Houdini 19.5 Nodes Dynamics nodes

Gas Build Collision Mask From Pieces dynamics node

A microsolver that builds a collision field for fluid simulations from instanced pieces.

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Since 19.5

This microsolver builds collision and collision velocity fields for use in fluid simulations, similar to Gas Build Collision Mask. Whereas Gas Build Collision Mask assembles these fields from other affector objects in the simulation, this DOP instances packed colliders from outside the simulation. This allows you to place and animate individual pieces by controlling the instance geometry, without the need to build the aggregate surface and velocity fields ahead of time.

To build packed collider sets from a rigid body simulation, you can use the Pyro Source Pack SOP with Input set to Rigid Body Pieces. This will generate a packed set for each piece (containing the piece’s signed distance field), which can then be instanced using the simulation points.

Each point of Instance Geometry corresponds to a collider piece. Point attributes of this geometry are used to identify the piece, its location, and to determine its motion. The specific attributes are listed below.

name

Name of the piece that’s being instanced. This string is matched against the name attribute on the packed colliders to identify the piece. This attribute is required.

P

Position of this instance in world space. This attribute is required.

orient

Orientation of this instance in world space.

pivot

The local pivot point for this piece (rotations are performed about this point).

v

Linear velocity of the piece.

w

Angular velocity of the piece.

Parameters

Collision Field

The scalar field of this name is set to be the (negated) signed distance field for the colliders. No work is done by this node if the Collision Field does not exist.

Collision Velocity Field

A vector field which is set to the velocity of the closest instanced piece at each point. If a field name is specified and the field does not exist, an attempt will be made to create it.

Stencil Field

A scalar field to use as a stencil for where to perform this node’s computations. Voxels whose stencil value strictly exceeds 0.5 will have the operation applied, while the rest will be left unchanged.

Note

If a stencil field isn’t provided or does not exist, the operation will be performed everywhere.

Clear Mask

When this toggle is enabled, incoming values in the Collision Field are ignored. If Clear Mask is disabled, the output fields are updated only in voxels where an instanced piece is closer than the incoming written value in Collision Field.

Note

This does not clear voxels that are masked out by the Stencil Field.

Starting Value

Starting value of the Collision Field when Clear Mask is turned on. Voxels that have no collider closer than this value will be assigned zero velocities (in the Collision Velocity Field). For example, if Starting Value is -2, then voxels that are more than 2 units outside all instanced pieces will be assigned zero collision velocities.

Reference Frame

The Input will be evaluated at this frame to get the packed colliders. If the packed sets are generated by the Pyro Source Pack SOP, then the value of this parameter should match Reference Frame on that node.

Input

Specifies where to fetch the packed colliders from. Context geometries refer to SOP nodes attached to inputs of the containing DOP network, while the SOP option can be used to select an arbitrary geometry node.

Surface SOP

Path of the SOP node containing the packed colliders. This is used when Input is set to SOP.

Instance Geometry

Name of the geometry data containing instance points. Each point of this geometry corresponds to a collider piece.

Surface Volume

Name of the signed distance field in each packed collider. If the packed sets are generated by the Pyro Source Pack SOP, then the value of this parameter should match Collision SDF on that node.

Inputs

All Inputs

Any microsolvers wired into these inputs will be executed prior to this node executing. The chain of nodes will thus be processed in a top-down manner.

Locals

channelname

This DOP node defines a local variable for each channel and parameter on the Data Options page, with the same name as the channel. So for example, the node may have channels for Position (positionx, positiony, positionz) and a parameter for an object name (objectname).

Then there will also be local variables with the names positionx, positiony, positionz, and objectname. These variables will evaluate to the previous value for that parameter.

This previous value is always stored as part of the data attached to the object being processed. This is essentially a shortcut for a dopfield expression like:

dopfield($DOPNET, $OBJID, dataName, "Options", 0, channelname)

If the data does not already exist, then a value of zero or an empty string will be returned.

DATACT

This value is the simulation time (see variable ST) at which the current data was created. This value may not be the same as the current simulation time if this node is modifying existing data, rather than creating new data.

DATACF

This value is the simulation frame (see variable SF) at which the current data was created. This value may not be the same as the current simulation frame if this node is modifying existing data, rather than creating new data.

RELNAME

This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP).

In this case, this value is set to the name of the relationship to which the data is being attached.

RELOBJIDS

This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP).

In this case, this value is set to a string that is a space separated list of the object identifiers for all the Affected Objects of the relationship to which the data is being attached.

RELOBJNAMES

This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP).

In this case, this value is set to a string that is a space separated list of the names of all the Affected Objects of the relationship to which the data is being attached.

RELAFFOBJIDS

This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP).

In this case, this value is set to a string that is a space separated list of the object identifiers for all the Affector Objects of the relationship to which the data is being attached.

RELAFFOBJNAMES

This value will be set only when data is being attached to a relationship (such as when Constraint Anchor DOP is connected to the second, third, of fourth inputs of a Constraint DOP).

In this case, this value is set to a string that is a space separated list of the names of all the Affector Objects of the relationship to which the data is being attached.

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.

See also

Dynamics nodes