glDetailTexFuncSGIS(3G)	       OpenGL Reference	       glDetailTexFuncSGIS(3G)



NAME
     glDetailTexFuncSGIS - specify detail texture scaling function


C SPECIFICATION
     void glDetailTexFuncSGIS( GLenum target,
			       GLsizei n,
			       const GLfloat *points )


PARAMETERS
     target  The target	to which the scaling function will be applied. Must be
	     GL_TEXTURE_2D.

     n	     The number	of scaling function samples in points.

     points  An	array of scaling function samples, each	of which is an (LOD,
	     function-value) pair.

DESCRIPTION
     The detail	texture	extension defines three	additional texture
     magnification filters.  These filters are selected	by choosing one	of the
     values GL_LINEAR_DETAIL_SGIS, GL_LINEAR_DETAIL_ALPHA_SGIS,	or
     GL_LINEAR_DETAIL_COLOR_SGIS for the current 2D texture's
     GL_TEXTURE_MAG_FILTER.

     All three filters sample the level	zero texture array exactly as it would
     be	sampled	with filter mode GL_LINEAR.  All three also sample the level
     zero texture array	of a second texture, known as the detail texture, when
     two conditions are	met.  The detail texture corresponding to texture
     GL_TEXTURE_2D is GL_DETAIL_TEXTURE_2D_SGIS.  The conditions are:

	  1.  The active texture must be GL_TEXTURE_2D.

	  2.  The internal formats of GL_TEXTURE_2D and
	      GL_DETAIL_TEXTURE_2D_SGIS	must have been specified identically.

	  3   The level	0 image	of GL_DETAIL_TEXTURE_2D_SGIS must have nonzero
	      width and	height.

	  4   If the internal formats of GL_TEXTURE_2D and
	      GL_DETAIL_TEXTURE_2D_SGIS	are one	of the dual or quad texture
	      storage formats as defined by SGIS_texture_select, then the
	      corresponding texture group selection must be specified
	      identically.

     If	these conditions are not met, it is as though the magnification
     texture filter was	GL_LINEAR.  (Although querying the magnification
     filter value will return the value	as specified.)	If they	are met, the
     level zero	array of the detail texture is also linearly sampled, using
     the following arithmetic:




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glDetailTexFuncSGIS(3G)	       OpenGL Reference	       glDetailTexFuncSGIS(3G)



	  n = log base 2 of the	width of the level zero	array of GL_TEXTURE_2D
	  m = log base 2 of the	height of the level zero array of GL_TEXTURE_2D
	  N = log base 2 of the	width of GL_DETAIL_TEXTURE_2D_SGIS
	  M = log base 2 of the	height of GL_DETAIL_TEXTURE_2D_SGIS
	  L = GL_DETAIL_TEXTURE_LEVEL_SGIS value of GL_TEXTURE_2D

	  u = s	* 2**(n-L)
	  v = t	* 2**(m-L)

	  i0 = floor(u - 1/2) mod 2**N
	  j0 = floor(v - 1/2) mod 2**M

	  i1 = (i0 + 1)	mod 2**N
	  j1 = (j0 + 1)	mod 2**M

	  A = frac(u - 1/2)
	  B = frac(v - 1/2)

	  Tdetail = (1-A) * (1-B) * detail[i0,j0] +
		       A  * (1-B) * detail[i1,j0] +
		    (1-A) *    B  * detail[i0,j1] +
		       A  *    B  * detail[i1,j1]


     Note that magnification corresponds to negative values of level-of-detail
     and minification corresponds to positive values.  Hence L,	the value of
     the GL_DETAIL_TEXTURE_LEVEL_SGIS parameter	of GL_TEXTURE_2D, must be
     negative. The absolute value of L can be thought of as the	number of
     levels that separate the layer zero image of GL_TEXTURE_2D	and the	image
     of	GL_DETAIL_TEXTURE_2D_SGIS, which is replicated as necessary to fill
     the appropriate number of texels.	For example, if	L is -2, the detail
     texture image is replicated as necessary in x and y to form a single
     image whose dimensions are	four times larger than the level zero array of
     GL_TEXTURE_2D.

     The texture value computed	from the primary texture (Ttexture) and	the
     value computed from the detail texture (Tdetail) are combined in one of
     two ways to compute the final texture value (T).  The values of Ttexture,
     Tdetail, and T are	treated	as though they range from 0.0 through 1.0 in
     these equations.

     If	GL_DETAIL_TEXTURE_MODE_SGIS of GL_TEXTURE_2D is	GL_ADD,	then

	  T' = Ttexture	+ F(LOD) * (2*Tdetail-1)

	  T = 0	 if T' < 0;
	      T' if 0 <= T' <= 1;
	      1	 if T' > 1.







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glDetailTexFuncSGIS(3G)	       OpenGL Reference	       glDetailTexFuncSGIS(3G)



     where F is	a function of the level-of-detail parameter LOD.  In effect,
     the detail	texture	is scaled and biased so	that its range is [-1,1].  The
     resulting signed value is scaled by a function of LOD, added to the base
     texture, and clamped to [0,1].

     If	GL_DETAIL_TEXTURE_MODE_SGIS of GL_TEXTURE_2D is	GL_MODULATE, then

	  T' = Ttexture	* (1 + F(LOD) *	(2*Tdetail-1))

	  T = 0	 if T' < 0;
	      T' if 0 <= T' <= 1;
	      1	 if T' > 1.


     Here again	the detail texture is scaled and biased	so that	its range is
     [-1,1].  The resulting signed value is scaled by a	function of LOD	and
     biased by 1.  This	result scales the base texture,	which is then clamped
     to	[0,1].

     glDetailTexFuncSGIS is used to specify the	scaling	function F.  target
     must be GL_TEXTURE_2D.  n specifies the number of pairs of	values in
     points.  points points to an array	of pairs of floating point values.
     The first value of	each pair specifies a value of LOD, and	the second
     value of each pair	specifies the corresponding function value.  The order
     in	which the points are specified is not significant.  The	n value	pairs
     in	points completely specify the function,	replacing any previous
     specification that	may have existed.

     The function F is evaluated by fitting a curve through the	sample points
     specified in points.  This	curve may be linear between adjacent points,
     or	it may be smoothed, but	it will	pass exactly through the points,
     limited only by the resolution of the implementation.  The	value pair
     with the lowest LOD value specifies the function value F for all values
     of	LOD less than or equal to that pair's LOD.  Likewise, the value	pair
     with the greatest LOD value specifies the function	value F	for all	values
     of	LOD greater than or equal to that pair's LOD.

     Since negative values of LOD correspond to	magnification and positive
     values correspond to minification,	the points should have negative	LOD
     values (although specifying a positive value does not generate an error).
     For example, an LOD of -4 corresponds to a	magnification by a factor of
     2**4, or 16.  The default function	points are (0,0) and (-4,1).

     If	the texture magnification filter is GL_LINEAR_DETAIL_SGIS, then	both
     the color and the alpha components	of T are computed as described in the
     equations above.  If the filter is	GL_LINEAR_DETAIL_COLOR_SGIS, then all
     components	of T other than	alpha are computed as described	above, and the
     alpha component of	T is computed as if the	texture	magnification filter
     were GL_LINEAR.  Finally, if the filter is	GL_LINEAR_DETAIL_ALPHA_SGIS,
     the alpha component of T is computed as described in the equations	above,
     and all other components of T are computed	as if the texture
     magnification filter were GL_LINEAR.



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glDetailTexFuncSGIS(3G)	       OpenGL Reference	       glDetailTexFuncSGIS(3G)



NOTES
     The detail	texture	itself is specified by calling glTexImage2D with
     target set	to GL_DETAIL_TEXTURE_2D_SGIS, level set	to 0, border set to 0,
     and the other parameters specified	to generate the	desired	image.

ERRORS
     GL_INVALID_ENUM is	generated if target is not GL_TEXTURE_2D.

     GL_INVALID_VALUE is generated if n	is negative.

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

ASSOCIATED GETS
     glGetTexParameter,	glGetDetailTexFuncSGIS.


MACHINE	DEPENDENCIES
     On	RealityEngine, RealityEngine2, and VTX systems detail texturing	may
     not be used when rendering	to pixmaps.


SEE ALSO
     glTexImage2D, glTexParameter, glTexSubImage2DEXT.






























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