Julio Jerez wrote:no that's not true, just imagine two bodies connected but a slider falling on a floor.
Well yes, but legs don't have sliders or any form of shock absorbers, so i was referring to your comparison of 'sloppy soft joints' vs. accurate stiff joints, with the former giving an unintended advantage.
my guess is that are using generalize coordinate system solvers
I saw a short YT video where they showed one of their computer simulations. It showed a simple robot visualization of ZMP, support polygon, things like that. Sadly i can't find it anymore.
The quality of the simulator was terrible. It did jitter like hell. Any shitty game is way better. I was pretty surprised.
Now i think the kind of quality we would expect did not matter to them, and making it looking nice was no goal either.
My own experience did kind of confirm this later. Details did not matter. I did not predict in which frame the foot would hit the floor as proposed above (that was just a game of thought to spur some inspiration eventually). Also things like unique joint / body states did not matter. My way of calculating a pose from pendulum was just hacks and lacked any accuracy. But it worked regardless to keep the ragdoll in balance.
All that mattered for balance is the summed COM of all bodies, and precise control of that. Small errors or perturbations coming from collisions have little effect on this.
But Newton moving the contact from the toe to the middle of the foot for just one frame was enough to make the ragdoll fall a moment later. That's a small error too, but this one had a huge impact on diverging from the prediction of the controller under motion with unstable balance. The only way to compensate such error would be to make a step, but i did not want to do this without prior understanding the true cause of the problem.
Maybe my method is more global, while yours is more about local detail. Thus we face different problems. I don't know. But what i try to say is: To me, the true cause and solution of a problem always turned out to be completely different from what i had expected. And i would not wonder if this happens to you too, although you are an expert on physics already.