glTexFilterFuncSGIS(3G) OpenGL Reference glTexFilterFuncSGIS(3G)
glTexFilterFuncSGIS - specify user-defined texture filtering function
void glTexFilterFuncSGIS( GLenum target,
GLenum filter,
GLsizei n,
const GLfloat *weights )
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.
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<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.
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.
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)
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.
glTexParameter
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