Texture filtering is an anti-aliasing technique used to refine file textures, reduce flickering, or to achieve special effects. The File texture’s Filter attributes (see File) scale the size of the filter and let you specify the amount of blur in the texture map.
If a texture map image is too sharp, it may reveal aliasing (staircase) effects in the final rendered image. By default, Filter is set to a value of 1.0 to help prevent such aliasing effects.
The effect of Filter is related directly to eye space. As the object moves further away from the eye, the more the texture blurs.