Physics Based Ragdoll Animation

Transcription

1 Physics Based Ragdoll Animation Arash Ghodsi & David Wilson Abstract Ragdoll animation is a technique used to add realism to falling bodies with multiple joints, such as a human model. Doing it right can be incredibly challenging, and this is why many professional video game companies choose to buy this technology from a third party. 1 Introduction Physics based animation can be really challenging to implement. It can also be overly taxing for even powerful modern computers. Due to these reasons many professional businesses in need of such technology choose to outsource the development to another company who specializes in this field. These specialists often will try to find shortcuts on the physics so that their product will run with as little overhead as possible. Clients then are free to use the remaining valuable processing time elsewhere.

2 2 Background In the recent past, true physics based hierarchical animation was not even possible in real time on most modest computers. As computers became more powerful, this started to change. Eventually companies began to capitalize on the growing interest in the technology. The Havok physics suite was an early success because its efficiency and ease of implementation made it desirable. Products using this technology got a lot of positive attention and soon it was common to see its use in many graphical applications, especially video games. Now, about a decade later, machines have become even more powerful and more companies are attempting to do their own ragdoll physics. But just as those who pioneered the method learned long ago, it is not so easy to pull off believably. 3 Method The first step to pulling off ragdoll physics is to make a model to demonstrate it on. A humanoid model is a

3 good choice since it has a number of limbs and a full compliment of joint styles. The next step is to build a skeleton that mirrors all the areas of the body you would like to have movement happen. For a human body, that could be the neck, shoulders, elbows, hips, knees, and more. From here you will build movement in to the skeleton in a hierarchical manner, or a sort of tree structure. In other words, have a top level like the torso, with immediate sublevels of the shoulders, then elbows and knees, and so on. Figure 1: Skeletal structure for hierarchical animation. In figure 1 you see an example of such a skeletal system. The elbow is relative to the shoulder which is relative to the body. As you add rotational movement

4 inside this structure, you begin to see the capacity this technique has for realism. But this is still just the beginning. With the overhead visible model matched up and tethered to the skeleton, it is time to start adding some actual physics. Gravity is a good place to start. It is also wise to add some angular springs at the joints to help simulate how joints tend to have a desired rest position. Springs combined with a dampening effect will go a long way later when other forces become involved. 4 Results This project accomplished successfully many aspects of ragdoll physics. The model, skeleton structure and joint system, as well as realistic physics were all implemented. When activated the model in this project will fall perfectly limp as a lifeless ragdoll would when the proper kinetic energy is applied.

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6 The series of images above show this ragdoll in various states of motion. 5 Future Work This ragdoll is fully functional in it s current state, but ragdolls are more fun if you do something interesting with them. In the future, we plan to add collision with the ground and environment objects. The doll will be put in to various situations where the ragdoll effects can shine. An example would be to shoot her with a projectile, or toss her in the air and watch her land etc. 6 Conclusion This paper has presented a basic starting point for advanced ragdoll physics. There is much more that can be done, and ragdoll physics will continue to become more sophisticated as increasing processing power allows for more realism. 7 References [1] Keith Peters. Advanced AS3 Animation. Available from

7 [2] Thomas Jakobsen. Advanced Character Physics. Available from [3] Chris Hecker. Inverse Kinematics. Available from [4] Chris Hecker. Rigid Body Dynamics. Available from [5] Chris Hecker. Five Physics Simulators for Articulated Bodies. Available from