I'm curious as to why no games or programs use quad-linear filtering with or without mip-mapping. This feature have been available in both hardware and drivers for mipmapping by Nvidia since the Geforce3 Ti series, and likely nearly as long by ATI. http://techreport.com/articles.x/3203/1
The xbox 1 was capable of perfroming quadlinear filtering as well, which makes sense since it had a geforce3 based gpu. Whether or not it was actually put to use is another question.
From what I've read there isn't to much dimminishing rate of return/ or performance penalty going from tri-linear to quadlinear filtering.
Here's an amazing quadlinear filtered image. You can see how well the colours blend together, the transition from one color to another is almost perfect.
http://graphics.idav.ucdavis.e.../return_jpg?pub_id=847