The rendering equation, the keystone of computer graphics rendering:

$$L_o(x, \vec{w}) = L_e(x, \vec{w}) + \int_\Omega f_r(x, \vec w', \vec w) L_i(x, \vec w') (\vec w' \cdot \vec n) \mathrm{d}\vec w'$$

(This is at the same time also a test for MathJax support on my weblog in order to provide correct mathematical typesetting using proper typography instead of some lame pixelated image.)