What should we do if the red point is one the side (or corner) of the mipmap? That means we cannot find 4 sample points around it
Can you clarify how bilinear filtering works? I can understand that trilinear filtering is another level on top that interpolates between mipmap levels -- bilinear filtering interpolates between texel samples, right? How does that show up in the formula above?
How do you compute fd for trilinear filtering?
Someone can correct me if I'm wrong, but I believe the premise here is that we can lerp over the result of two bilinear filters to get a trilinear filter. The idea being that if your projected texture demands a resolution at, say, 3/4 of the primary resolution, then you could lerp between the primary texture and a .5x downsample to approximate the desired texture. Fd then is basically the ratio between the desired texture resolution and the next highest mip resolution.