Articulated figures are called "rag dolls" by most people. The following is an excellent document explaining how they work, typed up by the most excellent MrElusive.

To create an articulated figure an md5 mesh with skeleton is required (.md5mesh). Furthermore an initial animation (.md5anim) is required with a single animation frame. This animation frame sets the model in a pose which is used to create the articulated figure. For human like characters this is usually a T-pose. The single frame of the initial animation also specifies the initial joint positions as they are modified by the bodies of the articulated figure.

Each articulated figure is specified in a separate .af file, stored in the base/af/ folder. To load and test an articulated figure in the game an entityDef is created in one of the .def files stored in the base/def/ folder. Such an entityDef typically looks like the following:

entityDef env_ragdoll_fatty {
  "editor_color"          "1 .5 0"
  "editor_mins"           "-8 -8 -8"
  "editor_maxs"           "8 8 8"
  "editor_usage"          "Ragdoll for monster_zombie_fat\n"
  "spawnclass"            "idAFEntity"
  "bleed"                 "1"
  "sleep"                 "1"
  "skin"                  "skins/models/monsters/fatty.skin"
  "model"                 "models/md5/monsters/zfat/zfat.md5mesh"
  "anim af_pose"          "models/md5/monsters/zfat/initial.md5anim"
  "articulatedFigure"     "monster_zombie_fat"
}

The "spawnclass" always points to "idAFEntity". The key "sleep" is set to 1 so the articulated figure will not immediately start moving when spawned. The "model" key points to the md5 mesh. The "anim" key with "af_pose" extension points to the file with the initial animation. The "articulatedFigure" key points to the .af file to be used.

A .af file first stores a section with general settings followed by a collection of bodies and constraints.

settings {
   key   params
}

body "name" {
   key  params
}

constraintType "name" {
   key  params
}

• ballAndSocketJoint
• universalJoint
• hinge
• spring

body1 "name"
body2 "name"

The body2 key may point to the special body called "world" and in this case the constraint will connect body1 to the world.

Other than the two connected bodies, each constraint has a number of settings specific to the constraint type as listed below.

• Minimize the number of bodies and constraints
• Use low complexity collision models
• Create one tree structure
• Don't use joint friction where possible, although joint friction might add stability
• Don't use self collision detection on bodies that don't need it

1. The articulated figure behaves weird or explodes.
   Make sure the mass of each body is within a reasonable range. Use af_showMass 1 to show the mass of each body. A body mass in the range [1, 100] usually works well. Connecting a very heavy body with a lightweight body often causes problems. It's usually also wise not to make the articulatd figure too heavy. The forces required to keep heavy bodies together can grow quite large and might not be represented accurately in the physics engine.

2. The articulated figure behaves weird or explodes.
   Make sure the inertia tensor of each body describes a reasonable balanced object. Use af_showInertia 1 to show the inertia tensor of each body. Wildly different diagonal elements of the inertia tensor may cause instability. The per body inertia scale may be used to scale the inertia tensor of a body such that the diagonal elements end up in a reasonable range relative to each other.

3. The articulated figure is jumpy and energy seems to enter the system.
   Make sure there are no contact points fighting with joint like constraints. When for instance three bodies are connected with two universal joints, and the bodies are relatively close to each other. The outer two bodies might be considered in contact. The figure will be pulled apart at the contact points while the universal joints try to keep the bodies together. Jumpy behaviour is often the result. Disabling self collision on the whole figure or on certain bodies can fix the problem.

Once an entity def is created for an articulated figure, the 'spawn' console command followed by the name of the entity def can be used to load the articulated figure in the game. The 'reloadAFs' console command can be used to reload all articulated figures. This command causes all articulated figures to snap back to the pose specified by the initial animation.

To visualize several physical properties of the articulated figures the following cvars can be used:

The skeleton of an md5 model can be visualized by setting the cvar 'r_showSkel' to 1.

To test articulated figures ingame it is useful to be able to pick them up and drag them around. When the cvar g_dragEntity is set to 1, an articulated figures can be selected and moved through the world. To select an articulated figure, the crosshair is pointed at a position somewhere on the visual mesh, and the attack button is pressed. While the attack button is pressed the articulated figure can be dragged through the world by walking around (or flying in noclip mode) and looking around.

The last articulated figure selected for dragging is contained in a yellow bounding box. This articulated figure can be removed from the game with the console command: 'deleteRagdoll'. Once articulated figures are positioned in a map they can be saved to the .map file by using the console command: 'saveRagdolls'. Articulated figures saved to a .map file are not deleted from the .map file with the 'deleteRagdoll' command. Any articulated figure can ofcourse still be deleted from a .map file in the level editor.