Houdini 16.5 Nodes Dynamics nodes

RBD Point Object dynamics node

Creates a simulation object at each point of some source geometry, similarly to how the Copy surface node copies geometry onto points.

On this page

The geometry for each of these objects depends on the instance attribute value of the point. The Position of the simulation object is set so that it is at the location and has the orientation of the source point.

This DOP can also inherit any other attributes from the source point. If the attribute names match the names of parameters in the RBD State data, the values can be used to set up these other aspects of the RBD Object (such as mass, bounce, and glue strength). Inherited attributes are imported as Position data, rather than Geometry data. They can be retrieved using DOP Import Records SOP.

Note

The angular velocity attribute is w, not angvel.

Using RBD Instanced Objects

  1. Select the geometry whose points will be used for instancing.

  2. Click the RBD Instanced Objects tool on the Rigid Bodies tab.

  3. Select the geometry to instance at each point.

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
v Point Vector

Defines a per-point velocity.

This can either be used to define the initial velocity of an RBD object if Inherit Velocity is selected, or the local deformation of the object is Use Per Point Velocities is turned on.

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

Parameter Use Menus

Many of the parameters for this DOP have a menu that controls how the values of that parameter are applied to the created RBD objects.

The channel name for each of these menus is the word use followed by the parameter name. So for the Position parameter, the menu channel name is uset.

Use Point Value

The value inherited from the source point is left unchanged. The value of the parameter on this DOP is ignored.

Add To Point Value

The value of this DOP parameter is added to the value inherited from the source point.

Override Point Value

The value of this DOP parameter is used for the RBD object. The source point attribute value is ignored.

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.

Point SOP Path

The SOP containing the geometry whose points will be used to create the RBD objects.

Geometry Path

By default, the instance point attribute (which can be defined with the Point SOP) is used to determine what geometry is instanced to each point.

This parameter can be used to override the instance attribute with a specific SOP path.

Extra Attributes

A mask that specifies which point attributes should be inherited from the source points. The position, rotation, velocity and angular velocity attributes are always inherited.

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.

Rotate to Normal

Determines if the position should be oriented to match the source point’s normal. If enabled, the point will determine an orientation using the normal and up vector attributes, if present. If no normal attribute is present, the velocity is used.

The behavior of this option matches that of the Copy SOP.

Use Particle Scale Attribute

Scales the geometry of each RBD object by the pscale attribute value on the source points.

Turning on this option causes each RBD object to make its own copy of its instanced geometry. Thus using this approach is much less efficient than creating a few differently scaled copies of the instanced geometry in SOPs, and having the source points instance those prescaled geometries.

Add Deformation Hint: Controls if the deformation hint_isgeometrydeforming is added to the geometry to store if the geometry is animated or not. Enabling this makes it easier for external code to know if geometry will be constant over time or not. However, it also prevents the geometry from instancing.

Create Active Objects

Sets the initial active state of the objects. An inactive object doesn’t 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.

Glue

Glue 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.

Collision Guide

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

This parameter controls the color of the guide geometry.

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. This box is always axis aligned in the DOP object’s local space, which is set by the position data.

Note

Use Object Transform bakes the object transform into the geometry’s transform, leaving the Position Data in world space. Turning this off causes the object transform to be send to the Position Data, which causes the object’s local space to be reoriented.

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 or VDB 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 or VDB From Polygons SOP. If it isn’t a true signed distance fields, unusual things may happen with RBD collisions.

Division Method

If Non Square is chosen, the specified size is divided into the given number of divisions of voxels. However, the sides of these voxels may not be equal, possibly leading to distorted simulations.

When an axis is specified, that axis is considered authoritative for determining the number of divisions. The chosen axis' size will be divided by the uniform divisions to yield the voxel size. The divisions for the other axes will then be adjusted to the closest integer multiple that fits in the required size.

Finally, the size along non-chosen axes will be changed to represent uniform voxel sizes. If the Max Axis option is chosen, the maximum sized axis is used.

When By Size is chosen, the Division Size will be used to compute the number of voxels that fit in the given sized box.

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.

Uniform Divisions

The resolution of the key axis on the voxel grid. This allows you to control the overall resolution with one parameter and still preserve uniform voxels. The Uniform Voxels option specifies which axis should be used as the reference. It is usually safest to use the maximum axis.

Division Size

The explicit size of the voxels. The number of voxels will be computed by fitting an integer number of voxels of this size into the given bounds.

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.

Tolerance

This specifies the tolerance used for ray intersections when computing the SDF. This value is multiplied by the size of the geometry and is scale invariant.

Proxy Volume

The geometry which will be used rather than the base geometry for computing the SDF. This can be a volume or VDB 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. This is always .simdata file format. Saving to a .bgeo extension will not save a .bgeo file.

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).

Bullet Data

Show Guide Geometry

Displays a visualization of the object’s collision shape, including the Collision Padding. This is useful for debugging problems with collision detection, but is typically slower than just displaying the object’s geometry.

Color

Specifies the color of the guide geometry.

Deactivated Color

Specifies the color of the guide geometry if the object is not moving and has been deactivated by the Bullet Solver.

Geometry Representation

The shape used by the Bullet engine to represent the object. The Show Guide Geometry option can be used to visualize this collision shape.

Convex Hull

Default shape for the object. The Bullet Solver will create a collision shape from the convex hull of the geometry points.

Concave

The Bullet Solver will convert the geometry to polygons and create a concave collision shape from the resulting triangles. This shape is useful when simulating concave objects such as a torus or a hollow tube. However, it is recommended to only use the concave representation when necessary, since the convex hull representation will typically provide better performance.

Box

Bounding box of the object.

Capsule

Bounding capsule of the object.

Cylinder

Bounding cylinder of the object.

Compound

Creates a complex shape consisting of Bullet primitives (including boxes, spheres, and cylinders). You will need to use the Bake ODE SOP.

Sphere

Bounding sphere of the object.

Plane

A static ground plane.

Create Convex Hull Per Set Of Connected Primitives

When Geometry Representation is Convex Hull, the Bullet Solver will create a compound shape that contains a separate convex hull collision shape for each set of connected primitives in the geometry.

AutoFit Primitive Boxes, Capsules, Cylinders, Spheres, or Planes to Geometry

When enabled, the object’s Geometry subdata will be analyzed instead of using the Position, Rotation, Box Size, Radius, and Length values.

When Geometry Representation is Box, Capsule, Cylinder, Sphere, or Plane, use the geometry bounds to create the shape.

Position

Position of the object shape in the Bullet world. Available when Geometry Representation is Box, Sphere, Capsule, Cylinder, or Plane.

Rotation

Orientation of the object shape in the Bullet world. Available when Geometry Representation is Box, Capsule, Cylinder, or Plane.

Box Size

The half extents of the box shape. Available when Geometry Representation is Box.

Radius

The radius of the sphere shape. Available when Geometry Representation is Sphere, Capsule, or Cylinder.

Length

The length of the capsule or cylinder in the Y direction. Available when Geometry Representation is Capsule or Cylinder.

Collision Padding

A padding distance between shapes, which is used by the Bullet engine to improve the reliability and performance of the collision detection. You may need to scale this value depending on the scale of your scene. This padding increases the size of the collision shape, so it is recommended to enable Shrink Collision Geometry to prevent the collision shape from growing.

This option is not available Plane geometry representations.

Shrink Collision Geometry

Shrinks the collision geometry to prevent the Collision Padding from increasing the effective size of the object.

This can improve the simulation’s performance by preventing initially closely-packed collision shapes from interpenetrating, and also removes the gap between objects caused by the Collision Padding.

When Geometry Representation is Box, Capsule, Cylinder, Compound, or Sphere, the radius and/or length of each primitive will be reduced by Shrink Amount.

When Geometry Representation is Convex Hull, each polygon in the representation geometry will be shifted inward by Shrink Amount.

This option is not available for the Concave or Plane geometry representations.

Shrink Amount

Specifies the amount of resizing done by Shrink Collision Geometry. By default, this value is equal to the Collision Padding so that the resulting size of the collision shape (including the Collision Padding) is the same size as the object’s geometry.

This option is not available for the Concave or Plane geometry representations.

Add Impact Data

When enabled, any impacts that occur during the simulation will be recorded in the Impacts or Feedback data. Enabling this option may cause the simulation time and memory usage to increase.

Enable Sleeping

Disables simulation of a non-moving object until the object moves again. The linear and angular speed thresholds are used to determine whether the object is non-moving. If the Display Geometry checkbox is turned off, you will see the color of the Guide Geometry change from the Color to the Deactivated Color.

Linear Threshold

The sleeping threshold for the object’s linear velocity. If the object’s linear speed is below this threshold for a period of time, the object may be treated as non-moving.

Angular Threshold

The sleeping threshold for the object’s angular velocity. If the object’s angular speed is below this threshold for a period of time, the object may be treated as non-moving.

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.

Bounce Forward

The tangential elasticity of the object. If two objects of bounce forward 1.0 collide, their tangential motion will be affected only by friction. If two objects of bounce forward 0.0 collide, their tangential motion will be matched.

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 objects created by this node are sent through the single output.

Locals

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.

Note

Most dynamics nodes have local variables with the same names as the node’s parameters. For example, in a Position node, you could write the expression:

$tx + 0.1

…to make the object move 0.1 units along the X axis at each timestep.

Examples

popswithrbdcollision Example for RBD Point Object dynamics node

Shows an RBD Simulation being attatched to a POP simulation to provide RBD style collisions to POPs.

The following examples include this node.

AutoFreezeRBD Example for Active Value dynamics node

This example shows a system for automatically detecting when RBD objects achieve a rest state and then turning off their active status. This will freeze them in place reducing computation time and jitter.

ConstrainedTeapots Example for Apply Relationship dynamics node

This example demonstrates how the Apply Relationship DOP can be used to create multiple constraints with the RBD Pin Constraint node.

SimpleField Example for Field Force dynamics node

This example demonstrates the use of the Field Force DOP. A group of RBD Objects are passed through a field which at first pulls the together, and then pulls them apart as they advance through the field.

MaskedField Example for Mask Field dynamics node

A Uniform Force is applied to a number of RBD Objects to demonstrate how the Mask Field can be used to define a region where the force will be applied.

ChoreographedBreakup

This example shows how one can control the break up of any glued object through the use of the RBD State node.

A torus, composed of spheres, is glued together. An additional sweep plane is defined. Any sphere which ends up on the wrong side of the sweep plane is broken off the torus and left to bounce on its own. This lets the break up of the torus to be controlled over many frames.

popswithrbdcollision Example for RBD Point Object dynamics node

Shows an RBD Simulation being attatched to a POP simulation to provide RBD style collisions to POPs.

Dynamics nodes