glMap2 Subroutine

Purpose

Defines a 2-dimensional (2D) evaluator.

Library

OpenGL C bindings library: libGL.a

C Syntax

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

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 the glVertex, glNormal, glTexCoord, and glColor subroutines, 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, such as B-spline surfaces, non-uniform rational B-spline surfaces (NURBS), and Bezier surfaces.

Evaluators define surfaces based on bivariate Bernstein polynomials. Define p(s, t) as follows:

Let p(s, t) = Bn0Bm0R00 + Bn1Bm0R01 + . . . + BnnBm0Rn0
   +Bn0Bm1R01 + . . . + BnnBm1Rn1
    .
    .
    .
   +Bn0BmmR0m + . . . + BnnBmmRnm

where Rij is a control point, Bni (s) is the ith Bernstein polynomial of degree:

n (uOrder = n +1)

See the following figure:

and Bmj (t) is the jth Bernstein polynomial of degree:

m (vOrder = m + 1)

See the following figure:

See the following figure:

The glMap2 subroutine 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 the glEnable and glDisable subroutines with the map name, which is one of the nine predefined values for the Target parameter. When the glEvalCoord2 subroutine presents values u and v, the bivariate Bernstein polynomials are evaluated using s and t, as in the following figure:

The Target parameter specifies a symbolic constant that indicates what kind of control points are provided in the Points parameter, and what output is generated when the map is evaluated. It can assume one of the following nine predefined values:

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 uOrder times vOrder 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 Rij 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 Rij to control point Ri(j+1).

Notes

For all GL subroutines 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.

Errors

Associated Gets

Associated gets for the glMap2 subroutine are as follows. (See the glGet subroutine for more information.)

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.

Files

Related Information

The glBegin or glEnd subroutine, glColor subroutine, glEnable or Disable subroutine, glEvalCoord subroutine, glEvalMesh subroutine, glEvalPoint subroutine, glGetMap subroutine, glIndex subroutine, glMap1 subroutine, glMapGrid subroutine, glNormal subroutine, glTexCoord subroutine, glVertex subroutine.

OpenGL Overview.