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Overview ¶
When you attach this solver to an object with Ripple Object
data (usually created by either a 
Ripple Object
dynamics node or a combination of 
Empty Object
and Ripple Configure Object),
it animates wave propagation across the object’s surface by
moving the surface’s points.
Two types of forces affect wave evolution:
-
The Wave speed value controls how fast waves move across the surface.
You can also paint per-point wave speed attributes (see the attributes section below) to make some areas of the surface communicate waves faster or slower than others. This lets you paint “islands” of zero wave speed. Waves will bounce off these areas and the surface within the area will remain at rest.
The local curvature of the surface determines the displacement speed. While the solver does displacement per-point, when it calculates curvature it uses the connectivity of the points. In the case of a height field, this has the effect of propagating ripples across the surface of the height field.
-
The Rest Spring value pulls the surface back towards its rest shape at each time step. Higher values will dissipate waves faster and make the entire surface lose wave energy and eventually settle down to the rest shape.
You can also paint per-point rest-spring attributes (see the attributes section below) to make some areas of the surface more resilient than others.
Ripple propagation, while easiest to understand on a height field, is not restricted to grid topology. Ripples can propagate along a curve, through a 3d lattice of points, or even across a complicated triangular mesh.
You can expand on the ripple solver’s behavior by using it as part of a multi-solver setup. You can combine the Ripple solver with additional SOP Solvers to add bumps or other effects on top of the ripple.
Tip
When the neighboring points don’t move with the ripple, they cause a boundary condition that triggers the waves to bounce off them. You can use a gradient of conservation to feather in to have a more absorbing boundary.
Attributes ¶
You can create attributes on the Ripple Object’s initial geometry to influence its behavior. Most of these attributes allow fine-tuning of the Ripple Solver by overriding default values set in this node.
| Name | Class | Type | Description |
|---|---|---|---|
v
|
Point | Vector |
The current velocity of the point. Momentum is passed from frame to frame through this. |
conservation
|
Point | Float |
Overrides the conservation parameter of the Wave Solver in a per-point manner. |
wavespeed
|
Point | Float |
Overrides the wavespeed parameter of the Wave Solver in a per-point manner. Note Adaptive substepping does not take overridden wavespeeds into account. Therefore, the value on the Wave Solver node should be the maximum of the values on the points. |
springk
|
Point | Float |
Overrides the rest spring parameter of the Wave Solver on a per-point manner. |
Parameters ¶
Adaptive Substep
For high wave speeds and small edge lengths, the computation can become unstable. To prevent this, an optimal substep is computed by comparing the wave speed with the minimum edge length.
Because computing the proper substep size can be
expensive, after one has fixed the topology and parameters one
can turn off adaptive substep and just set the minimum substep to the
proper calculated value. (You can get the calculated value from the
wave2d_sop/foreach1/Number Range parameter.)
Note
This does not take into account painted wave speeds - the wave speed should be set to the maximum of any painted wave speeds.
CFL Condition
Adjusts how conservative the adaptive substep should be. Values less than 1 will produce smaller substeps than are required, larger than 1 will produce larger.
Larger values of CFL are likely dangerous as the automatically computed value is close to the maximum.
Min Substep
Regardless of the adaptive substep computation, this number of substeps will be performed. This is also the number of substeps to use if adaptive substep is turned off.
If high rest springs are present, it might be useful to increase the minimum substep to reduce dampening.
Max Substep
No more than this number of substeps will be performed. However, if the adaptive calculation is requiring more than these, it likely will be unstable.
Conservation
How much energy is conserved in the system. The velocity is modified by this rate directly. It measures the fraction of velocity that will be kept after one second of animation.
1 means no velocity is lost, 0.5 means 50% after one second, and 0 means the velocity is zeroed out every frame.
Wave Speed
How fast the ripples travel from point to point. If Use Edge Len is off, this is measured in terms of topology - distance between each edge is considered 1. Otherwise, it is proportional to the average edge length at the node.
Rest Spring
The strength of the springs tying the points to their rest positions. Because the wave speed calculation only looks at curvature of the displacement field, it will not restore the geometry to the rest state.
The total displacement will be preserved by the wave speed. Adding rest springs will drag the points back to their origin, and possibly induce new ripples in doing so.
The spring is solved implicitly so large values can be used with small time steps. However, doing so will introduce considerable dampening in the spring behavior - if this is undesired, a lower minimum substep can be used.
Use Normal Frame
By default, the stored velocities and curvature computations
are done in world space. Alternatively, one can use the rnml
attribute to define all displacements in terms of the rest
geometry’s normal. This allows ripples to properly bend around
curves in the surface.
Use Rest Data
Governs if the rest and rnml attribute should be used or if a
separate Geometry data should be used as the rest geometry.
Rest Data
Name of the Geometry data which is attached to this object that should be used as the rest data.
Parameter Operations
Each data option parameter has an associated menu which specifies how that parameter operates.
Use Default
Use the value from the Default Operation menu.
Set Initial
Set the value of this parameter only when this data is created. On all subsequent timesteps, the value of this parameter is not altered. This is useful for setting up initial conditions like position and velocity.
Set Always
Always set the value of this parameter. This is useful when specific keyframed values are required over time. This could be used to keyframe the position of an object over time, or to cause the geometry from a SOP to be refetched at each timestep if the geometry is deforming.
You can also use this setting in
conjunction with the local variables for a parameter value to
modify a value over time. For example, in the X Position, an
expression like $tx + 0.1 would cause the object to
move 0.1 units to the right on each timestep.
Set Never
Do not ever set the value of this parameter. This option is most useful when using this node to modify an existing piece of data connected through the first input.
For example, an 
RBD State
DOP may want to animate just the mass of an
object, and nothing else. The Set Never option could be used
on all parameters except for Mass, which would use Set
Always.
Default Operation
For any parameters with their Operation menu set to Use Default, this parameter controls what operation is used.
This parameter has the same menu options and meanings as the Parameter Operations menus, but without the Use Default choice.
Make Objects Mutual Affectors
All objects connected to the first input of this node become mutual affectors.
This is equivalent to using an 
Affector
DOP to create an affector relationship between
* and * before connecting it to this node. This option makes it
convenient to have all objects feeding into a solver node affect
each other.
Group
When an object connector is attached to the first input of this node, this parameter can be used to choose a subset of those objects to be affected by this node.
Data Name
Indicates the name that should be used to attach the data to an object or other piece of data. If the Data Name contains a “/” (or several), that indicates traversing inside subdata.
For example, if the 
Fan Force DOP has the default Data Name
“Forces/Fan”. This attaches the data with the name “Fan” to an
existing piece of data named “Forces”. If no data named “Forces”
exists, a simple piece of container data is created to hold the
“Fan” subdata.
Different pieces of data have different requirements on what names should be used for them. Except in very rare situations, the default value should be used. Some exceptions are described with particular pieces of data or with solvers that make use of some particular type of data.
Unique Data Name
Turning on this parameter modifies the Data Name parameter value to ensure that the data created by this node is attached with a unique name so it will not overwrite any existing data.
With this parameter turned off, attaching two pieces of data with the same name will cause the second one to replace the first. There are situations where each type of behavior is desirable.
If an object
needs to have several Fan Forces blowing on it, it is
much easier to use the Unique Data Name feature to ensure that
each fan does not overwrite a previous fan rather than trying to
change the Data Name of each fan individually to avoid
conflicts.
On the other hand, if an object is known to have RBD
State data already attached to it, leaving this
option turned off will allow some new RBD State
data to overwrite the existing data.
Solver Per Object
The default behavior for solvers is to attach the exact same solver to all of the objects specified in the group. This allows the objects to be processed in a single pass by the solver, since the parameters are identical for each object.
However, some objects operate more logically on a single object at
a time. In these cases, one may want to use $OBJID expressions
to vary the solver parameters across the objects. Setting this
toggle will create a separate solver per object, allowing $OBJID
to vary as expected.
Setting this is also required if stamping the parameters with a Copy Data DOP.
Inputs ¶
First Input
This optional input can be used to control which simulation objects are modified by this node. Any objects connected through this input and which match the Group parameter field will be modified.
If this input is not connected, this node can be used in conjunction with an Apply Data node, or can be used as an input to another data node.
All Other Inputs
If this node has more input connectors, other data nodes can be attached to act as modifiers for the data created by this node.
The specific types of subdata that are meaningful vary from node to
node. Click 
an input connector to see a list of available
data nodes that can be meaningfully attached.
Outputs ¶
First Output
The operation of this output depends on what inputs are connected to this node. If an object stream is input to this node, the output is also an object stream containing the same objects as the input (but with the data from this node attached).
If no object stream is
connected to this node, the output is a data output. This data
output can be connected to an 
Apply Data DOP,
or connected directly to a data input of another data node, to
attach the data from this node to an object or another piece of
data.
Locals ¶
channelname
This DOP node defines a local variable for each channel and parameter on the Data Options page, with the same name as the channel. So for example, the node may have channels for Position (positionx, positiony, positionz) and a parameter for an object name (objectname).
Then there will also be local variables with the names positionx, positiony, positionz, and objectname. These variables will evaluate to the previous value for that parameter.
This previous value is always stored as part of the data attached to the object being processed. This is essentially a shortcut for a dopfield expression like:
dopfield($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 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
The simulation time for which the node is being evaluated.
Depending on the settings of the 
DOP Network
Offset Time and Scale Time parameters,
this value may not be equal to the current Houdini time
represented by the variable T.
ST 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 $FF == 1.
SF
The simulation frame (or more accurately, the simulation time step number) for which the node is being evaluated.
Depending on the settings of the DOP Network parameters,
this value may not be equal to the current Houdini frame number
represented by the variable F. Instead, it is equal to
the simulation time (ST) divided by the simulation timestep size
(TIMESTEP).
TIMESTEP
The size of a simulation timestep. This value is useful for scaling values that are expressed in units per second, but are applied on each timestep.
SFPS
The inverse of the TIMESTEP value. It is the number of timesteps per second of simulation time.
SNOBJ
The number of objects in the simulation. For nodes that
create objects such as the Empty Object DOP,
SNOBJ increases for each object that is evaluated.
A good way to guarantee unique object names is to use an expression
like object_$SNOBJ.
NOBJ
The number of objects that are evaluated by the current node during this timestep. This value is often different from SNOBJ, as many nodes do not process all the objects in a simulation.
NOBJ may return 0 if the node does not
process each object sequentially (such as the 
Group
DOP).
OBJ
The index of the specific object being processed by the node. This value always runs from zero to NOBJ-1 in a given timestep. It does not identify the current object within the simulation like OBJID or OBJNAME; it only identifies 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
is -1 if the node does not process objects sequentially (such
as the Group DOP).
OBJID
The unique 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. This is very useful in situations where each object needs to be treated differently, for example, to produce a unique random number for each object.
This value is also the best way to look up information on an object using the dopfield expression function.
OBJID is -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
The simulation time (see variable ST) at which the current object was created.
To check if an object was created
on the current timestep, the expression $ST == $OBJCT should
always be used.
This value is zero if the node does not process
objects sequentially (such as the Group DOP).
OBJCF
The simulation frame (see variable SF) at which the current object was created. It is equivalent to using the dopsttoframe expression on the OBJCT variable.
This value is zero if the node does not process objects
sequentially (such as the Group DOP).
OBJNAME
A string value containing the name of the object being processed.
Object names are not guaranteed to be unique within a simulation. However, if you name your objects carefully so that they are unique, the object name can be a much easier way to identify an object than the unique object identifier, OBJID.
The object name can
also be used to treat a number of similar objects (with the same
name) as a virtual group. If there are 20 objects named “myobject”,
specifying strcmp($OBJNAME, "myobject") == 0 in the activation field
of a DOP will cause that DOP to operate on only those 20 objects.
This value is the empty string if the node does not process objects
sequentially (such as the Group DOP).
DOPNET
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 DOP,
you could write the expression:
$tx + 0.1
…to make the object move 0.1 units along the X axis at each timestep.
Examples ¶
RippleGrid Example for Ripple Solver dynamics node
This example demonstrates how to use the Ripple Solver and Ripple Object nodes. Bulge SOPs are used to deform a grid to create initial geometry and rest geometry for the Ripple Object which is then piped into the Ripple Solver.
