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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
-
Select the geometry whose points will be used for instancing.
-
Click the
RBD Instanced Objects tool on the Rigid Bodies tab. -
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
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.
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 Representation
Chooses between colliding points against volume or colliding
edges against volume. Optionally, the point attributes 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). 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.
Collisions
Volume
Surface
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.
Bullet Data
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
ConstrainedTeapots Example for Apply Relationship dynamics node
SimpleField Example for Field Force dynamics node
MaskedField Example for Mask Field dynamics node
popswithrbdcollision Example for RBD Point Object dynamics node
