Applying Velocity Fields to Debris Simulation

In this lesson, you will learn how to steer the movement of fractured shards after a collision using directional velocity fields to create realistic scattering effects.

Understanding Velocity Fields

When you shatter a Geometry Collection using the Chaos physics system, the shards initially inherit the energy of the impact. However, to simulate environmental forces like an explosive blast wave or a gust of wind, you need to apply persistent forces. A Velocity Field acts as a vector-based "push" that influences the velocity of every shard within its influence area, overriding or adding to their existing momentum.

Setting Up a Linear Force

To apply a directional push, we use the Field System Actor. By utilizing a Uniform Vector field, you can define a constant direction and magnitude that pushes all debris along a specific axis.

Think of this like a "wind tunnel" for your rubble. If you want debris to scatter away from an explosion center, you combine your Radial Falloff field with a Vector field.

Here is how you structure the field logic in your Blueprint graph:

1. Create a Field System Actor: This acts as the container for your physics forces.
2. Define the Vector: Use a Make Vector node to define the direction (e.g., 0, 0, 1000 for a strong upward lift).
3. Apply the Field: Use the Apply Physics Field node. Connect your Uniform Vector field to the Target and set the Field Operation to Multiply or Add to blend this force with the existing physics simulation.

// Logic flow for applying directional force to Chaos shards
FieldSystem->ApplyPhysicsField(
    Enabled = true, 
    Target = Field_UniformVector, 
    Operation = Field_Add, 
    Magnitude = 500.0f
);

Controlling the Flow

By adjusting the Falloff parameter, you ensure the force doesn’t apply globally to everything in your scene. A Radial Falloff ensures the velocity field is strongest at the point of the blast and dissipates as shards move further away, preventing the debris from flying off into infinity. You are essentially using math to "paint" motion onto your physics objects, giving your destruction sequence that polished, cinematic feel.

Key Takeaway: Velocity fields allow you to dictate the narrative of your destruction by applying precise, directional physics forces that guide how debris scatters after a fracture occurs.