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Squishy Object dynamics node

The Squishy Object DOP converts a geometry object into a dynamic object that behaves like a soft body in the DOP environment.

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Parameters
Outputs
Locals

Essentially, it converts the geometry points into particles with sufficient tension between them to prevent the geometry from losing its shape under the influence of DOP forces. The resulting squishy body is ready for use by the POP Solver.

This operator is a superset of the POP Object DOP.

Tip

To avoid clamping, try increasing DOP substepping.

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.

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 particular object externally.

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

Initial Geometry

The path to a SOP which will be used as the initial state for the POP Object. If the specified SOP contains geometry other than particles, the POP Solver can be configured to automatically convert these other primitives into particle systems.

Use Deforming Geometry

Lets the software know whether to watch for geometry whose shape changes (or might change) before being brought into the DOP simulation. Expect faster performance when this toggle is off.

Use Object Transform

Specifies whether or not the transform of the object containing the Initial Geometry should be embedded in the Geometry data.

Collisions

Tolerance

When colliding with the surface of another object, this tolerance value is used by the ray intersection code.

Any time a point gets within this distance of the surface it is counted as a collision.

Volume Offset

When colliding points against a Volume representation, the surface of the Volume is effectively pushed out by this amount.

Like the Tolerance value above, it causes a collision to be generated if a point comes within this distance of the real Volume.

Physical

Bounce

The elasticity of the particles. If two objects of bounce 1.0 collide, they will rebound without losing energy. If two objects of bounce 0.0 collide, they will come to a standstill. If the particles have a point attribute called bounce, that point attribute value overrides this value.

Friction

The coefficient of friction of the particles. A value of 0 means the object is frictionless.

This governs how much the tangential velocity is affected by collisions and resting contacts. If the particles have a point attribute called friction, that point attribute value overrides this value.

Dynamic Friction Scale

An object sliding may have a lower friction coefficient than an object at rest. This is the scale factor that relates the two. It is not a friction coefficient, but a scale between zero and one.

A value of one means that dynamic friction is equal to static friction. A scale of zero means that as soon as static friction is overcome the object acts without friction. If the particles have a point attribute called friction, that point attribute value overrides this value.

Softbody

Restoration Strength

Controls how quickly the points will move to their rest positions. A low value will only slowly try to restore the rest shape, while a large enough value makes the object almost rigid.

This is the multiplier to move the points towards their goal positions in a single step. Higher numbers will result in a faster restoration of the base state. Note that values greater than the frame rate will be clamped to ensure stability.

Squash Resistance

Controls how much the group of points will be restored from squashing effects. Lower values let the object become squashed into a pancake, while higher values will always try to keep the object in its original dimensions. In any case, volume is preserved by applying a stretch to the non-squashed direction.

In addition to purely rigid transforms, the shape matching may also discard squash/stretch transforms. The Squash Resistance is the amount to restore the goal to a rigid state in one second. Again, it is clamped to the frame rate.

Frame Drag Strength

Acts as a damping effect on the simulation.

The restoration of the goal shape uses spring-like forces. This would result in oscillation of displaced points. The Frame Drag will apply a drag force with respect to how they are not obeying rigid motion. This cancels out these oscillations without slowing the entire object down.

Outputs

First

The simulation object created by this node is sent through the single output.

Locals

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).

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.