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
     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.


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