Implementing Physics Constraints for Character Joints

You will apply angular and linear limits to constraints to restrict skeletal movement and prevent unrealistic joint rotation in ragdoll simulations.

In this lesson, you will learn how to apply angular and linear limits to character constraints to prevent your ragdolls from bending in impossible, "jelly-like" directions.

Understanding Physics Constraints

When a character enters a ragdoll state, Unreal Engine uses the Physics Asset to simulate individual body parts as rigid bodies connected by constraints. By default, these constraints might allow joints to rotate 360 degrees, leading to limbs twisting unnaturally. To achieve realism, we must define the range of motion for every joint, such as restricting an elbow to only bend forward or limiting the rotation of a neck.

Applying Angular Limits

The most critical settings are found within the Physics Asset Editor. When you select a constraint (the wireframe visual between two bones), you will see the Angular Limits section in the Details panel.

Swing 1/2 Motion: These control the rotation along the two axes perpendicular to the joint's primary axis (like the side-to-side motion of a shoulder). Setting these to Limited allows you to define an angle.
Twist Motion: This restricts the rotation along the bone's primary axis (like rotating your forearm).
Limit Angles: By setting these values (in degrees), you create a "cone" of movement. For an elbow, you might set the Swing 1 limit to 0 and the Swing 2 limit to 145, effectively locking it from bending backward.

Setting Linear Limits

While Angular Limits handle rotation, Linear Limits control the distance between the two connected bones. For standard character joints, these should usually be set to Locked on all three axes (X, Y, and Z). If they are set to Free, your character’s limbs will detach and float away from the body during the simulation, which is almost never the desired look for a humanoid.

Constraint Settings:
- Linear Limit: Locked (Prevents limb detachment)
- Angular Limit Type: Limited (Restricts rotation range)
- Swing Limit: 45.0 (Restricts lateral motion)
- Twist Limit: 0.0 (Prevents unnatural bone spinning)

By carefully defining these boundaries, you transform a chaotic ragdoll into a believable physical representation of your character.

Key Takeaway: Use Angular Limits to define the physical "cone" of joint movement and set Linear Limits to 'Locked' to keep your character’s skeletal structure intact during simulation.