glTexFilterFuncSGIS(3G)	       OpenGL Reference	       glTexFilterFuncSGIS(3G)



NAME
     glTexFilterFuncSGIS - specify user-defined	texture	filtering function


C SPECIFICATION
     void glTexFilterFuncSGIS( GLenum target,
			       GLenum filter,
			       GLsizei n,
			       const GLfloat *weights )


PARAMETERS
     target   Specifies	the texture for	which filter weights will be defined.
	      The allowable values are GL_TEXTURE_1D and GL_TEXTURE_2D.

     filter   Specifies	the filter whose weights will be defined.  Must	be
	      GL_FILTER4_SGIS.

     n	      The number of filter weight values in weights.

     weights  Specifies	an array of floating-point values which	define the
	      filter weights.

DESCRIPTION
     Texture filtering is the process by which one or more adjacent texels are
     fetched from a texture image and then combined to form a final texture
     color.  The algorithm for combining texels	multiplies the texels by a set
     of	weights, and then sums the results.  The function which	determines the
     weights is	known as the texture filtering function.

     For most texture filtering	methods, the texture filtering function	is
     defined implicitly.  For example, when using GL_NEAREST filtering,	only
     one texel is fetched from the texture image, and the filtering function
     is	simply the constant 1.	When using 1D GL_LINEAR	filtering, two texels
     are fetched from the texture image, and the weights applied to the	texels
     are inversely proportional	to the distance	from their centers to the
     texture sample point.  (If	the sample point falls precisely on a texel
     center, then that texel's weight will be 1	and the	weight applied to the
     other texel will be 0.  If	the sample point falls one-third of the	way
     from the first texel center to the	second,	the first texel	will be
     weighted by 2/3 and the second texel will be weighted by 1/3, and so on.)

     For GL_FILTER4_SGIS filtering, however, glTexFilterFuncSGIS allows	the
     filtering function	f to be	specified explicitly.  The filtering function
     is	expressed as a table that maps distances from the texture sample point
     into weights.

     The default filtering function is implementation-dependent.  The
     filtering function	is specified in	table format by	calling
     glTexFilterFuncSGIS with weights pointing to an array of n	floating point
     values.  The value	n must equal 2**m+1 for	some nonnegative integer value
     of	m.



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glTexFilterFuncSGIS(3G)	       OpenGL Reference	       glTexFilterFuncSGIS(3G)



     The array weights contains	samples	of the filtering function f(x),	0<x<2.
     Each element weights[i] is	the value of f((2*i)/(n-1)), 0<iNOTES
     glTexFilterFuncSGIS is part of the	SGIS_texture_filter4 extension,	not
     part of the core GL command set.  If GL_SGIS_texture_filter4 is included
     in	the string returned by glGetString when	called with argument
     GL_EXTENSIONS, extension SGIS_texture_filter4 is supported	by the
     connection.  See glIntro for more information about using extensions.

ERRORS
     GL_INVALID_ENUM is	generated if target or filter do not have one of the
     allowable values.

     GL_INVALID_VALUE is generated if n	does not equal 2**m+1 for some
     nonnegative integer value of m.

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





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glTexFilterFuncSGIS(3G)	       OpenGL Reference	       glTexFilterFuncSGIS(3G)



ASSOCIATED GETS
     glGetTexFilterFuncSGIS
     glGetTexParameter with argument GL_TEXTURE_FILTER4_SIZE_SGIS


MACHINE	DEPENDENCIES
     The SGIS_texture_filter4 extension	is supported only on InfiniteReality
     systems.

     On	InfiniteReality	systems, GL_INVALID_VALUE is generated if the value of
     the argument n to the command glTexFilterFuncSGIS does not	equal the
     value of the implementation dependent constant Size, which	can be
     determined	by calling glGetTexParameter with argument
     GL_TEXTURE_FILTER4_SIZE_SGIS.  This restriction will be removed in	a
     future release.


SEE ALSO
     glTexParameter




































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