Anisotropic texture filtering
Anisotropy is the distortion visible in the texels of a 3D object whose surface is oriented at an angle with respect to the plane of the screen. When a pixel from an anisotropic primitive is mapped to texels, its shape is distorted. Direct3D measures the anisotropy of a pixel as the elongation - that is, length divided by width - of a screen pixel that is inverse-mapped into texture space.
You can use anisotropic texture filtering in conjunction with linear texture filtering or mipmap texture filtering to improve rendering results.
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