Houdini 20.5 Nodes Dynamics nodes

Ripple Object dynamics node

Creates an object from existing geometry that will be deformed with the ripple solver.

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Overview

This node takes existing geometry, usually from a Geometry container object, and configures it (using an internal Ripple Configure Object node) with the data needed by the Ripple Solver. This lets the ripple solver deform the surface by simulating the propagation of waves across it.

See the Ripple Solver help for general information on ripple simulations.

A Ripple Object consists of two surfaces:

  • The rest surface. This is the “normal” shape of the surface without any waves on it. Waves deform the surface up and down away from this rest shape.

  • The initial surface. This is the starting state of the surface. The difference between this surface and the rest surface causes the first set of waves across the surface.

The current ripple state and the rest geometry must have the same topology (number of points and connectivity) to get reasonable results. The surface will blow up into an interesting random wavy shape if the initial and rest surfaces don’t have the same topology.

(When you create a Ripple Object node using the tab menu in a dynamics network, it starts pre-configured with a pair of internal “demo” surfaces. Just change Initial SOP Path and Rest SOP Path to point to your geometry.)

The easiest way to set up a ripple object is to create surface geometry, deform it using surface nodes (for example, using Edit surface nodes), and then point this node’s Rest SOP Path parameter to the surface containing the original surface, and Initial SOP Path to the downstream node with the deformed surface.

You can also specify the rest state by…

If you specify more than one rest surface in different ways, Ripple Object will prefer attached rest data because it provides greater flexibility.

Tip

The Ripple Object and Ripple Solver have no relation to the Ripple surface node.

Attributes

You can create/paint 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 this node’s parameters.

You can paint “islands” of zero wave speed. Waves will bounce off these areas and the surface within the area will remain at rest.

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

Creation Frame Specifies Simulation Frame

Determines if the creation frame refers to global Houdini frames ($F) or to simulation specific frames ($SF). The latter is affected by the offset time and scale time at the DOP network level.

Creation Frame

The frame number on which the object will be created. The object is created only when the current frame number is equal to this parameter value. This means the DOP Network must evaluate a timestep at the specified frame, or the object will not be created.

For example, if this value is set to 3.5, the Timestep parameter of the DOP Network must be changed to 1/(2*$FPS) to ensure the DOP Network has a timestep at frame 3.5.

Number of Objects

Instead of making a single object, you can create a number of identical objects. You can set each object’s parameters individually by using the $OBJID expression.

Object Name

The name for the created object. This is the name that shows up in the details view, and is used to reference this object externally.

Note

It is possible to have many objects with the same name, but this complicates writing references, so it is recommended to use something like $OBJID in the name.

Solve On Creation Frame

When turned on, newly created objects are solved by the solver on the timestep in which it was created.

This parameter is usually turned on if this node is creating objects in the middle of a simulation rather than creating objects for the initial state of the simulation.

Allow Caching

By preventing a large object from being cached, you can ensure there is enough room in the cache for the previous frames of its collision geometry.

This option should only be set when you are working with a very large sim. It is much better just to use a larger memory cache if possible.

Initial SOP Path

The path to a SOP (or an Object, in which case the display SOP is used) which will be the initial ripple geometry.

Rest SOP Path

The path to a SOP which will be the rest state of the wave equation.

Rest springs will pull the ripple geometry towards this state, and the solver uses it to determine the curvature for propagating ripples from point to point.

Use Deforming Rest

Reloads the rest geometry every timestep. By animating the rest geometry, you can add new bumps to the system without directly editing the current state.

Use Object Transform

The transform of the object containing the chosen SOP is applied to the geometry. This is useful if the initial location of the geometry is defined by an object transform.

If you want to transfer an object whose movement is defined at the object level, you should use the Object Position DOP instead.

Position

Position in world space of the object. This can be animated.

Rotation

Initial orientation of the object. This is in RX/RY/RZ format. You can animate this.

Pivot

Pivot in world space of the object. You can animate this.

Outputs

First

The Ripple object created by this node.

Locals

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.

Dynamics nodes