AnimDynamics

The AnimDynamics Animation Blueprint node is a light-weight physics simulation solution, that you can use to apply physics-based, secondary animation to parts of a character's Skeletal Mesh at runtime. Unlike the RigidBody node that uses the character's Physics asset, the AnimDynamics node simulates its own physics bodies, to increase your projects performance.

With the AnimDynamics node, you can apply simulated-physics motion to parts of your character's skeletal mesh, such as, necklaces, bracelets, wires, or other items, in conjunction with character motion.

Here, the AnimDynamics node is used to dynamically drive the motion of the character's antenna, to react to the motion of the character's head during animation playback.

Overview

The AnimDynamics node is a low-performance cost physics-solver that is processed on the AnimGraph at runtime. In order to achieve a low performance cost, the AnimDynamics node makes a few approximations that are important to consider:

• Simulated boxes are used instead of the character's bones or physics bodies, to calculate inertia for each segment.

• Collision is not calculated; instead, constraints can be used to restrict movement.

The AnimDynamics node supports Linear, Angular and Planar constraints to simulate physically-influenced motion. Linear and Angular constraints can be driven by springs to provide a more bouncy feel, while Planar constraints can be used to create a plane that the object will not cross. You can toggle each of these constraints, and adjust their associated properties in the AnimDynamics node's Details panel.

Single-Body Simulation

Using the AnimDynamics node you can apply simulated-physics motion to any object on your character using a bone as a reference point. Here is an example of the AnimDyamics node being used to add dynamic motion to a drum that is reacting to the motion of the character's running animation.

By selecting the drum's bone as the Bound Bone, you can control the physics-driven motion by defining a bounding box with the Box Constraints property. The Local Joint Offset is an offset from the Bound Bone's joint, that you can use to determine the reference point of the constraints.

By leaving the Box Constraints and the Local Joint Offset properties as their default values, you may not see any movement. This is because the constraints are preventing any motion on the object. By reducing the bounding box's dimensions, you will see motion.

Here, the AnimDynamics's Debug properties are enabled. You can use these debug drawings to visually see the Bounding Box and Local Joint Offset properties as they influence the mesh.

In this example, Rotation Constraints are also set, so the drum can only move along a set axis, within a specified range. The applied Rotation Constraint along the Y axis is represented by the green angle overlay, and is useful to prevent the mesh from overlapping on itself.

For more information about creating single-body physics simulations using the AnimDynamics node, see the Creating Dynamic Animations workflow guide.

External Force

You can also set an External Force to the physics-simulate motion, to weight the object being simulated, or apply more inertia for more reactive motion. By default, an External Force vector can be applied using the pin in the AnimGraph.

For more information about using an External Force to create physics simulations using the AnimDynamics node, see the Creating Dynamic Animations workflow guide.

Chains

By enabling the Chain property in the Details panel, you can use the AnimDynamics node is to simulate the motion and collision of a chain of objects.

Chain simulation is a much more resource intensive simulation, than a single-body simulation. With chain simulation, linked constraints now need to be solved, which requires many more iterations to correctly converge. Iteration counts are also configurable per-node using the Num Solver Iterations Pre Update and the Num Solver Iterations Post Update properties in the Details panel.

By enabling the Chain property and selecting a Bound Bone and Chain End, the AnimDynamics will generate a chain using any bones in between. Each bone within the chain, excluding the Bound Bone, will have a constraint box generated around it, to simulate motion and collision with the other bones in the chain. These constraint boxes will need to be tweaked manually to achieve good results.

Overlapping constraints can cause undesirable results, like flailing bones and uncontrolled motion. If you are experiencing these kinds of results, ensure the chain constraints are not overlapping.

Collision Simulation

Using Planer and Spherical limits, you can enable simple collision simulation with the AnimDyanmic node's simulated physics structures.

Planer Limits

With planar limits, you can set flat boundaries the physically-simulated structures are unable to cross. You can use these planer constraints to prevent the simulated structures from overlapping with other parts of the character's Skeletal Mesh or the ground.

In the AnimDynamics Details panel, you can enable planer limits with the Use Planer Limits property. Then, using Add (+) to create an Index, you can set individual planer limits to control the simulated structure's behavior. With each index, you can set a Driving Bone as a reference point for the location of the planer limit. Then, using the Plane Transform properties, you can set the planer limit's Location, Rotation, and Scale properties to establish planer boundaries that the simulated structure is unable to cross.

For example, adding a planer limit to the character's root bone, can create a boundary that represents the ground, that simulated objects are not able to cross.

Spherical Limits

With spherical limits, you can create sphere boundaries, that surround points on the AnimDynamics physics-simulated structures, to act as a simple collision prevention.

After enabling the Use Spherical Limits property in the AnimDynamics Details panel, you can use Add (+) to create an Index where you can set up sphere limits. Within each Index you can set a Driving Bone to act as a reference point for the location of the sphere and the offset its location on the X, Y, and Z axes, with the Sphere Local Offset property. You can also set the sphere's Limit Radius to determine how large the sphere is, and set the Limit Type to decide how the sphere interacts with the Skeletal Mesh.

By setting the Limit Type on a sphere limit to Outer, you can use sphere limits to prevent structures from colliding, by not allowing objects to pass through the sphere. You can also use sphere limits to contain structures, by setting the Limit Type to Inner.

Property Reference

Here, you can reference a list of the AnimDynamics properties and a description of their functions: