Houdini 16.5 Nodes Dynamics nodes

Source Volume dynamics node

A microsolver that imports and directly applies SOP volume data.

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The Source Volume DOP is a microsolver used in building larger fluid simulations. The Fluid Solver and Smoke Solver DOPs allow microsolvers to be added before or after the main solver step to extend or tweak the simulation. Alternatively, advanced users may attempt to build an entire new solver out of microsolvers.

The Source Volume DOP imports volumes and binds these to the specified DOP fields. Any combination of volumes can be loaded and mapped. A set of default combinations are presented as presets. These work together with the Fluid Source operator. Multiple Volume Source DOP microsolvers can be used to perform specific tasks, such as applying sources, sinks, or pumps directly from volumes created in SOPs, instead of relying on DOP specified relationships between objects.

Note

If the Smoke Object that the Source Volume operates on has instancing enabled, the sampled volumes are separated based on volumename_instancenumber. The volume name is specified under SOP To DOP Bindings and the current instance number is fetched from the DOP object being processed.

This object should have data associated with it under the name init_cluster. The init_cluster name is unique to every object and is used to fetch the correct volume for sourcing. This data can be generated using the Cluster SOP.

If the correct volume is not found, nothing is sourced. If the init_cluster number doesn’t match the correct volume to source from, a different volume might get used.

Parameters

Initialize

Configures the Source Volume microsolver according to the selected preset. Helps clarify and visualize the volume-to-field relationships.

Volume Path

The volume sample location. The specified node should contain the SOP volumes.

Activation

Controls if the source will be applied or not. Mostly useful for liquid sources that can’t be smoothly feathered in or out by controlling the scale source volume.

Scale Source Volume

Controls the scale of the (scalar) volume specified as a source volume to be added to scalar field X.

Scale Temperature

Controls the scale of the (scalar) volume specified as a temperature volume to be added to scalar field Y.

Scale Velocity

Controls the scale of the (vector) volume specified as a velocity volume to be added to vector field X,Y, and Z.

Use Object Transform

Takes into account animated objects.

Volume operation

Source Volume

Controls how to add or blend the SOP source volume on to the specified DOP field.

Temperature Volume

Controls how to add or blend the SOP temperature volume on to the specified DOP field.

Velocity

Controls how to add or blend the SOP velocity volume on to the specified DOP field.

"Blended average" is a very non-realistic pump designed to to make it easy to blend in a desired goal velocity without overshooting or hard-constraint effects. It blends the speed (rather than velocity) of the source velocity with the current fluid velocity. Basically, the node tries to match the speeds of the source and the simulated fluid.

Target Speed

Available when Velocity is "Blended Average". When the fluid is moving faster than this (m/s in in world space), the node will stop adding velocity.

Target Influence

Available when Velocity is "Blended Average". A per-frame normalized value, 0..1, where 0.5 means blend 0.5 times the source speed into the fluid velocity until the target speed is reached.

Max Acceleration

Available when Velocity is "Blended Average". The maximum percentage of Target speed the node can add per frame. After the amount of speed to add has been computed through the Target influence, that delta is then clamped to targetspeed * max_acceleration.

Particle Operation

Source Particles

Add the points from the SOP specified by Volume Path and the point group specified in Source Group.

Source Group

The point group from which to source particles.

Particle Fluid Object

The Particle Fluid Object node for the simulation to be affected.

Source Volume uses this node to obtain information such as fluid particle attributes, parameters from the Particle Fluid Object node, etc.

Time Offset Scale

Offset any new particles by adding their scaled velocity multiplied by a random portion of the current timestep. This can help ensure the emitted stream of particles appears regular even at lower simulation substeps.

Time Offset Seed

The seed used to randomly offset the particles by their velocities.

Life Expectancy

If enabled, set the particles' life attribute to control how long the particle will live. If sourcing particles to the FLIP Solver, then Apply POP Solver and Reap Particles must be enabled on the solver for this attribute to have an effect.

Life Variance

Particles will live the number of seconds in Life Expectancy, plus or minus this number of seconds. Use 0 for no variance.

Kill Inside

Kill any particles within the specified DOP field SDF. If Source Particles is enabled, only particles added in the current timestep are eligible to be killed. If Source Particles is disabled, all particles are eligible to be killed.

Stream Name

Any emitted particles will be placed in this group.

Masks

Field to multiply the sourced volume with. By enabling Absolute only parts of the source volume with a Mask Field value lower then 0 are being considered (but not multiplied).

SOP To DOP Bindings

Volumes

Specifies the SOP volume name to look for. In the case of Temperature and Density this must be a scalar volume, such as X. Velocity must be of a type vector, such as X.X, X.Y, X.Z.

Add To Field

Controls what field to add the sampled volume to. Source and Temperature volumes should be added to a scalar field. Velocity should be added to a vector field.

Instancing

Instancing

When performing clustered simulations it is important that each cluster receives its own individual sources. This is done by affixing the cluster number to the named volume that is sourced.

This option controls if this affixing of source names is performed. Auto-detect will attempt to determine it based on the smoke object’s instancing setting. However, some times one wishes to use the same source across many instances, in which cases this should be overridden to Off.

Clear

Fields to clear

A space-separated list of fields to clear after sourcing operations. Not clearing tempvel or source enables visualizing them later on.

Inputs

First Input

This optional input can be used to control which simulation objects are modified by this node. Any objects connected through this input and which match the Group parameter field will be modified.

If this input is not connected, this node can be used in conjunction with an Apply Data node, or can be used as an input to another data node.

All Other Inputs

If this node has more input connectors, other data nodes can be attached to act as modifiers for the data created by this node.

The specific types of subdata that are meaningful vary from node to node. Click an input connector to see a list of available data nodes that can be meaningfully attached.

Outputs

First Output

The operation of this output depends on what inputs are connected to this node. If an object stream is input to this node, the output is also an object stream containing the same objects as the input (but with the data from this node attached).

If no object stream is connected to this node, the output is a data output. This data output can be connected to an Apply Data DOP, or connected directly to a data input of another data node, to attach the data from this node to an object or another piece of data.

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.

Examples

The following examples include this node.

fieldforce Example for Field Force dynamics node

This example demonstrates the use of the Field Force DOP. It shows how to use a particle system to blow around smoke.

DensityViscosity Example for FLIP Solver dynamics node

This example demonstrates two fluids with different densities and viscosities interacting with a solid object.

FlipColumn Example for FLIP Solver dynamics node

This example demonstrates how a mixture of fluid colours can have their colour changed by a collision with a static object.

SpinningFlipCollision Example for FLIP Solver dynamics node

This scene shows how to create FLIP fluids based on the velocity of geometry by generating new particles from points scattered on the original geometry based on the velocity vectors. It also shows how to set up the original geometry to act as a collision object for the fluid.

VariableViscosity Example for FLIP Solver dynamics node

This example demonstrates interaction between three fluids of varying viscosity and a moving collision object.

EqualizeLiquid Example for Gas Equalize Volume dynamics node

This example demonstrates how the Gas Equalize Volume dop can be used to preserve the volume in a fluid simulation.

UpresRetime Example for Gas Up Res dynamics node

This example demonstrates how the Up Res Solver can now be used to re-time an existing simulation. The benefit of this is that one can simply change the speed without affecting the look of the sim. On the up-res solver there is a tab called Time. The Time tab offers various controls to change the simulation’s speed.

AdvectByVolume Example for POP Advect by Volumes dynamics node

This example demonstrates how to use POP Advect by Volumes to advect particles using the velocity from a smoke simulation.

Open CL smoke Example for Smoke Object dynamics node

Demonstrates a simple Open CL accelerated smoke sim that can be used as a starting point for building optimized GPU accelerated smoke sims. See the Use OpenCL parameter on the Smoke solver.

For fastest speeds, the system needs to minimize copying to and from the video card. This example demonstrates several methods for minimizing copying.

  • Turns off DOPs caching. Caching requires copying all the fields every frame. Useful if you want to scrub and inspect random fields, not if you want maximum speed.

  • Only imports density to SOPs. This means copying only one field from the GPU to CPU each frame.

  • Saves to disk in background. This gives you the best throughput.

  • Uses a plain Smoke solver.

Displaying the simulated output in the viewport requires a GPU → CPU → GPU round trip, but this is required in general to support simulating on a card other than your display card.

ColourAdvect Example for Fluid Source geometry node

This example demonstrates how you can use the Fluid Source SOP to source and advect colours from an additional volume into a smoke simulation.

CoolLava Example for Fluid Source geometry node

This example demonstrates how to cool Lava using the Cool Within Object shelf tool.

See also

Dynamics nodes