Ragdoll physics is a type of procedural animation used by physics engines, which is often used as a replacement for traditional static death animations in video games and animated films. As computers increased in power, it became possible to do limited real-time physical simulations, which made death animations more realistic.
Early video games used manually created animations for a character’s death sequences. This had the advantage of low CPU utilization, as the data needed to animate a "dying" character was chosen from a set number of pre-drawn frames. In contrast, a ragdoll is 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 constraints that restrict how the bones may move relative to each other. When the character dies, their body begins to collapse to the ground, honouring these restrictions on each of the joints' motion, which often looks more realistic.
The term ragdoll comes from the problem 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 rag doll, often into comically improbable or compromising positions. Modern use of ragdoll physics goes beyond death sequences.
The Jurassic Park licensed game exhibited ragdoll physics in 1998 but received very polarised opinions; most were negative, as the game had a large number of bugs. It was remembered, however, for being a pioneer in video game physics.[1]
There are fighting games where the player controls one part of the body of the fighter and the rest follows along, such as Rag Doll Kung Fu, as well as racing games such as the FlatOut series.
Recent procedural animation technologies, such as those found in NaturalMotion's Euphoria software, have allowed the development of games that rely heavily on the suspension of disbelief facilitated by realistic whole-body muscle/nervous ragdoll physics as an integral part of the immersive gaming experience, as opposed to the antiquated use of canned-animation techniques. This is seen in Grand Theft Auto IV, Grand Theft Auto V, Red Dead Redemption, Max Payne 3 and Red Dead Redemption 2 as well as titles such as LucasArts' and Puppet Army Faction's Kontrol, which feature 2D powered ragdoll locomotion on uneven or moving surfaces.
Ragdolls have been implemented using Featherstone's algorithm and spring-damper contacts.[2] An alternative approach uses constraint solvers and idealized contacts.[3] While the constrained-rigid-body approach to ragdolls is the most common, other "pseudo-ragdoll" techniques have been used:
used by and popularized by Thomas Jakobsen,[4] 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.
traditionally used in non-realtime media (film/TV/etc), this technique (used in the Medal of Honor series starting from European Assault onward) employs the use of multi-layered physical models in non-playing characters (bones / muscle / nervous systems), and deformable scenic elements from "simulated materials" in vehicles, etc. By removing the use of pre-made animation, each reaction seen by the player is unique, whilst still deterministic.