The glMap2d and glMap2f functions define a two-dimensional evaluator.
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
);
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 OpenGL 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 OpenGL 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(û,) as
where R (ij) is a control point, (û) is the ith Bernstein polynomial of degree
n (uorder = n + 1)
and () is the jth Bernstein polynomial of degree m (vorder = m + 1)
Recall that
The glMap2 function 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 above. When glEvalCoord2 presents values u and v, the bivariate Bernstein polynomials are evaluated using û and , where
The target parameter 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.
The ustride, uorder, vstride, vorder, and points parameters define the array addressing for accessing the control points. The points parameter 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 uorderxvorder control points in the array. The ustride parameter tells 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) . The vstride parameter tells 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)) .
As is the case with all OpenGL commands that accept pointers to data, it is as if the contents of points were copied by glMap2 before it returned. Changes to the contents of points have no effect after glMap2 is called.
The following functions retrieve information related to glMap2:
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
The following are the error codes generated and their conditions.
Error Code | Condition |
---|---|
GL_INVALID_ENUM | target was not an accepted value. |
GL_INVALID_VALUE | u1 was equal to u2, or if v1 was equal to v2. |
GL_INVALID_VALUE | either ustride or vstride was less than the number of values in a control point. |
GL_INVALID_VALUE | either uorder or vorder was less than one or greater than GL_MAX_EVAL_ORDER. |
GL_INVALID_OPERATION | glMap2 was called between a call to glBegin and the corresponding call to glEnd. |
Windows NT: Use version 3.5 and later.
Windows: Use Windows 95 and later.
Windows CE: Unsupported.
Header: Declared in gl.h.
Import Library: Link with opengl32.lib.
glBegin, glColor, glEnable, glEnd, glEvalCoord, glEvalMesh, glEvalPoint, glMap1, glMapGrid, glNormal, glTexCoord, glVertex