We propose a framework for online generation of efficient gaits on uneven terrain. A set of dynamically optimal leg swing motion primitives for irregular terrains is first constructed offline-this is done by generating minimum torque motions for various starting and ending ground configurations, extracting dominant principal components, and forming basis functions. Gaits are then generated online via linear interpolation of the principal component basis functions, using a distance metric on SO(3) to select the components. The algorithm is verified via dynamic simulations involving the STriDER, a three legged passive-walking robot. Our results show that gaits using only knee-actuated leg swings are able to traverse uneven terrain of bounded variation.