glMap1 Subroutine

Purpose

Defines a 1-dimensional (1D) evaluator.

Library

OpenGL C bindings library: libGL.a

C Syntax

void glMap1d(GLenum Target, 
   GLdouble u1, 
   GLdouble u2, 
   GLint Stride, 
   GLint Order, 
   const GLdouble *Points)

void glMap1f(GLenum Target, 
   GLfloat u1, 
   GLfloat u2, 
   GLint Stride, 
   GLint Order, 
   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 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 splines used in computer graphics, such as B-splines, Bezier curves, and Hermite splines.

Evaluators define curves based on Bernstein polynomials. Define p(t) as the following:

Let p(t) = Bn0(t)R0 + Bn1(t)R1 + . . . + Bnn(t)Rn

where Ri is a control point and Bni(t) is the ith Bernstein polynomial of degree:

n (Order = n+1)

See the figure:

See the figure:

The glMap1 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, one of the nine predefined values for the Target parameter. The glEvalCoord1 subroutine evaluates the 1D maps that are enabled. When glEvalCoord1 presents a value u, the Bernstein functions are evaluated using 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 Stride, Order, and Points parameters 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. Order is the number of control points in the array. Stride tells how many float or double locations to advance the internal memory pointer to reach the next control point.

Notes

As is the case with all GL subroutines that accept pointers to data, it is as if the contents of Points were copied by glMap1 before it returned. Changes to the contents of Points have no effect after glMap1 is called.

Errors

Associated Gets

Associated gets for the glMap1 subroutine are as follows. (See the glGet subroutine for more information.)

glGetMap

glGet with argument GL_MAX_EVAL_ORDER

glIsEnabled with argument GL_MAP1_VERTEX_3

glIsEnabled with argument GL_MAP1_VERTEX_4

glIsEnabled with argument GL_MAP1_INDEX

glIsEnabled with argument GL_MAP1_COLOR_4

glIsEnabled with argument GL_MAP1_NORMAL

glIsEnabled with argument GL_MAP1_TEXTURE_COORD_1

glIsEnabled with argument GL_MAP1_TEXTURE_COORD_2

glIsEnabled with argument GL_MAP1_TEXTURE_COORD_3

glIsEnabled with argument GL_MAP1_TEXTURE_COORD_4.

Files

Related Information

The glBegin or glEnd subroutine, glColor subroutine, glEnable or glDisable subroutine, glEvalCoord subroutine, glEvalMesh subroutine, glEvalPoint subroutine, glGetMap subroutine, glIndex subroutine, glMap2 subroutine, glMapGrid subroutine, glNormal subroutine, glTexCoord subroutine, glVertex subroutine.

OpenGL Overview.