I haven't gotten around to writing interesting samples or uncovering any deep XNA truths lately. Instead I've been tinkering on a little beat-em-up game with stylized stick men to do the fighting. These little actors are entirely procedural, so the game itself generates the geometry and animations rather than using models created by artists. Obviously nothing can replace a good artist and this proved painfully true when it came to the animations.

The animations are generated by a particle-based physics system (described here), which works by applying force to the attacking limb towards the victim, checking collisions and letting the simulation run its course. The base skeleton displayed below is set up easily enough, but without additional contraints the resulting movements are far from natural. As noted in the original article, a lot of tweaking can also be done using the mass of particles to get some control over how easily particles (i.e. joints) can move.

So if the skeleton and animations are that hard to tweak, you might be wondering what good this procedural technique is then. The beauty of this -admittedly simple- physics based rendering setup is that you essentially get inverse kinematics for free. If I want to hit my opponent with a hand, I just apply some force on the hand towards where I want to hit him. With sufficient tweaking, this produces convincing animations for accurately hitting the victim anywhere with any part of the attacking actor. Headbutts, kicks and more exotic attacks are just a matter of picking target and subject particles on either side.

Another nice benefit of this procedural approach is that the geometry is very accessibly to the program and thus can be altered in a variety of ways. With a few minutes of tinkering, style variations like those below are easily implemented.

The project is still a pretty long way off from becoming a playable game, but it's already made its way around the office for passive-aggressive stress relief   I'm afraid I can't put a playable build out anytime soon, but in the meantime here's a little movie (WMV, 7mb) showing some basic pummeling and the style so far. Since a lot of tweaking is involved and much of the style is still up in the air, comments and/or suggestions would be much appreciated.