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RBD Workflows in Houdini


Happy Holidays. I did a poll on my Youtube channel to see what people were interested in. You all chose RBD Workflows. So here you go.

What is RBD, and How Do I Use It?

RBD, more commonly known as Rigid Body Dynamics, is a system inside Houdini.

This system allows for the motion, fracturing, and collision of objects to be handled within solvers. As well as a combination with SOP tools. It handles objects as if they were hard objects, such as buildings, bricks, or other structures.

RBD can handle objects in two states. Active or Passive. Active objects can interact with collisions and forces. While passive objects can collide with active objects, they don’t move.

RBD also has several different material fracturing options. This allows you to generate accurate fractures for different types of objects. These fracturing methods can be found in the RBD Material Fracture SOP, and in the Voronoi Fracture SOP. The RBD Material Fracture SOP allows for you to create wood, concrete and glass fractures. Then it will pre fractures the geometry. It is an ideal node when working with multiple constraints, glues, and meshes.

The Voronoi Fracture SOP is great for small scale RBD sims. As well as generating fractures in concrete. The node is also on it’s 2.0 version(As of 2021) so there have been several improvements to it.

There are also other tools that go along with the Voronoi fracture SOP to improve its use. As well as your RBD simulation as a whole. It has two inputs. The first is for the mesh you would like to fracture, and the second input is for the points it needs to fracture the mesh. You can create these points with scatter SOPS, or points from volume SOPS. Or even the Add SOP.

In combination with the node above, you can also use the Voronoi Fracture Points SOP to generate points for it to use. This node calculates data from the incoming mesh and a set of points to generate impact points for the fractures.

Internally, the node generates volumes from the chosen impact points from metaballs that are copied to each point. However, you can customize this metaball generation by adding metaballs into its second input. These metaballs will be merged into the metaballs the node generates, and then together they will form the impact volume. The volume is then used to divide the geometry into three regions.

These regions are: The Surface Region, The Interior Region, and the Exterior Region. The Surface Region is the area where the object intersects with the impact volume. The Interior Region is the intersection of the volume of the object, and the impact volume. Finally, The Exterior Region is the area of the volume that is outside the area of impact.

You can also use the Voronoi Spilt SOP to selectively cut up your geometry. It does this by separating and breaking the geometry by a set of polylines. Each line cuts along its midpoint. In order to create these lines, to get the node to work, you can use the Tetrahedralize SOP.

The RBD Interior Detail SOP, is also one of these nodes. It can be used to create high resolution pieces on your mesh on both their inside and outside.

The RBD Cluster SOP can be used to create large clusters of fractured pieces.

Probably the best preview tool in your RBD arsenal is the RBD Exploded View SOP. This tool will explode and separate your fractured pieces so you can see how they will fracture. It’s also great for matching high res and low res geometry together to see how the different resolutions can have an impact on the simulation.

This node also has different visualization options. A few of them are:

Geometry Outside Proxy: Colors areas red to show where your geometry is poking out beyond your proxy geometry.

Volume: Shows the volume distribution amongst the pieces.

Pieces: Displays the fracture pieces in different colors.

Strength and Stiffness Constraints: This let's you visualize the strength values, glue constraints, and spring constraints if they are applied to your model.

How Do I Start To Build a Project Using RBD?

There are several different ways to set up a RBD simulation. But let’s try and simplify it from start to finish.

#1 Prepare your objects. Fracture your models.Create any groups you might need to control the fractures and constraints.

#2 You can also do this before step one, depending on your model.You’ll need to clean your objects. So basically, make sure the geometry is closed, check for holes, wrong normals, and any unused points.

#3 Check your UVs. After simulating, your model’s UVs might be incorrect. This can be caused by incorrect UVs at the start of your simulation.

#4 Fracture your model.Choose your preferred way.

#5 Create the UVs for the inside of your now fractured model. If you don;t do this, and the inside pieces of your debris faces the camera, the materials will appear warped and inaccurate.

#6 Cache the object.

#7 Create your constraints if needed.

#8 Slap down a RBD configure node.

#9 RBD Solver. It’s time to simulate, baby!

#10 Drop down a RBD I/O. This is to cache the simulation.

#11 Do whatever you need to do next.

When setting up these scenes it’s important to consider these things.

Aesthetics: How realistic does your fracturing and overall simulation look?

Feedback: Can you change this setup without altering the end result too much if someone asks you to do so?

Further Flexibility: How does this setup work with certain geometry resolutions, or different geometry shapes? How functional is the setup? Can you easily alter the simulation without breaking it?

Performance: How long does the scene take to cache out? How long does it take to simulate? Can you improve cache, render, and simulation times?

Extensibility and Procce-durablity: Can you hand this build off to other users and have them easily understand it? Will someone have to break or add things to your build in order to use it?

Node by node this is pretty much what you should be doing:

#1 Get your Geometry
#2 Add a Convex Decomposition if you need it.
#3 RBD Material Fracture: Choose your material type.
#4 RBD Configure
#5 RBD Pack
#6 Add Velocity (If needed). This can be done with a wrangle.
#7 RBD Unpack (Turn on transfer attributes and add the attributes you want)
#8 And RBD or Bullet Solver
#9 You can now drop down an RBD Exploded view. You can also drop down this node to preview your
simulation and geometry at any stage of building your simulation.

Also keep in mind that when trying to fracture different objects you will also need to consider this different ideas, as well as research them:

How can you fracture the object in the most efficient method?
Is a particle system enough to create the debris you need, or do you need to use a full RBD system? Sometimes you can fake things, but your supervisor might know better. ;)
Will you have to create any secondary simulations for this project to make it look realistic?

Workflow For Glue Constraints

Constraints can be tricky. And even trickier to setup. So here is a breakdown on Glue Constraints and how you can approach them.

Long story short. Houdini allows you to glue RBD pieces together. These objects will stay together until they are told to break apart, or the strength of the “glue” is overcome.

There are two main workflows for using glue.

First one:

- Glue an object with no fracture groups or customized glue attributes together.

Second One:

- Get your object.

- Create fracture groups

- Import these chunks as pieces glued together.

Glue can be a great way to get your model to break apart in chunks and have a delay in its destruction. Or make the object fall piece by piece.

The way RBD is progressing in Houdini 19, to quote Mr CGwiki: RBD is copying vellum. It is becoming more SOP based, and very much so in the way it handles it’s constraint networks. Also, keep in mind that sometimes the SOP RBD tools work better without packing the geometry.

One easy way to set up your constraints before they enter your solver, is to use a RBD constraint property SOP.This node can help you set up your attribute constraints for bullet simulations.

Can I Use RBD To Influence Other Types of Simulations?

Absolutely! In order to make your final simulation look realistic, you’ll need to combine it with particles, dust, more pyro, and other simulation types.

But you might want to consider making the moving pieces of your RBD simulation be the emitters for your particle and pyro simulations. This will allow the other simulations to inherit the velocity from the pieces and move with the debris accurately. It can also make the individual pieces look smokey or hot because they are emitting steam.

When using fully simulated RBD debris as an emitter , keep in mind you might not need all of it. Delete the pieces and chunks you don’t need, and then work with the simulations from there.

You might also have to advect RBD pieces with other simulation types as well. Mainly with the velocity fields generated from them. Let’s use this example. Let’s say you want to simulate a tornado going through a building, and the pieces of the building being sucked up into the tornado. The tornado itself will probably be a pyro simulation. This means you will have a velocity attribute generated from this simulation. Blast away all the other attributes, and then plug that vel attribute into the solver of your scattered debris pieces. Then dive inside, and drop down a node that will allow you to advect the debris up into the air. This can be done with a POP advect by volumes.

Other Facts About RBD

Here are some further breakdowns of RBD and tips and tricks you can use to help you and your simulations.

Let’s talk more about fracturing.

Glass fractures, which are now generated with the RBD Material Fracture SOP, follow a set of rules. This fracture type follows the exact fractography of glass. The fracturing is customized by impact points which generate the starting area of the cracks. These cracks radiate outwards using voronoi methods.
Wood fracturing is also generated with the RBD Material Fracture SOP. The main features of wood in reality is its defined grains and splinters in parallel directions. Then the method uses booleans to calculate how the results fracture.

A few new tools for RBD in the past 2-3 years have also helped increase the functionality in RBD. Especially when it comes to fracturing. ¾ of the battle when you are setting up RBD sim is the emitter.

The RBD Recursive Fracture SOP is one of these tools. It works in combination with the Invoke SOP, which works along with TOPs. This node uses a combination of both Voronoi and Boolean Fractures.

The second input and output of the RBD Material Fracture SOP is for the constraints.

Houdini has also made the use of constraints easier. They have integrated constraints into the fracturing process. Some constraints are immediately created upon fracturing. You can also use the RBD Convert Constraints SOP to rewire constraints based on new fracture points in your geometry. As well as preserve tagged connections.

Now, inside the RBD solver, collisions can be tricky to work with. So let’s talk about how the static object DOP samples objects. There are different modes for sampling collisions. The higher resolution the sample size, the better and more accurate your collisions will be. However, your simulation times will increase. So if you are working with a high resolution collider and voxel size, cache it out.

There are different sampling modes inside the static object. One of them is Volume Sample, which samples volumes. (Also probably the best method) After choosing which method you’d like then focus on the division method. The division method you can view as the resolution of the volume. The lower the value, the higher the res. The higher the value, the lower resolution you will sample. You can also set the proxy volume path if you wish to customize it.

One thing to note is that is that with the new and old RBD solver there are a few differences. The new Bullet solver does not handle volumes for collisions correctly. Whereas the old systems can.

New to Houdini is Guided Simulations. Guided simulations allow for you to drive the motion of high resolution geometry with low resolution geometry. Let’s say you have a very high resolution and complex piece of geometry that you would like to destroy. However, RBD might not be able to handle it. So by lowering the resolution and using a guided simulation, you can avoid some major simulation issues, and long simulation times.

You can create guided simulations by turning the Guided Simulation tab on the solver, and then wire your high res geometry into the third input of the solver. The low res geometry gets plugged into the first input. This whole process sounds a lot more intimidating than it looks, but speaking from experience, it's very easy. (Thank you Tim for letting me know I was wasting my time by doing it the wrong way. ;))

The guided simulation tab also has several useful features. You can specify the frame range, add VEX expressions, or customize the guide strength in particular areas of the simulation. There is also a guide release so you can choose when the geometry will stop following the simulation forces.

There is also a RBD Deform Pieces SOP you can use. This handles your high resolution geometry and bends it like metal using the proxy geometry. This node looks for constraints to see which pieces are still attached to the fractures. Then it takes the pieces that are still attached together, and smooths them.

Now for some helpful tips:

- Fracture per Piece is useful if you would like to target and create custom cutters to specific pieces. This will help you cut up the pieces into finer details before being fractured.

- The RBD Deforming to Animated Geometry Node unpacks geometry and proxy geometry. Then it freezes it at a reference frame, repacks it , and animates the packed transformations by extracting the transforming from the incoming deformed geometry. This node is great if you need to offset the timing of a RBD simulation, and add more collisions into the simulations.

- The RBD Disconnected Faces SOP determines if the connected faces in the geometry have become separated. This can help diagnose issues in glasses fractures, or fractures where you have a lot of overlapping interior detail. The overlapping fractures can leave holes in the geometry before you simulate it. This can cause issues in rendering and simulating.

- The best way to scatter an object is by taking the whole model and cutting it up into large chunks(You can use a Cookie SOP for this), and then breaking it down into smaller pieces from there. Before making the smaller pieces, place each large chunk in groups and then merge the chunks back together. This will make the pieces better to manipulate.

- One way to make groups out of disconnected chunks is by using the Connectivity SOP followed by the
Partition SOP. Then create the groups.
There are three main constraint types in Houdini. Glue, Soft Body, and Hard constraints.

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