Bilateral upsampling can be used as a cheat method to increase the size of a texture you got without bluriness over the edges. Normally you do bilinear sampling to get a smooth result when upsizing a texture but this approch also considers the normals and heights. The result is an image with smooth result but crisp edges.

It works like a normal bilinear sampling but not only uses the distance between the sample pixels as weights but also calculate a weight factor based on difference in height and normals.

It can be implemented like the following in a GLSL shader function:

TODO

Here’s a paper about soft global illumination that uses bilateral upsampling to save some processing. It also includes a formel for the bilateral upsampling.
http://www.ppsloan.org/publications/ProxyPG.pdf

And here’s a more in detail description about bilateral upsampling:
http://www.cs.duke.edu/~tomasi/papers/tomasi/tomasiIccv98.pdf