RBD Object

Creates an RBD Object from SOP Geometry.

The RBD Object DOP creates an RBD Object inside the DOP simulation. It creates a new object and attaches the subdata required for it to be a properly conforming RBD Object.

Using RBD Objects

Select the geometry to convert to an RBD Object.
Click the RBD Object tool on the Rigid Bodies tab.

Note

Once you convert geometry to an RBD object you can only transform, rotate, and scale it when it is on the first frame.

Attributes

You can create attributes on the RBD object’s geometry to influence its behavior. Most of these attributes allow fine-tuning of the RBD by overriding default values set in this node.

Name	Class	Type	Description
friction	Point	Float	Defines a per-point friction. This will override the friction set in the physical parms page.
dynamicfriction	Point	Float	Defines a per-point dynamic friction. This will override the dynamic friction set in the physical parms page.
bounce	Point	Float	Defines a per-point bounce value. This will override the bounce value set in the physical parms page.
nopointvolume	Point	Integer	Points with this attribute set to true will not be included in the collision information when point sampling is chosen.
noedgevolume	Vertex	Integer	Edges with this attribute set to true will not be included in the collision information when edge sampling is chosen.

Parameters

Creation Frame Specifies Simulation Frame	Determines if the creation frame refers to global Houdini frames (`$F`) or to simulation specific frames (`$SF`). The latter is affected by the offset time and scale time at the DOP network level.
Creation Frame	The frame number on which the object will be created. The object is created only when the current frame number is equal to this parameter value. This means the DOP Network must evaluate a timestep at the specified frame, or the object will not be created. For example, if this value is set to 3.5, the Timestep parameter of the DOP Network must be changed to `1/(2*$FPS)` to ensure the DOP Network has a timestep at frame 3.5.
Number of Objects	Instead of making a single object, one can create a number of identical objects. You can set each object’s parameters individually by using the `$OBJID` expression.
Object Name	The name for the created object. This is the name that shows up in the details view and is used to reference this particular object externally. Note While it is possible to have many objects with the same name, this complicates writing references, so it is recommended to use something like `$OBJID` in the name.
Solve On Creation Frame	For the newly created objects, this parameter controls whether or not the solver for that object should solve for the object on the timestep in which it was created. Usually this parameter will be turned on if this node is creating objects in the middle of a simulation rather than creating objects for the initial state of the simulation.
SOP Path	The path to a SOP (or an Object, in which case the display SOP is used) which will be the geometry for this object. This parameter can also be a list of Object or SOP paths, and can include wild card specifications or operator groups or bundles. If multiple Objects or SOPs match this string, a separate simulation object will be created for each matching SOP.
Use Deforming Geometry	Causes the geometry for the object to be pulled from the chosen SOP at each timestep. If the SOP contains animated geometry, the RBD object’s geometry will also animate. If this option is used, the Use Point Velocity parameter of the RBD Solver should also be turned on to take into account the deformations when calculating collision responses.
Use Object Transform	The transform of the object containing the chosen SOP is applied to the geometry. This is useful if the initial location of the geometry is defined by an object transform. If you want to transfer an object whose movement is defined at the object level, you should use the Object Position DOP instead.
Create Active Object	Sets the initial active state of the object. An inactive object does not react to other objects in the simulation.
Display Geometry	Controls if the geometry is displayed in the viewport. Does not reset the simulation when it is changed.

Initial State

Position	Initial position in world space of the object.
Rotation	Initial orientation of the object. This is in RX/RY/RZ format.
Velocity	Initial velocity of the object.
Angular Velocity	Initial angular velocity of the object. This is the axis of rotation times the rate of rotation. Speed of rotation is measured in degrees per second, so a multiplier of 360 will cause the object to rotate once per second.
Inherit Velocity from Point Velocity	When one brings in a moving piece of geometry from an external source one does not always know the precise velocity and angular velocity. If this toggle is set, the point velocity attribute of the geometry will be used to calculate the estimated velocity and angular velocity of the object. This allows one to effect a smooth hand off even if the source geometry came from a sequence of geometries rather than a simulation.

Glue

Glue to Object	The name of an object to glue to. If this is blank, the object is glued to no other object and acts normally. If it is the name of another RBD Object which it mutually affects, this object becomes glued to the other object. Its relative position to the other object is maintained by the solver.
Glue Strength	The amount of accumulated force required to break a glue bond. A value of -1 will prevent the bond from ever breaking. A value of 0 will cause the bond to break with the first external force.
Glue Impulse HalfLife	The number of seconds for the glue impulse to decay by one half. Whenever a glued object gets hit, it accumulates a glue impulse force. This controls how fast that force decays.

Collisions

Volume

Use Volume Based Collision Detection

Turning on this option causes the RBD solver to use a volume representation of this object for collision detection.

The volume representation results in very fast collision detection and very robust results that are tolerant of temporary interpenetrations. The disadvantage is that a volume representation cannot be used to represent a flat object such as a grid, or a hollow sphere.

When this toggle is turned off, the collision detection is geometry-based rather than volume-based. In this case, the collision code will track the trajectories of moving objects over time to find out whether collisions occurred. This allows more accurate results than volume-based collision detection. For this to work, Cache Simulation must be enabled on the dop network.

Mode

Ray Intersect	Use ray intersection with the geometry to create an accurate volumetric representation of the geometry.
Meta Balls	Instead of using rays to determine if points are inside or outside, evaluate the metaball field. This should be used with Laser Scanning turned off on geometry that consists solely of metaballs.
Implicit Box	Calculate the bounding box for the geometry, and create a volumetric representation that precisely fills that bounding box.
Implicit Sphere	Calculate the bounding sphere for the geometry, and create a volumetric representation that precisely fills that bounding sphere.
Implicit Plane	Calculate the bounding box for the geometry, and create a volumetric representation that divides that box along its smallest axis. Everything below that plane is considered inside, and everything above is outside. This mode is primarily useful for creating ground planes or immovable walls.
Minimum	Use the distance to the surface or curve. If the Offset Surface is 0, no volume will be made. A positive offset surface will create just that - an offset volume around the object’s surface. This is useful for turning thin objects or wires into actual solids.
Volume Sample	The divisions are ignored in this mode, instead they are computed from the first volume primitive in the geometry. The computed divisions are chosen to match the voxel size of the source volume. The volume primitive is sampled raw and treated as a signed distance field. The assumption is that the source is the output of an Iso Offset SOP. If it isn’t a true signed distance fields, unusual things may happen with RBD collisions.

Divisions

Controls the creation of the volumetric representation of this object. This should be set fine enough to capture the desired features of the geometry.

Laser Scan

In laser scan mode the volumetric representation is built by sending rays along the primary axes. Only the closest and farthest intersections are used. The space between these two points is classified as inside, and the rest outside.

The laser scan mode will work even with geometry which has poorly defined normals, self intersects, or is not fully watertight. The disadvantage is that interior features can’t be represented as they are not detected.

When laser scanning is turned off, the volumetric representation is still built by sending rays along the primary axes. All intersections are found, however. Each pair of intersections is tested to see if the segment is inside or outside. This relies on the normal of the geometry being well defined (i.e., manifold, no self intersections), and the geometry being watertight. Complicated shapes with holes can be accurately represented, however.

Fix Signs

Even with the best made geometry, numerical imprecision can result in incorrect sign choices. This option will cause the volumetric representation to be post-processed to look for inconsistent signs. These are then made consistent, usually plugging leaks and filling holes.

This takes time, and can be turned off in cases where the volumetric representation is known to generate without problems.

Force Bounds

The Fix Signs method alone will smooth out, and usually eliminate, sign inversions. However, it is possible for regions of wrong-sign to become stabilized at the boundary of the volumetric representation. This option will force all voxels on the boundary to be marked as exterior. The Fix Signs method will be much less likely to stabilize incorrectly then.

Invert Sign

If you want a hollow box, one method is to build one box inside the other and not use Laser Scanning. A more robust method is to just specify the inner box and use sign inversion. This treats everything outside of the box as inside, allowing the more robust Laser Scanning method to be used.

Sign Sweep Threshold

After the fix signs process is complete there can still be inconsistent areas in the SDF. Large blocks can become stabilized and stick out of the SDF. A second sign sweep pass can be performed to try to eliminate these blocks.

The sign sweep threshold governs how big of a jump has to occur for a sign transition to be considered inconsistent. If the values of the sdf change by more than this threshold times the width of the cell, it is considered an invalid sign transition. The original geometry is then ray intersected to determine inside/outside and the result used to determine which sign is correct. The correct sign is then propagated forward through the model.

Max Sign Sweep Count

The sign sweeps are repeated until no signs are flipped (ie, all transitions are within the threshold) or this maximum is reached. Too low of a sign sweep threshold may prevent the process from converging. Otherwise, it tends to converge very quickly.

Offset Surface

A constant amount to offset the signed distance field by. This can be used grow the object slightly or shrink it. Note that it can’t be grown much beyond its original size or it will hit the bounding box of the signed distance field.

Show Collision Guide Geometry

The internal representation used for collision detection is converted to visible geometry. This is useful for debugging problems with collision detection.

Collision Guide Color

This parameter controls the color of the guide geometry.

Proxy Volume

The geometry which will be used rather than the base geometry for computing the SDF. This can be a volume in the case of Volume Sample mode to allow one better control over the cached data.

File Mode

Controls the operation for this object’s volume data.

Automatic	If a file with the specified name exists already, it is read from disk. Otherwise the volume is created based on the other parameters on this page, and the specified file is created on disk. This file will never be deleted automatically, even when exiting the application.
Read Files	The specified file is read from disk.
Write Files	The volume is created using the other parameters on this page, and is then written to the specified file on disk.
No Operation	The file is never read or written. The parameters on this page are used to create the volume.

File

The name of the file to access according to the choice of File Modes above.

Surface

Surface Representation	Chooses between colliding points against volume or colliding edges against volume. Optionally, the point attributes “nopointvolume” and “noedgevolume” may be added to the geometry to disable individual points/edges from participating in collision detection against a volume object. An edge is disabled if either of its endpoints is disabled.
Convert To Poly	This enables conversion of primitives (such as spheres) in the geometry into polygons. Only polygons are used for collision detection.
Triangulate	When this flag is turned on, polygons in the geometry are triangulated.
LOD	This controls the Level Of Detail of the triangulation. It is used to specify the point density in the U and V directions.
Add Barycenters	The barycenters of each polygon can be included in the collision detection as points or edges (connected to the vertices of the primitive).

ODE Primitive

Display ODE Primitive	Toggles the display of the ODE approximation, drawn in red wireframe.
Object Type	ODE uses simplified collision objects. This selects what collision object type will be used.
Automatically Fit Geometry	The parameters for the object type will be determined by a rough bounding calculation of the object geometry. This fit may be smaller or greater than the actual objects geometry.
Position	Offset of the object type relative to the center of the rbd object.
Rotation	Rx/Ry/Rz rotation angles for the object.
Box Size	Dimensions of the object if in Box mode.
Radius	The uniform radius used for sphere, tube, or capsule mode.
Cylinder Length	The length of the cylinder for tubes and capsules.

Physical

Compute Center of Mass	Determines if the center of the object should be found automatically from the object’s volumetric representation and glued sub-objects.
Center of Mass	If the center of mass is not computed automatically, this value becomes the center of the mass. The center of mass can be thought of as the pivot point about which the object will rotate.
Compute Mass	Determines if the mass will be calculated automatically from the object’s volumetric representation and glued sub-objects.
Density	The mass of an object is its volume times its density.
Mass	The absolute mass of the object.
Rotational Stiffness	When an object receives a glancing blow, it will often acquire a spin. The amount of spin acquired depends on the shape and mass distribution of the object, known as the inertial tensor. The Rotational Stiffness is a scale factor applied to this. A higher stiffness will make the object less liable to spinning, a lower value will make it more ready to spin.
Bounce	The elasticity of the object. If two objects of bounce 1.0 collide, they will rebound without losing energy. If two objects of bounce 0.0 collide, they will come to a standstill.
Friction	The coefficient of friction of the object. A value of 0 means the object is frictionless. This governs how much the tangential velocity is affected by collisions and resting contacts.
Dynamic Friction Scale	An object sliding may have a lower friction coefficient than an object at rest. This is the scale factor that relates the two. It is not a friction coefficient, but a scale between zero and one. A value of one means that dynamic friction is equal to static friction. A scale of zero means that as soon as static friction is overcome the object acts without friction.
Temperature	Temperature marks how warm or cool an object is. This is used in gas simulations for ignition points of fuel or for buoyancy computations. Since this does not relate directly to any real world temperature scale, ambient temperature is usually considered 0.

Outputs

First

The RBD object created by this node is sent through the single output.

Local variables

ST	This value is the simulation time for which the node is being evaluated. This value may not be equal to the current Houdini time represented by the variable T, depending on the settings of the DOP Network Offset Time and Time Scale parameters. This value is guaranteed to have a value of zero at the start of a simulation, so when testing for the first timestep of a simulation, it is best to use a test like `$ST == 0` rather than `$T == 0` or `$FF == 1`.
SF	This value is the simulation frame (or more accurately, the simulation time step number) for which the node is being evaluated. This value may not be equal to the current Houdini frame number represented by the variable F, depending on the settings of the DOP Network parameters. Instead, this value is equal to the simulation time (ST) divided by the simulation timestep size (TIMESTEP).
TIMESTEP	This value is the size of a simulation timestep. This value is useful to scale values that are expressed in units per second, but are applied on each timestep.
SFPS	This value is the inverse of the TIMESTEP value. It is the number of timesteps per second of simulation time.
SNOBJ	This is the number of objects in the simulation. For nodes that create objects such as the Empty Object node, this value will increase for each object that is evaluated. A good way to guarantee unique object names is to use an expression like `object_$SNOBJ`.
NOBJ	This value is the number of objects that will be evaluated by the current node during this timestep. This value will often be different from SNOBJ, as many nodes do not process all the objects in a simulation. This value may return 0 if the node does not process each object sequentially (such as the Group DOP).
OBJ	This value is the index of the specific object being processed by the node. This value will always run from zero to NOBJ-1 in a given timestep. This value does not identify the current object within the simulation like OBJID or OBJNAME, just the object’s position in the current order of processing. This value is useful for generating a random number for each object, or simply splitting the objects into two or more groups to be processed in different ways. This value will be -1 if the node does not process objects sequentially (such as the Group DOP).
OBJID	This is the unique object identifier for the object being processed. Every object is assigned an integer value that is unique among all objects in the simulation for all time. Even if an object is deleted, its identifier is never reused. The object identifier can always be used to uniquely identify a given object. This makes this variable very useful in situations where each object needs to be treated differently. It can be used to produce a unique random number for each object, for example. This value is also the best way to look up information on an object using the dopfield expression function. This value will be -1 if the node does not process objects sequentially (such as the Group DOP).
ALLOBJIDS	This string contains a space separated list of the unique object identifiers for every object being processed by the current node.
ALLOBJNAMES	This string contains a space separated list of the names of every object being processed by the current node.
OBJCT	This value is the simulation time (see variable ST) at which the current object was created. Therefore, to check if an object was created on the current timestep, the expression `$ST == $OBJCT` should always be used. This value will be zero if the node does not process objects sequentially (such as the Group DOP).
OBJCF	This value is the simulation frame (see variable SF) at which the current object was created. This value is equivalent to using the dopsttoframe expression on the OBJCT variable. This value will be zero if the node does not process objects sequentially (such as the Group DOP).
OBJNAME	This is a string value containing the name of the object being processed. Object names are not guaranteed to be unique within a simulation. However, if you name your objects carefully so that they are unique, the object name can be a much easier way to identify an object than the unique object identifier, OBJID. The object name can also be used to treat a number of similar objects (with the same name) as a virtual group. If there are 20 objects named “myobject”, specifying `strcmp($OBJNAME, "myobject") == 0` in the activation field of a DOP will cause that DOP to operate only on those 20 objects. This value will be the empty string if the node does not process objects sequentially (such as the Group DOP).
DOPNET	This is a string value containing the full path of the current DOP Network. This value is most useful in DOP subnet digital assets where you want to know the path to the DOP Network that contains the node.