Houdini 17.0 Nodes Dynamics nodes

Bullet Data dynamics node

Attaches the appropriate data for Bullet Objects to an object.

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The Bullet Data DOP takes a simulation object and attaches the data which is needed for it to be used by a Bullet Solver.

This DOP tells the Bullet engine how a certain object should be interpreted.

Parameters

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.

Inputs

First

The simulation objects to turn into Bullet objects by attaching the appropriate data.

Outputs

First

The simulation objects which were passed into this node are output with the data required for them to be considered Bullet Objects attached.

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

The following examples include this node.

FluidGlass Example for Particle Fluid Solver dynamics node

This example demonstrates how to get a smooth fluid stream to pour into a glass.

Dynamics nodes