Even though trilinear filtering goes a long way towards improving the image quality of a texture mapped 3D surface, there are cases when trilinear filtering fails to provide good results. Both bilinear and trilinear filtering are used to solve problems that arise due to the difference in size of the source texture and the size of the surface that the texture needs to be mapped on. However, they do not take into account the difference in shape and perspective between the source texture and the target surface. However, when the surface is oblique to the viewer, filtering causes loss of detail. This degradation in image quality is most obvious when the texture being mapped contains text.
Text typically has areas which are as low as one pixel wide.
Applying bilinear or trilinear filtering on text that has been mapped obliquely will result in the characters becoming blurry and blending with adjacent characters and therefore almost unreadable depending on the perspective. Anisotropic filtering is used to get around this problem by taking extra samples from the texture map. Anti-Aliasing Technique of blurring pixels. This is commonly used to hide the staircase effect on lines or edges when scenes are rendered at lower resolutions. Bi-linear filtering When a small texture is used as a texture map on a large polygon, a stretching will occur and large blocky pixels will appear.