The equation to graph a nice set of breasts in 3D space.

z=x4 + y4 - 4xy + 1

If you're using this equation for the Tomb Raider Model, the third term must be increased with the release. Thus the breast of Tomb Raider 1 will be smaller than Tomb Radar 2 and so on. It's amazing what 5 college calculus class, some free time, and a dirty mind and come up with.