Bilinear Filtering

Bilinear filtering is a texture filtering method used to smooth textures when displayed larger or smaller than they actually are.

Most of the time, when drawing a textured shape on the screen, the texture is not displayed exactly as it is stored, without any distortion. Because of this, most pixels will end up needing to use a point on the texture that's 'between' texels, assuming the texels are points (as opposed to, say, squares) in the middle (or on the upper left corner, or anywhere else; it doesn't matter, as long as it's consistent) of their respective 'cells'. Bilinear filtering uses these points to perform bilinear interpolation between the four texels nearest to the point that the pixel represents (in the middle or upper left of the pixel, usually).