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In computer Game Engine s, ragdoll physics are a type of Procedural Animation that is often used as a replacement for traditional static death Animations . Early computer games, going as far back as Computer Graphics themselves, used hand-created death sequences for characters. This had the advantage of low CPU usage as all a dying character had to do was choose from a pre-set Finite number of animations.

As computers increased in power, it became possible to do limited Real-time Physical Simulation s. A ragdoll is therefore a collection of multiple Rigid Bodies (each of which is ordinarily tied to a Bone in the graphics engine's Skeletal Animation system) tied together by a system of Constraint s that restrict how the bones may move relative to each other.

The term ''ragdoll'' comes from the fact that the articulated systems, due to the limits of the solvers used, tend to have little or zero joint/ Skeletal Muscle stiffness, leading to a character collapsing much like a toy Ragdoll , often into an improbable and comedic position.

The first game to exhibit ragdoll physics was the '''', which received very polar opinions, though most were negative. The game had terrible bugs but was remembered for being a pioneer in video-game physics engines. This is mostly due to the game's production schedule being hurried; the scope being drastically changed from survival horror to a more simplified shooter approach.


RAGDOLL ADVANTAGES/DISADVANTAGES


Due to the computationally expensive nature of performing simulations, most games using ragdolls use very simple approximations of characters:

  • Extremity bones such as fingers often go unsimulated.

  • Simple joints are used instead of actual constraints imposed by a true Skeleton . (For example, human Knee joints are often modelled as a rigid Hinge even though an actual human knee allows some rotation.)

  • Simplified Collision Hull s are used to detect contact with other rigid bodies rather than detecting collision with the mesh.


The chief advantage ragdolls offer over traditional animations is that they allow much more correct interaction with the surrounding environment. Where it would be absurd to try and hand-craft custom animations for all conceivable circumstances, ragdolls fill in and generate a mostly-correct interpretation of events on the fly.


OTHER TYPES OF RAGDOLL


While the constrained-rigid-body approach to ragdolls is the most common, other " Pseudo -ragdoll" techniques have been used:

  • '' and popularized by Thomas Jakobsen, this technique models each character bone as a point connected to an arbitrary number of other points via simple constraints. Verlet constraints are much simpler and faster to solve than most of those in a fully modelled rigid body system, resulting in much less CPU consumption for characters.


  • '', this technique relies on playing a pre-set death animation and then using inverse kinematics to force the character into a possible position after the animation has completed. This means that, during an animation, a character could wind up Clipping through world geometry, but after he has come to rest, all of his bones will be in valid space.


  • Blended ragdoll: this technique works by playing a pre-made animation, but constraining the output of that animation to what a physical system would allow. This helps alleviate the ragdoll feeling of characters suddenly going limp, but offers correct environmental interaction as well. This requires both animation processing and physics processing, thus making it even slower than traditional ragdoll alone.



TRY RAGDOLL PHYSICS YOURSELF

Ragdoll Masters. Fighting game with ragdolls. ( Shareware download)

Ski stunt simulator (java)

Falling woman (flash). Use your mouse to manipulate her, if she gets stuck.

President Bush as a ragdoll (flash) Same program as Falling Woman, above.

Thomas Jakobsen's article about the integration of Verlet Physics. (Link to Gamasutra.com, but can be found all over the web)

Stair Dismount and Truck Dismount. Try to score points by hurting the ragdoll. (download)


SEE ALSO