Often in rigid body simulations, you want a solid object to break into pieces because of some impact or force. For example, you might want an earthquake to destroy a house, with the concreate walls fracturing, the wood door splintering, and glass windows shattering. Or you might want a swinging demolition crane ball to cave in a wall.

• Most fracturing tools in Houdini support a pre-fracturing workflow, where you break the geometry into pieces in SOPs, with the pieces held together by glue constraints. Pre-fracturing gives you full artistic control over the look of the destruction (for example, do you want big blocky pieces or small jagged pieces). The object will crumble when a force overcomes the glue strength, or you can manually animate the glue off when you want the object to break down.

  The high-level tool for pre-fracturing geometry is the RBD Material Fracture SOP, with plenty of controls over different types of fracturing. There are many lower-level SOPs if you need even more control over fracturing.

  

• You can also do dynamic fracturing during the DOP simulation. See the help for Make Breakable tool for more information.

1. Use the RBD Material Fracture node to pre-fracture your model in SOPs.

2. RBD Material Fracture automatically creates glue constraints between the fractured pieces. On the Constraints tab you can set the Primary strength to the initial glue strength you want.

   After you import the fractured object into an RBD simulation (see the next step), you can go back and edit the settings on the RBD Material Fracture and edit the glue strength to control how much the pieces stick together.

   The value depends on the size and weight of your pieces, and what effect you want to achieve. A value of 1 will usually fall apart immediately.

   If you want the model to stay together until it’s hit by another RBD object, you will need to fine-tune the glue strength to a level where it’s high enough to keep the model stays together on its own, but not so high it prevents an impact from breaking the constraints.

3. Go up to the object level and select the fractured object. On the Rigid Bodies shelf tool, click the RBD Objects tool to import the object into DOPs as rigid body pieces.

See the rest of this page for more information on the available tools.

• Use the Group node to name groups of primitives. For example, the door, individual windows, and walls. This will allow you to fracture them individually.

• If you see pieces spinning/wobbling in the simulation, you can particle drag to freeze them.

• You can use the Output for view RMB menu on RBD SOPs to choose which output is displayed. By default, the first output (Geometry) is displayed. However, you can also toggle between Constraint Geometry and Proxy Geometry.

• Clicking MMB on outputs of the RBD SOPs will show the data for each individual output. Additionally, if you click MMB on the inputs, it will trace up to the geometry data coming in.

The RBD Material Fracture node and several other RBD SOP nodes share a consistent set of inputs and outputs, that let you send constraint geometry and proxy geometry through the network alongside the fractured geometry.

• The first input/output is the high-resolution display geometry.

• The second, colored input/output is constraint network geometry.

• The third input/output is low-res proxy geometry.

The RBD Material Fracture tool not only fractures your geometry, but it also creates constraints for you between those pieces. If you want to create more constraints, there are 3 SOPs that allow you to do so, separate from the fracturing process. They follow the same 3 input/3 output RBD workflow, and give you a lot of control for building constraints. This is useful when you're fracturing two different types of materials, and want to create boundary constraints between them.

All three constraint nodes will create simple primitives with a rest length attribute. The RBD Constraint Properties SOP node can be used afterward to create constraint groups and set up constraint types (such as glue or soft constraints). The help for the Constraint Network DOP has more background information about constraint networks (Constraint Network is the low-level DOP node responsible for converting SOP constraint geometry into the equivalent DOP constraints).

The RBD Material Fracture node can work on fast low-res proxy geometry. You need to set up high-res and low-res geometry with the same named pieces (for example, by breaking up the high-res geometry into named pieces and then copying and reducing the number of polygons to create the proxy).

Clustering refers to grouping fractured pieces into bigger clumps. There are two main clustering workflows:

• If you just want a bunch of pieces to stick together permanently, give them all the same name attribute. Nodes that work on pieces will treat them as once piece.

  This can be useful, for example, with wood splintering, where you often want to group small splinters into bigger jagged chunks.

• For certain directable crumbling effects, you will often want to work with bigger pieces early in the shot and have them break down into smaller pieces later in the shot. You can do this with a hierarchy of glue constraints. You can animate higher-level constraints off to break up bigger pieces into smaller pieces.

The RBD Material Fracture node provides clustering controls when the Material type is “Wood”. You can do manual clustering with the RBD Cluster node.

The Rigid bodies shelf has tools for importing geometry objects into the DOP simulation.

Other rigid body simulation tools:

The following nodes work with the RBD Material Fracture node. They all have the same inputs and outputs as RBD Material Fracture.

The RBD Material Fracture node uses these lower-level nodes internally. You can use them separately if you want to.

These are lower-level nodes, many of which are used inside the RBD Material Fracture SOP to provide functionality. You may find them useful if you are doing complex custom fracturing.