Houdini 16.5 Nodes Dynamics nodes

Gas Up Res 2.0 dynamics node

Up-scales and/or modifies a smoke, fire, or liquid simulations.

On this page

Using Up-res

  1. Select the fluid container object you want to up-res.

  2. On the Fluid Containers shelf tab, click Up-res Container.

The tool creates a new DOP network containing the nodes for up-rezzing the selected container.

The up-res network

If you use the shelf tool to upres a container, it will set up the up-res container and solver for you automatically. The tool creates the nodes in a new DOP node network to keep the separation between the "base" network and the "up-res" version.

If you are setting up an up-res network from scratch, use the same object (Smoke object DOP or Fluid object DOP) you used in the base simulation. The Up Res solver works with all known container types. You should reference or copy the base simulation settings (such as the scale) but change the division size (resolution).

This solver makes use of various field subdata on the object. The required fields vary based on the simulation type.

  • With a smoke (pyro or smoke) base simulation, the object should have a scalar field density for the density of the smoke.

  • With a fire (pyro) base simulation, the object should have a scalar field heat for the flames and possibly density for the smoke.

  • With a liquid base simulation, the object should have a scalar field surface representing the liquid’s surface.

  • The object should have a vector field vel for the velocity at each voxel.

  • Optionally, the object can have a scalar field temperature for internal buoyancy and ignition calculations.

Previous versions

Houdini 12 introduced a new up-rezzing workflow. In versions prior to 12, an Up Res Object was used to represent the modified smoke, fire or liquid. Houdini 12 uses the default container types as input objects to an Up Res solver.

Parameters

Simulation

These parameters control how the up-rezzed simulation develops, based on the velocity field from the base simulation.

Use Lower Source Resolution

Instead of recomputing the base simulation’s source at the higher resolution, use the original source resolution, then resimulates everything else at the higher resolution. This is faster, and also preserves the "fuzziness" of the original source. Turn this off to recompute the source at the higher resolution, giving a tighter smoke effect.

Source From Imported Fields

Use the already simulated source fields for computing (for example) smoke dissipation or fire ignition. This is faster, but can give sampling artifacts. You should usually leave this turned off.

Tip

Copy and use the original source operators! Simply connect them to the 5th (sourcing) input of the Up Res Solver. This is done by default when using the Up Res Container shelf tool.

Note

What fields to supply the Up Res Solver with greatly depends on whether or not Source From Imported Fields is enabled. When disabled (default), the same fields used in the base simulation’s source calculations are required. If enabled, the base simulation’s already calculated fields are automatically loaded and linked so no initial fields are required. This could save time but also introduces possible artifacts sampling a (most likely) lower resolution simulation object.

Source Fields are most likely found when dealing with a smoke or fire simulation, in the form of the field not being considered the end result (like heat, density or the liquid’s surface.) Temperature is used to ignite fuel or control buoyancy but doesn’t get rendered as smoke. This field could be imported from the base simulation. Same applies to fuel, which is used to start a fire (together with temperature) but isn’t rendered as flames. This field could also be imported from the base simulation.

Initial Data

Parameters under this tab help control and specify the data used as a base for the Up Res simulation.

Fluid Type

Controls the naming and loading of fields for the simulation type specified.

Source

Where the object looks to find the base simulation. It can be a collection of named volumes in a SOP Path, or read directly from a .bgeo sequence.

Note

The base simulation’s Dop IO Node is generally used for fetching the necessary files.

Low Res Group

Specifies a specific group of geometry to import from.

Combustion

The parameters on this tab are similar to the ones found on the Pyro Solver. When using the shelf these are referenced if the solver found is a Pyro Solver and combustion is enabled. By referencing the parameters the same output can be guaranteed. See Show Simulation Links. By changing the parameters the actual combustion can be modified.

Shape

Similar to the Shape Operators found under the Pyro Solver’s Shape tab. Although the source velocity is used to steer behavior, turbulence can be added to add intrinsic detail. It is advisable not to add any forces at this stage.

Dissipation

By Default Dissipation is referenced if the solver found is a Pyro Solver. By referencing the parameters the same output can be guaranteed. See Show Simulation Links. Changing the dissipation parameters changes the decay of smoke (or the field specified).

Turbulence

Turbulence adds detail to the simulation using turbulent noise, applied at a small enough scale to avoid affecting the bulk motion of the smoke. The amplitude and position is defined through information sampled from the base simulation and set using the turbulence Source

Enable Turbulence

Enables turbulence.

Turbulence Scale

An overall scale to the added turbulence.

Source

Controls how the turbulence field is built. The turbulence field defines a per-voxel scale for the amount of turbulence to add to the upres simulation.

Constant

No special turbulence scale calculation is done. However, if a turbulence scale field is provided, it will be multiplied with the amplitude. This allows one to modulate the turbulence according to space.

Wavelets

A wavelet decomposition is performed on the low resolution velocity field and the power of the finest detail coefficients used as a multiplier for turbulence. This will only introduce turbulence where there existed some in the coarser smoke simulation, thereby keeping smooth areas smooth.

Curl

The magnitude of the curl of the velocity field is used to scale the turbulence. This matches the scale used for vortex confinement, for example, so will add extra turbulence where vortices are detected.

Compute Frequency Cutoff

Because the goal of the upres stage is not to affect the low resolution simulation, this automatic frequency cut off can have the solver properly determine what frequency will add detail without changing the base simulation’s behavior.

Blur Radius

Sharp changes in the turbulence amplitude will result in kinks in the resulting velocity field. The Blur Radius allows one to smooth out the turbulence field by the given world distance.

Noise Frequency

Specifies the size (swirl size / frequency) of the noise to be added. This can either be based on your original simulation or set manually.

Size

The size of the low resolution smoke. This is used only in calculating the cut off frequency.

Low Res Division Size

The division size of the low resolution smoke. This is used to determine the cutoff frequency for the turbulent noise.

Frequency Scale

The calculated frequency is multiplied by this scale. Values less than one will introduce lower frequencies (bigger swirls), possibly overwriting frequencies already present in the base simulation. However, because numerical diffusion has tended to dampen those frequencies already, it is often useful to dial this down to 0.7 to avoid their being an apparent frequency gap, or lacunarity, between the base sim and the upres sim.

Temporal Frequency

Controls how quickly the turbulence changes over time.

Absolute Frequency

Only available when Compute Frequency Cutoff is turned off. Let’s the user specify a frequency not related or based on the original simulation’s division size. It’s advisable to enable Compute Frequency Cutoff and control the frequency through the Frequency Scale parameter.

Noise Settings

Specifies noise type specifics.

Noise Type

What form of noise basis to use for the curl noise.

Turbulence

Controls how many octaves of noise are added. Each octave doubles the effective resolution of the grid, so this should be set to the number of doublings. For example, if the base smoke is 32^3 and the high res 128^3, there are two doublings so 2 octaves is necessary. If you have a frequency scale of 0.5, you will need an extra doubling to account for it. Turbulence above this amount will show up as chatter at the finest detail level, which may or may not be what you are looking for.

Roughness

The amount of influence added bands of Turbulence have, relative to the initial Frequency.

Attenuation

Controls the contrast.

Step Size

Controls the resolution of the curl function used to generate the noise. A lower value will lead to tighter spirals in the noise.

Turbulence Settings

Controls where turbulence is being applied. The original turbulence is first computed using the method specified under Turbulence Source. The resulting output is then mapped on to the fluid using the settings specified here.

Remap Turbulence Scale

The turbulence field is sent through this mapping to get the final amplitude. This allows one to sculpt the fall off or perform an edge detection.

Turbulence Range

The to be normalized min and max values of the computed turbulence.

Tip

Play with these settings! First figure out the right range, then remap the actual values. The turbulence field can be visualized with the parameters available under Visualization. The computed turbulence can take on any form but generally resides around a max value of 10-20.

Turbulence Field Ramp

Remaps the previously computed and normalized turbulence field.

Control Settings

The controls on this tab let you vary the amount of turbulence across the container based on the values in a field.

Control field

When enabled, the force exerted is scaled by the content of this field.

Control Influence

A scaling factor on the control field’s influence on the effect. A value of 0 makes the field have no influence.

Control Range

Map from this range of values in the control field.

Remap Control Field

Enables or disables the control field ramp.

Control field ramp

The ramp’s vertical axis is amount of turbulence and the horizontal axis is the value in the control field. For example, the default ramp’s shape, which is low on the left side and high on the right side, adds more turbulence where the density field is high.

Visualization

Turn this on to see a viewport visualization of the forces applied to the velocity field by this effect.

Relationships

Prior to Houdini 12, the Pyro solver used DOP relationships to associate sources, pumps, sinks, and collision geometry with a fluid container, using the Merge DOP and/or Apply relationship DOP to create the relationship. The preferred method in Houdini 12 and later is to use SOP networks to create sources, pumps, sinks, and collision geometry and import them using the Source volume DOP.

If you want to use the old relationship method to set up sources, sinks, etc., you can enable relationships using the parameters on this tab. By default, relationships are turned off, and the solver ignores relationship data.

You can use both methods (import SOP geometry and attach it to the solver’s "sourcing" input, as well as set up DOP object relationships). When relationships are enabled, the solver will combine the sources, sinks, etc. from both methods.

Enable Relationships

Use object relationship data to add sources, pumps, sinks, and collision geometry to the simulation (in addition to imported data connected to the sourcing input, if any).

Advanced

You should generally not need to change these parameters.

Frames Before Solve

Specifies the number of frames to wait before a full solve cycle is computed. On those frames only operations applied to the last solver input (sources) are computed. This enables the sourcing and resizing of data without actually computing a full solve on the maximum grid.

Rest

Auto Regenerate Rest Field

The turbulence needs a rest field so it tracks naturally with the smoke. One option is to use the rest field from the base simulation, in which case this should be turned off. If this is turned on, a new rest field is created. This rest field is partly-updated on demand, so is not useful for shading purposes.

Rest Regeneration Threshold

How stretched the rest field can get before it is reset. Making this too near one will just cause the rest field to always be reset.

Rest Advection Speed

How quickly the rest field moves compared to the rest of the fluid sim. Slowing down the rest field reduces the rate it stretches, but can cause the turbulence to appear to stick or lag.

Advection

Advection Type

The algorithm to use for advecting the fields.

Single stage

Equivalent to the Gas Advect DOP, where each point is back traced through the velocity field once to find the new voxel value.

BFECC and Modified MacCormack

Run a second basic advection stage, resulting in a sharper fluid that doesn’t disperse as much.

Clamp Values

The error correction of the BFECC and Modified MacCormack advection types can move voxel values outside the container, leading to strange effects such as negative density values. This parameter lets you choose a method to avoid this problem. The default is "Revert".

None

Do not attempt to prevent error correction from moving values outside the container.

Clamp

Restrict each voxel to the range of values possible given its eight original values.

Revert

If the error-corrected voxel is out of range, return it to the single-stage value.

Reverting can avoid checker artifacts where the error correction breaks down.

Blend

Apply a smooth blend between non-clamped and clamped values as the advected field approaches the clamping limit. Particularly with the Revert option, applying a small amount of Blend (e.g. 0.05 - 0.1) can reduce grid artifacts in the advected field at the cost of some additional smoothing of the field.

Vel Advection Type

The algorithm to use for advecting the velocity field. Higher types in the list will reduce the apparent viscosity of the field, but may add energy or cause chatter.

Advection Method

Controls particle tracing.

Single step

Takes the velocity at each voxel and makes a single step in that direction for the time step. This is fastest and is independent of the speed of the velocity field, but will start to break up for large time steps.

Trace

Ensures the backtracking does not move more than a single voxel before its velocity is updated, allowing for larger time steps.

Trace Midpoint

Like Trace but uses second order advection for more accuracy but slower simulation.

HJWENO

A non-lagrangian integrator, this allows for theoretically more accurate advection of divergent fields. Unfortunately, if too large substeps are taken, it will explode.

Upwind

A faster but less accurate non-lagrangian integrator.

Trace RK3

Like Trace but uses third order advection for more accuracy but slower simulation.

Trace RK4

Like Trace but uses fourth order advection for more accuracy but slower simulation.

Advection CFL

When tracing the particles, this controls how many voxels the particles can move in a single iterations. Higher values give faster tracing and faster advection, but more errors.

Bindings

Do not change the values on this tab unless you really know what you're doing.

Clear Fields

Fields to Clear

Zeros out the specified types of fields after the solve step. This ensures the .sim files, which store the complete state of the simulation, do not have any information not needed, reducing their size and save time.

None

No special clearing of fields is done.

Hidden

Fields not needed for the next time step that do not have guide parameters are cleared.

Static

Fields not needed for next time step are cleared. Note that some guides will thus start showing up as zero as the underlying field got cleared.

Additional

A space separated list of extra fields that should be cleared post-solve.

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

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.

Dynamics nodes