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 GLTEXTURE1D and GLTEXTURE2D.
filter Specifies the filter whose weights will be defined. Must be
GLFILTER4SGIS.
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 GLNEAREST filtering, only
one texel is fetched from the texture image, and the filtering function
is simply the constant 1. When using 1D GLLINEAR 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 GLFILTER4SGIS 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<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
GLTEXTUREFILTER4SIZESGIS. 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 GLTEXTUREBORDERCOLOR is used instead of the unspecified
value.
NOTES
glTexFilterFuncSGIS is part of the SGIStexturefilter4 extension, not
part of the core GL command set. If GLSGIStexturefilter4 is included
in the string returned by glGetString when called with argument
GLEXTENSIONS, extension SGIStexturefilter4 is supported by the
connection. See glIntro for more information about using extensions.
ERRORS
GLINVALIDENUM is generated if target or filter do not have one of the
allowable values.
GLINVALIDVALUE is generated if n does not equal 2**m+1 for some
nonnegative integer value of m.
GLINVALIDOPERATION 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 GLTEXTUREFILTER4SIZESGIS
MACHINE DEPENDENCIES
The SGIStexturefilter4 extension is supported only on InfiniteReality
systems.
On InfiniteReality systems, GLINVALIDVALUE 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
GLTEXTUREFILTER4SIZESGIS. This restriction will be removed in a
future release.
SEE ALSO
glTexParameter
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