glTexGen.3gl

Name

  glTexGend, glTexGenf,	glTexGeni, glTexGendv, glTexGenfv, glTexGeniv -
  control the generation of texture coordinates

C Specification

  void glTexGend( GLenum coord,
		  GLenum pname,
		  GLdouble param )
  void glTexGenf( GLenum coord,
		  GLenum pname,
		  GLfloat param	)
  void glTexGeni( GLenum coord,
		  GLenum pname,
		  GLint	param )

Parameters

  coord	  Specifies a texture coordinate.  Must	be one of the following:
	  GL_S,	GL_T, GL_R, or GL_Q.

  pname	  Specifies the	symbolic name of the texture-coordinate	generation
	  function.  Must be GL_TEXTURE_GEN_MODE.

  param	  Specifies a single-valued texture generation parameter, one of
	  GL_OBJECT_LINEAR, GL_EYE_LINEAR, or GL_SPHERE_MAP.

C Specification

  void glTexGendv( GLenum coord,
		   GLenum pname,
		   const GLdouble *params )
  void glTexGenfv( GLenum coord,
		   GLenum pname,
		   const GLfloat *params )
  void glTexGeniv( GLenum coord,
		   GLenum pname,
		   const GLint *params )

Parameters

  coord
       Specifies a texture coordinate.	Must be	one of the following: GL_S,
       GL_T, GL_R, or GL_Q.

  pname
       Specifies the symbolic name of the texture-coordinate generation
       function	or function parameters.	 Must be GL_TEXTURE_GEN_MODE,
       GL_OBJECT_PLANE,	or GL_EYE_PLANE.

  params
       Specifies a pointer to an array of texture generation parameters.  If
       pname is	GL_TEXTURE_GEN_MODE, then the array must contain a single
       symbolic	constant, one of GL_OBJECT_LINEAR, GL_EYE_LINEAR, or
       GL_SPHERE_MAP. Otherwise, params	holds the coefficients for the
       texture-coordinate generation function specified	by pname.

Description

  glTexGen selects a texture-coordinate	generation function or supplies
  coefficients for one of the functions.  coord	names one of the (s,t,r,q)
  texture coordinates, and it must be one of these symbols: GL_S, GL_T,	GL_R,
  or GL_Q.  pname must be one of three symbolic	constants:
  GL_TEXTURE_GEN_MODE, GL_OBJECT_PLANE,	or GL_EYE_PLANE. If pname is
  GL_TEXTURE_GEN_MODE, then params chooses a mode, one of GL_OBJECT_LINEAR,
  GL_EYE_LINEAR, or GL_SPHERE_MAP. If pname is either GL_OBJECT_PLANE or
  GL_EYE_PLANE,	params contains	coefficients for the corresponding texture
  generation function.

  If the texture generation function is	GL_OBJECT_LINEAR, the function

			     g=p x +p y	+p z +p	w
				1 o  2 o  3 o  4 o

  is used, where g is the value	computed for the coordinate named in coord,
  p , p	, p , and p  are the four values supplied in params, and x , y , z ,
   1   2   3       4                                              o   o   o
  and w  are the object	coordinates of the vertex.  This function can be used
       o
  to texture-map terrain using sea level as a reference	plane (defined by p ,
                                                                           1
  p , p	, and p	). The altitude	of a terrain vertex is computed	by the
   2   3       4
  GL_OBJECT_LINEAR coordinate generation function as its distance from sea
  level; that altitude is used to index	the texture image to map white snow
  onto peaks and green grass onto foothills, for example.

  If the texture generation function is	GL_EYE_LINEAR, the function

			   g=p'x' + p'y	 + p'z  + p'w
			      1 e    2 e    3 e	   4 e

  is used, where
                                                        -1
		   (p'	p'  p'  p') = (p   p   p   p )·M
		     1	 2   3   4	1   2	3   4

  and x	, y , z	, and w	 are the eye coordinates of the	vertex,	p , p ,	p ,
       e   e   e       e                                         1   2   3
  and p  are the values supplied in params, and M is the modelview matrix
       4
  when glTexGen	is invoked.  If	M is poorly conditioned	or singular, texture
  coordinates generated	by the resulting function may be inaccurate or
  undefined.

  Note that the	values in params define	a reference plane in eye coordinates.
  The modelview	matrix that is applied to them may not be the same one in
  effect when the polygon vertices are transformed. This function establishes
  a field of texture coordinates that can produce dynamic contour lines	on
  moving objects.

  If pname is GL_SPHERE_MAP and coord is either GL_S or GL_T, s and t
  texture coordinates are generated as follows.	Let u be the unit vector
  pointing from	the origin to the polygon vertex (in eye coordinates). Let n'
  be the current normal, after transformation to eye coordinates. Let
                  T
  f = (f   f   f )  be the reflection vector such that
	x   y	z

                                             T
				f = u - 2n'n' u

		      _____________
		     | 2  2       2
  Finally, let m = 2\|f +f +(f +1) . Then the values assigned to the s and
                       x  y   z
  t texture coordinates	are

				       f
				        x   1
				  s = --- + -
				       m    2

				       f
				        y   1
				  t = --- + -
				       m    2

  A texture-coordinate generation function is enabled or disabled using
  glEnable or glDisable	with one of the	symbolic texture-coordinate names
  (GL_TEXTURE_GEN_S, GL_TEXTURE_GEN_T, GL_TEXTURE_GEN_R, or GL_TEXTURE_GEN_Q)
  as the argument. When	enabled, the specified texture coordinate is computed
  according to the generating function associated with that coordinate.	When
  disabled, subsequent vertices	take the specified texture coordinate from
  the current set of texture coordinates. Initially, all texture generation
  functions are	set to GL_EYE_LINEAR and are disabled.	Both s plane
  equations are	(1,0,0,0), both	t plane	equations are (0,1,0,0), and all r
  and q	plane equations	are (0,0,0,0).

Errors

  GL_INVALID_ENUM is generated when coord or pname is not an accepted defined
  value, or when pname is GL_TEXTURE_GEN_MODE and params is not	an accepted
  defined value.

  GL_INVALID_ENUM is generated when pname is GL_TEXTURE_GEN_MODE, params is
  GL_SPHERE_MAP, and coord is either GL_R or GL_Q.

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

Associated Gets

  glGetTexGen
  glIsEnabled with argument GL_TEXTURE_GEN_S
  glIsEnabled with argument GL_TEXTURE_GEN_T
  glIsEnabled with argument GL_TEXTURE_GEN_R
  glIsEnabled with argument GL_TEXTURE_GEN_Q

See Also

  glTexEnv, glTexImage1D, glTexImage2D,	glTexParameter

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