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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.
The Whitewater Solver DOP is usually a superior approach for non-oriented, ballistic whitewater effects.
This operator modifies the
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.
This is activation of the node as a whole. You can’t use this parameter to deactivate the node for certain particles.
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.
The source of the liquid simulation volumes. The context geometry refer to the inputs of this DOP Network.
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
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.
Specifies the SDF value for the top of the liquid surface. Decreasing this value will make objects float deeper within the liquid.
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.
The buoyancy force applied to any particles below the surface as specified by the Isosurface and Falloff Distance.
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.
A uniform scale to apply to the velocity field before any of the force applications.
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.
In Update Velocity mode, the amount to mix in the field velocity every timestep.
How to re-orient particles:
Instantaneously face the desired orientation. Useful for initializing orientations, but jerky if applied in animation.
Re-orient the particle but cap the total it can spin to a maximum number of degrees per second.
Applies a torque on the particle for it to spin into the desired direction.
Scales the torque applied to orient the particle’s Align Reference to match the liquid surface normal.
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.
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.
A scale for the vorticity of the liquid simulation’s velocity field, used to spin particles.
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.
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
style expressions in VEX are evaluated with respect to. By
., 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.
The 0-1 amount of float forces applied to the particle.
The goal alignment direction for the particle, corresponding the surface normal of the liquid surface.
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.
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.
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.
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.
In this case, this value is set to the name of the relationship the data to which the data is being attached.
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.
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.
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.
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.
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.
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).
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.
This value is the inverse of the TIMESTEP value. It is the number of timesteps per second of simulation time.
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
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).
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).
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).
This string contains a space separated list of the unique object identifiers for every object being processed by the current node.
This string contains a space separated list of the names of every object being processed by the current node.
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 == $OBJCT should
always be used. This value will be zero if the node does not process
objects sequentially (such as the Group DOP).
This value is the simulation frame (see variable SF) at which the current object was created.
This value is equivalent to using the dopsttoframe expression on the OBJCT variable. This value will be zero if the node does not process objects sequentially (such as the Group DOP).
This is a string value containing the name of the object being processed.
Object names are not guaranteed to be unique within a simulation. However, if you name your objects carefully so that they are unique, the object name can be a much easier way to identify an object than the unique object identifier, OBJID.
The object name can
also be used to treat a number of similar objects (with the same
name) as a virtual group. If there are 20 objects named "myobject",
strcmp($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).
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.
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.