glSharpenTexFuncSGIS(3G)       OpenGL Reference	      glSharpenTexFuncSGIS(3G)



NAME
     glSharpenTexFuncSGIS - specify sharpen texture scaling function


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


PARAMETERS
     target  The target	to which the scaling function will be applied.	Must
	     be	one of GL_TEXTURE_1D, GL_TEXTURE_2D, or	GL_TEXTURE_3D_EXT.

     n	     The number	of scaling function samples in points.

     points  An	array of scaling function samples, each	of which is a (level-
	     of-detail,	function-value)	pair.

DESCRIPTION
     The sharpen texture extension defines three additional texture
     magnification filters.  These filters are selected	by choosing one	of the
     values GL_LINEAR_SHARPEN_SGIS, GL_LINEAR_SHARPEN_ALPHA_SGIS, or
     GL_LINEAR_SHARPEN_COLOR_SGIS for the current 1D, 2D, or 3D	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.  If	both texture levels 0 and 1
     are complete, the level 1 array of	the texture is also linearly sampled,
     just as though mipmap minification	was being performed with LOD (the
     level of detail parameter)	valued near 1.0.  If levels 0 and 1 are	not
     complete, it is as	though the magnification texture filter	was GL_LINEAR.
     (Although querying	the magnification filter value will return the value
     as	specified.)

     The texture value computed	from the level 0 array (T0) and	the value
     computed from the level one array (T1) are	combined to compute the	final
     texture value (T):

	  T' = ((1 + F(LOD)) * T0) - (F(LOD) * T1)

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


     glSharpenTexFuncSGIS is used to specify the scaling function F.  target
     must be GL_TEXTURE_1D, GL_TEXTURE_2D, or GL_TEXTURE_3D_EXT.  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



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glSharpenTexFuncSGIS(3G)       OpenGL Reference	      glSharpenTexFuncSGIS(3G)



     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_SHARPEN_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_SHARPEN_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_SHARPEN_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.

ERRORS
     GL_INVALID_ENUM is	generated if target is not one of the allowable
     values.

     GL_INVALID_VALUE is generated if n	is negative.

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

ASSOCIATED GETS
     glGetTexParameter,	glGetSharpenTexFuncSGIS.


MACHINE	DEPENDENCIES
     On	RealityEngine, RealityEngine2, and VTX systems sharpen texturing may
     not be used with 3D textures or when rendering to pixmaps.

     The default scaling function is incorrect on InfiniteReality systems.
     This will be fixed	in a future release.






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glSharpenTexFuncSGIS(3G)       OpenGL Reference	      glSharpenTexFuncSGIS(3G)



SEE ALSO
     glTexImage1D, glTexImage2D, glTexImage3DEXT, glTexParameter,
     glTexSubImage2DEXT.




















































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