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.
     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<i<n-1.

     The filtering function is stored and used by GL as a set of samples
     f((2*i)/(Size-1)), 0<i<Size-1, where Size is an implementation-dependent
     constant.  If n equals Size, the array weights is stored directly in GL
     state.  Otherwise, an implementation dependent resampling method is used
     to compute the stored samples.

     To determine the value of Size, use glGetTexParameter with argument
     GL_TEXTURE_FILTER4_SIZE_SGIS.  For best results, call glTexFilterFuncSGIS
     with n equal to Size.

     In the 1D filtering case, the four texels closest to the texture sample
     point will be fetched, and the distances from their centers to the sample
     point will be computed.  Each texel i will be weighted by f(distance(i))
     and the results will be summed to form the final texel value.  Note that
     the distance from the texel center to the sample point is not signed, so
     identical weights will be assigned to texels at identical distances left
     and right of the sample point.  Thus, the filter is symmetric.

     The 2D filtering case is best considered as four separate applications of
     the 1D filter to the four texels in each of the four rows closest to the
     sample point, followed by a final application of the 1D filter to the
     four resulting texel values.  That is, the filter is separable.  The
     final texel value is a combination of 16 adjacent texel values in the
     texture image.

     As always, if any of the texels used in this process refer to a border
     texel with unspecified value, then the border color given by the current
     setting of GL_TEXTURE_BORDER_COLOR is used instead of the unspecified
     value.

NOTES
     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.

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