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Texture filtering 212
Texture filtering
In computer graphics, texture filtering or texture smoothing is the method used to determine the texture color for a
texture mapped pixel, using the colors of nearby texels (pixels of the texture). Mathematically, texture filtering is a
type of anti-aliasing, but it filters out high frequencies from the texture fill whereas other AA techniques generally
focus on visual edges. Put simply, it allows a texture to be applied at many different shapes, sizes and angles while
minimizing blurriness, shimmering and blocking.
There are many methods of texture filtering, which make different trade-offs between computational complexity and
image quality.
The need for filtering
During the texture mapping process, a 'texture lookup' takes place to find out where on the texture each pixel center
falls. Since the textured surface may be at an arbitrary distance and orientation relative to the viewer, one pixel does
not usually correspond directly to one texel. Some form of filtering has to be applied to determine the best color for
the pixel. Insufficient or incorrect filtering will show up in the image as artifacts (errors in the image), such as
'blockiness', jaggies, or shimmering.
There can be different types of correspondence between a pixel and the texel/texels it represents on the screen. These
depend on the position of the textured surface relative to the viewer, and different forms of filtering are needed in
each case. Given a square texture mapped on to a square surface in the world, at some viewing distance the size of
one screen pixel is exactly the same as one texel. Closer than that, the texels are larger than screen pixels, and need
to be scaled up appropriately - a process known as texture magnification. Farther away, each texel is smaller than a
pixel, and so one pixel covers multiple texels. In this case an appropriate color has to be picked based on the covered
texels, via texture minification. Graphics APIs such as OpenGL allow the programmer to set different choices for
minification and magnification filters.
Note that even in the case where the pixels and texels are exactly the same size, one pixel will not necessarily match
up exactly to one texel - it may be misaligned, and cover parts of up to four neighboring texels. Hence some form of
filtering is still required.
Mipmapping
Mipmapping is a standard technique used to save some of the filtering work needed during texture minification.
During texture magnification, the number of texels that need to be looked up for any pixel is always four or fewer;
during minification, however, as the textured polygon moves farther away potentially the entire texture might fall
into a single pixel. This would necessitate reading all of its texels and combining their values to correctly determine
the pixel color, a prohibitively expensive operation. Mipmapping avoids this by prefiltering the texture and storing it
in smaller sizes down to a single pixel. As the textured surface moves farther away, the texture being applied
switches to the prefiltered smaller size. Different sizes of the mipmap are referred to as 'levels', with Level 0 being
the largest size (used closest to the viewer), and increasing levels used at increasing distances.