Houdini 16.5 Nodes Dynamics nodes

POP Float by Volumes dynamics node

A POP node that floats particles on the surface of a liquid simulation.

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The Float by Volumes POP is designed to make it easy to float oriented particles on the surface of a liquid simulation for debris or other effects. Because the particles are oriented, geometry instanced onto the particles with the Instance POP will appear to float properly in the liquid. This DOP applies several operators to the particle system:

Often the liquid simulation will be simulated as a separate pass and the velocity fields read off disk. However, the particles can be live-linked to an existing simulation.

Note

The Whitewater Solver DOP is usually a superior approach for non-oriented, ballistic whitewater effects.

This operator modifies the force, vel, P, torque, w, targetw, spinresist, and orient attributes.

Parameters

Activation

Turns this node on and off. The node is only active if this value is greater than 0. This is useful to control the effect of this node with an expression.

Note

This is activation of the node as a whole. You can’t use this parameter to deactivate the node for certain particles.

Group

Only affect a group of points (created with, for example, a Group POP or Collision Detection POP) out of all the points in the current stream.

Ignore Mass

Ignores any mass on the input particles.

Since forces are stored as force rather than accel (acceleration), this is done by multiplying the force by the mass attribute. This will then be canceled out by the solver.

airresist will also be similarly multiplied.

Ignoring mass ensures that small pieces of an RBD object move at the same speed as big pieces. This makes for a more controllable simulation.

Field Source

The source of the liquid simulation volumes. The context geometry refer to the inputs of this DOP Network.

SOP

The SOP path which has an SDF volume and three volumes to be interpreted as the x, y, and z components of the velocity field. These are often the result of importing the surface and velocity fields from a liquid simulation. You can directly link to a fluid simulation with something like /obj/AutoDopNetwork:smoke/vel.

DOP Object

The name of a DOP Object in the same simulation as this.

Surface Field Name

The name of the surface SDF field that tracks the interior of the liquid in the liquid simulation.

Velocity Field Name

The name of the velocity fields that contain the velocity of the liquid simulation.

Floating

Isosurface

Specifies the SDF value for the top of the liquid surface. Decreasing this value will make objects float deeper within the liquid.

Falloff Distance

The scale of any operators applied by this POP will start falling off at this distance below the surface. Below this depth the particles are fully floating.

Buoyancy

The buoyancy force applied to any particles below the surface as specified by the Isosurface and Falloff Distance.

Advection

Advection Type

There are several different ways to update a particle system in response to an external velocity field. They are broadly categorized as Force, Velocity, and Position updates.

The Update Force will adjust the acceleration of the particles according to the force scale, and the velocity field. This acts like the Force POP.

Update Velocity directly changes the velocity of the particles by blending in the velocity field, causing them to swiftly match the velocity field.

Update Position directly moves the particle positions. This is useful as it allows the particles own velocity to be considered independently of the bulk fluid velocity. Update Position is similar to what the Gas Advect DOP would perform.

Velocity Scale

A uniform scale to apply to the velocity field before any of the force applications.

Air Resistance

How strong of an influence to have on the particle. Higher values will cause it to match the wind velocity faster. This is also used to do a weighted average when competing winds are applied to the same particle.

Velocity Blend

In Update Velocity mode, the amount to mix in the field velocity every timestep.

Spin

Type

How to re-orient particles:

Immediate

Instantaneously face the desired orientation. Useful for initializing orientations, but jerky if applied in animation.

Turn

Re-orient the particle but cap the total it can spin to a maximum number of degrees per second.

Spin

Applies a torque on the particle for it to spin into the desired direction.

Align Scale

Scales the torque applied to orient the particle’s Align Reference to match the liquid surface normal.

Align Resistance

How strong an influence the alignment force should have on the particle. Higher values will cause it to orient to the surface normal faster.

Align Degrees Per Second

How fast, in degrees per second, that the particle’s Align Reference should orient to match the liquid surface normal.

Align Reference

The direction in particle space that should be aligned to the liquid surface normal. For example, a value of 0,1,0 will make the Y-axis of the particle float upright on the surface.

Spin Scale

A scale for the vorticity of the liquid simulation’s velocity field, used to spin particles.

Spin Resistance

How strong an influence the vorticity force should have on the particle. Higher values will cause it to match the field vorticity faster.

Spin Angular Velocity Blend

The amount to mix the field vorticity into the particle angular velocity every timestep.

Bindings

Geometry

The name of the simulation data to apply the POP node to. This commonly is Geometry, but POP Networks can be designed to apply to different geometry if desired.

Evaluation Node Path

For nodes with local expressions, this controls where ch() style expressions in VEX are evaluated with respect to. By making this ., you can ensure relative references work. It is important to promote this if you are embedding a node inside an HDA you are also exporting the local expressions.

Delete Internal Attributes

To save memory, this POP deletes internal particle attributes that can be useful as debugging aids or for further processing, in which case they should be removed from this list.

floatscale

The 0-1 amount of float forces applied to the particle.

floatdir

The goal alignment direction for the particle, corresponding the surface normal of the liquid surface.

Inputs

First Input

This optional input has two purposes.

First, if it is wired to other POP nodes, they will be executed prior to this node executing. The chain of nodes will be processed in a top-down manner.

Second, if the input chain has a stream generator (such as POP Location, POP Source, or POP Stream), this node will only operate on the particles in that stream.

Outputs

First Output

The output of this node should be wired into a solver chain.

Merge nodes can be used to combine multiple solver chains.

The final wiring should go into one of the green inputs of a full-solver, such as POP Solver or FLIP Solver.

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 the data 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