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Anisotropic filtering 9
An improvement on isotropic MIP mapping
Hereafter, it is assumed the reader is familiar with
MIP mapping.
If we were to explore a more approximate anisotropic
algorithm, RIP mapping, as an extension from MIP
mapping, we can understand how anisotropic filtering
gains so much texture mapping quality. If we need to
texture a horizontal plane which is at an oblique angle
to the camera, traditional MIP map minification
would give us insufficient horizontal resolution due to
the reduction of image frequency in the vertical axis.
This is because in MIP mapping each MIP level is
isotropic, so a 256 × 256 texture is downsized to a 128
× 128 image, then a 64 × 64 image and so on, so
resolution halves on each axis simultaneously, so a
MIP map texture probe to an image will always
sample an image that is of equal frequency in each
axis. Thus, when sampling to avoid aliasing on a
high-frequency axis, the other texture axes will be
similarly downsampled and therefore potentially
blurred.
An example of ripmap image storage: the principal image on the top
left is accompanied by filtered, linearly transformed copies of reduced
With RIP map anisotropic filtering, in addition to downsampling to 128 × 128, images are also sampled to 256 × 128
and 32 × 128 etc. These *anisotropically* downsampled images can be probed when the texture-mapped image
frequency is different for each texture axis and therefore one axis need not blur due to the screen frequency of
another axis and aliasing is still avoided. Unlike more general anisotropic filtering, the RIP mapping described for
illustration has a limitation in that it only supports anisotropic probes that are axis-aligned in texture space, so
diagonal anisotropy still presents a problem even though real-use cases of anisotropic texture commonly have such
screenspace mappings.
In layman's terms, anisotropic filtering retains the "sharpness" of a texture normally lost by MIP map texture's
attempts to avoid aliasing. Anisotropic filtering can therefore be said to maintain crisp texture detail at all viewing
orientations while providing fast anti-aliased texture filtering.
Degree of anisotropy supported
Different degrees or ratios of anisotropic filtering can be applied during rendering and current hardware rendering
implementations set an upper bound on this ratio. This degree refers to the maximum ratio of anisotropy supported
by the filtering process. So, for example 4:1 (pronounced 4 to 1) anisotropic filtering will continue to sharpen more
oblique textures beyond the range sharpened by 2:1.
In practice what this means is that in highly oblique texturing situations a 4:1 filter will be twice as sharp as a 2:1
filter (it will display frequencies double that of the 2:1 filter). However, most of the scene will not require the 4:1
filter; only the more oblique and usually more distant pixels will require the sharper filtering. This means that as the
degree of anisotropic filtering continues to double there are diminishing returns in terms of visible quality with fewer
and fewer rendered pixels affected, and the results become less obvious to the viewer.
When one compares the rendered results of an 8:1 anisotropically filtered scene to a 16:1 filtered scene, only a
relatively few highly oblique pixels, mostly on more distant geometry, will display visibly sharper textures in the
scene with the higher degree of anisotropic filtering, and the frequency information on these few 16:1 filtered pixels
size.