This Houdini HDA takes two Unreal meshes as inputs: the object to be fractured and an impact object. The HDA exposes parameters in Unreal that allow the user to position the impact sphere and art direct the velocity and direction of the resulting simulation.

To demonstrate the principle, the example uses two simple spheres: one serves as the fracture object, while the other acts as the impact object. This provides a straightforward demonstration of how the impact position and simulation parameters can be adjusted directly within Unreal.

Creating the Initial Impact Points

First, move the Impact object to the location where you want the fracture to originate.

The red intersecting sphere is then used to group the primitives on the fractured object that fall within the sphere’s area of intersection. Everything outside of this group is deleted, leaving only the selected primitives.

Finally, the remaining group of primitives is used as the surface for a Scatter operation, generating points across the impact area. These scattered points will serve as the initial impact points for the fracture.

Controlling Fracture Detail

We can add as many scatter points as necessary to control the amount of detail in the fracture. Each point has the potential to generate an individual fracture piece, so increasing the number of points generally creates a more detailed and complex result.

Depending on the desired level of detail, however, less can often be better. Using fewer points can produce larger, cleaner fracture pieces and help keep the simulation and resulting geometry more manageable.

Creating the Fracture

We can then fracture the object, creating a higher concentration of pieces in the area where the points were scattered. This makes the object appear as though the fracture originated from the impact area, with the most detailed breakup concentrated around the point of impact.

Calculating and Applying Velocity

We can calculate the velocity by taking the newly transformed point position and subtracting the original point position, then storing the result as a velocity attribute. This creates an aimed velocity based on the direction and distance the impact object has traveled.

The velocity can then be transferred from the points to the fractured pieces before running them through an RBD simulation. This allows the fractured geometry to inherit the art-directed direction and velocity, creating a more controlled and believable impact simulation.

A More Detailed Version of the Tool

The following example shows a more detailed version of the tool, using the same principles described above. The impact object defines the fracture origin, scattered points control the concentration of fracture pieces, and the calculated velocity drives the direction and motion of the resulting RBD simulation.

Creating Bursting Fractures

For walls and other objects with sections that burst outward, the same principles described above still apply. We begin by moving the red intersection sphere to the location where we want the hole or impact area to originate.

The same process is then used to create the impact points and generate the fracture pieces, concentrating the smaller and more detailed pieces around the area of impact.

Next, the red intersection sphere is used to create two groups: an inside group, containing the fracture pieces within the sphere’s area, and an outside group, containing the remaining pieces outside of the sphere. These groups can then be used to control how the different sections of the fractured object behave during the simulation.