1 Answers
Trilinear filtering is an extension of the bilinear texture filtering method, which also performs linear interpolation between mipmaps.
Bilinear filtering has several weaknesses that make it an unattractive choice in many cases: using it on a full-detail texture when scaling to a very small size causes accuracy problems from missed texels, and compensating for this by using multiple mipmaps throughout the polygon leads to abrupt changes in blurriness, which is most pronounced in polygons that are steeply angled relative to the camera.
To solve this problem, trilinear filtering interpolates between the results of bilinear filtering on the two mipmaps nearest to the detail required for the polygon at the pixel. If the pixel would take up 1/100 of the texture in one direction, trilinear filtering would interpolate between the result of filtering the 128×128 mipmap as y1 with x1 as 128, and the result of filtering on the 64×64 mipmap as y2 with x2 as 64, and then interpolate to x = 100.
The first step in this process is to determine how big in terms of the texture the pixel in question is. There are a few ways to do this, and the ones mentioned here are not necessarily representative of all of them.