glMap2.3gl
Digital Equipment Corporation OpenGL man pages

glMap2.3gl




Name

  glMap2d, glMap2f - define a two-dimensional evaluator


C Specification

  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


  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 is evaluated by the equations
	   specified by	this command.

  ustride  Specifies the number	of floats or doubles between the beginning of
	   control point R   and the beginning of control point	R        ,
                          ij                                     (i+1) j
	   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.

  uorder   Specifies the dimension of the control point	array in the u axis.
	   Must	be positive.

  v1, v2   Specify a linear mapping of v, as presented to glEvalCoord2,	to v,
	   one of the two variables that is evaluated by the equations
	   specified by	this command.

  vstride  Specifies the number	of floats or doubles between the beginning of
	   control point R   and the beginning of control point	R        ,
			  ij                                     (i+1) j
	   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.

  vorder   Specifies the dimension of the control point	array in the v axis.
	   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 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


                    n   m
		   --- ---
	   	   \   \   n    m
        p(u,v)  =   \   \ B (u)B (v) R
		    /   /  i    j     ij
		   /   /
		   --- ---
		   i=0 j=0

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

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

       m
  and B (v') is the jth Bernstein polynomial of degree m (vorder = m+1)
       j
			    m	     |m| j     m-j
			   B (v)  =  | |v (1-v)
			    j	     |j|

  Recall that  

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

  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

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

					v - v1
				 v  =  -------
				        v2 - v1


  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.  The current index is not	updated	with
			   the value of	these glIndex commands,	however.

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

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

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

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

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

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

  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
  uorderxvorder	control	points in the array.  ustride tells how	many float or
  double locations are skipped to advance the internal memory pointer from
  control point	R   to control point R	    .  vstride tells how many float
                 ij                   (i+1)j
  or double locations are skipped to advance the internal memory pointer from
  control point	R   to control point R	    .
		 ij		      i(j+1)


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 it returned.
  Changes to the contents of points have no effect after glMap2	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, 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	one
  or greater than GL_MAX_EVAL_ORDER.

  GL_INVALID_OPERATION is generated if glMap2 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_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

See Also

  glBegin, glColor, glEnable, glEvalCoord, glEvalMesh, glEvalPoint, glMap1,
  glMapGrid, glNormal, glTexCoord, glVertex



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Last Edited: Fri Dec 6 11:18:03 EST 1996 by AFV
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