control point to control point . vstride specifies how many float or double locations are skipped to advance the internal memory pointer from control point to control point .

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.

GL_INVALID_OPERATION is generated if glMap2 is called and the value of GL_ACTIVE_TEXTURE is not GL_TEXTURE0.

Associated Gets

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