glMap1.3gl




Name

  glMap1d, glMap1f - define a one-dimensional evaluator


C Specification

  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


  target  Specifies the	kind of	values that are	generated by the evaluator.
	  Symbolic constants GL_MAP1_VERTEX_3, GL_MAP1_VERTEX_4,
	  GL_MAP1_INDEX, GL_MAP1_COLOR_4, GL_MAP1_NORMAL,
	  GL_MAP1_TEXTURE_COORD_1, GL_MAP1_TEXTURE_COORD_2,
	  GL_MAP1_TEXTURE_COORD_3, and GL_MAP1_TEXTURE_COORD_4 are accepted.

  u1, u2  Specify a linear mapping of u, as presented to glEvalCoord1, to u,
	  the variable that is evaluated by the	equations specified by this
	  command.

  stride  Specifies the	number of floats or doubles between the	beginning of
	  one control point and	the beginning of the next one in the data
	  structure referenced in points.  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.

  order	  Specifies the	number of control points.  Must	be positive.

  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 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	splines	used in	computer graphics,
  including B-splines, Bezier curves, Hermite splines, and so on.

  Evaluators define curves based on Bernstein polynomials.  Define p(u) as

				 n
				---
				\   n
		       p(u)  =   \ B (u)R
				 /  i    i
				/
				---
				i=0


                                   n
  where	R  is a	control	point and B (u) is the ith Bernstein polynomial of
         i                         i
  degree n (order = n+1):


			    n	     |n|  i      n-i
			   B (u)   = | | u  (1-u)
			    i	     |i|



  Recall that


			     0	       |n|
			    0  = 1 and | |  = 1
				       |0|

  glMap1 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.  glEvalCoord1 evaluates the one-dimensional
  maps that are	enabled.  When glEvalCoord1 presents a value u,	the Bernstein
  functions are	evaluated using	u, where

				       u - u1
				  u =  -------
				       u2 - u1

  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_MAP1_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_MAP1_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_MAP1_INDEX		   Each	control	point is a single floating-point
			   value representing a	color index.  Internal
			   glIndex commands are	generated when the map is
			   evaluated.  The current index is not	updated	with
			   the value of	these glIndex commands,	however.

  GL_MAP1_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.  The current color	is not
			   updated with	the value of these glColor4 commands,
			   however.

  GL_MAP1_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.	 The current
			   normal is not updated with the value	of these
			   glNormal commands, however.

  GL_MAP1_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.  The current texture
			   coordinates are not updated with the	value of
			   these glTexCoord commands, however.

  GL_MAP1_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.  The current texture
			   coordinates are not updated with the	value of
			   these glTexCoord commands, however.

  GL_MAP1_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.  The current texture
			   coordinates are not updated with the	value of
			   these glTexCoord commands, however.

  GL_MAP1_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.	 The current
			   texture coordinates are not updated with the	value
			   of these glTexCoord commands, however.

  stride, order, 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.  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 commands 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

  GL_INVALID_ENUM is generated if target is not	an accepted value.

  GL_INVALID_VALUE is generated	if u1 is equal to u2.

  GL_INVALID_VALUE is generated	if stride is less than the number of values
  in a control point.

  GL_INVALID_VALUE is generated	if order is less than one or greater than
  GL_MAX_EVAL_ORDER.

  GL_INVALID_OPERATION is generated if glMap1 is executed between the
  execution of glBegin and the corresponding execution of glEnd.

Associated Gets

  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



See Also

  glBegin, glColor, glEnable, glEvalCoord, glEvalMesh, glEvalPoint, glMap2,
  glMapGrid, glNormal, glTexCoord, glVertex

Introduction | Alphabetic | Specification

Last Edited: Mon, May 22, 1995

AFV