How Ragdoll Physics Works

The Engines of Animation

Bodies go flying in “Max Payne.”
Bodies go flying in “Max Payne.”
Screen capture by HowStuffWorks staff

For decades now, video game makers have been trying to recreate real-world physics. They use simulated physics engines to build in principles of gravity, velocity, collision detection and momentum that affect your racecars, planes and even Mario as he jumps and scrambles through the underworld. Without these elements, there are no rules or boundaries to gameplay that make any real sense.

The same goes for character deaths. With primitive games, characters always died to exactly the same pre-scripted, static animation. That was fine and dandy in simpler times, but improved hardware made room for better all-around graphics performance. Dedicated graphics processing cards took some of the burden from the CPU, allowing for more sophisticated gameplay and, you guessed it, better death animations.

Games like the "Halo" series and "Max Payne" feature realistic movements as bodies are jolted by bullets. And Rockstar Games has made a name for itself with its "Grand Theft Auto" series, which is filled with natural-looking lighting effects and human motion that's almost startling in its accuracy. Thanks in part to ragdoll physics, instead of canned graphics, programmers make characters that respond in real time to other onscreen elements, from walls to bombs to bullets.

"Hitman: Codename 47" was one of the first games to employ ragdoll physics. After you incapacitated an opponent, you could drag the lifeless, rolling body and steal its clothes as a disguise. Bullets slammed into bodies with ridiculous force. The elements weren't altogether convincing, but they added a new layer of believability that had been missing from gameplay.

"Hitman" used Verlet integration, an algorithm used to incorporate Newton's equations of motion into applications such as computer animation. Each part of an animated skeleton is defined as points connected to other points with some basic rules as guidelines. The comparative simplicity of this algorithm means it uses less CPU processing time than other techniques.

Blended ragdoll physics combines real-time physics processing with premade animations, in games such as "Jurassic Park: Trespasser." The static animations interact more realistically with the environment; animated characters don't just flop down. They crash and bend more like actual human beings. But there are still visual flaws that don't make sense to the human brain. It doesn't look natural enough.

Procedural animation is the latest and most immersive type of game physics. There are no predetermined animations here. Instead, all of the characters and much of the environment is continually responsive to in-game physics. That applies to death animations, of course, but it also makes every other aspect of the game more convincing, too.