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Impact Analysis dynamics node

Stores filtered information about impacts on an RBD object.

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Setup
Parameters
Outputs
Locals

This node creates ImpactAnalysis geometry sub-data on the RBD object for each physical point of impact.

You can filter impact information based on time, impact strength, impact distance, or the other object, preventing the list of impacts from becoming cluttered or filled with useless impacts.

The impact points are useful in surface networks. For example:

You can use the points as clustered fluid simulation source points, so you can raise dust clouds wherever debris hits the ground.
You can create geometry centered on the points, then use a Cookie node to remove those shapes from the ground geometry, to create craters, divots, bullet holes, or whatever.

Setup

To...	Do this
Set up Impact Analysis using the shelf	Select the RBD object you want to record impacts. On the Rigid Bodies tab, click RBD Impacts, then press `Enter`. This will have no visible effect in the viewer. It simply sets up the nodes necessary to record the RBD object’s impacts. See below for how to use that information in a surface network.
Add the Impact Analysis node to the network manually	This node is a solver. You must apply this solver after the RBD solver so Impact Analysis can see the RBD solver’s collision. For example use a Multiple Solver with the RBD solver as the first green (data) input and this node as the second green input.
Import the impact points into a surface network	In the surface network, create a Dop Import node. Set the DOP Network to the path of the DOP network containing the RBD object (for example, `/obj/AutoDopNetwork`). Set the Object Mask to the name of the RBD object (for example, `sphere_object1`). To fetch the impact data from multiple objects at once, you can use a pattern here (for example, `impact_object`). Set Import Style* to "Fetch Geometry from DOP Network". Set the Geometry Data Path to `ImpactAnalysis`. You may need to turn on Points in the display toolbar on the right side of the viewer to see the imported impact points.
Access information about the impact points	Impact points have the following attributes. `P` Position of the impact. `impacttime` Time of the impact. `avgposition` The average position of all unfiltered impact points at the same time sharing the same object. `impulse` Force of the impact. `v` Speed of the impact. `avgimpulse` The average impulse of all unfiltered impact points at the same time sharing the same object. `impactobject` The DOP object ID of the object being hit. `impactprimnum` The primitive number of the object being hit. This can be used to identify impacts for individual pieces of an RBD Packed Object. `parentobject` The DOP object ID of the object recording the impact. `parentprimnum` The primitive number of the object recording the impact. `N` The impact normal. `distance` Distance from the previous filtered impact point. To access fields on the DOP objects that collided, for example to get the mass of the impacted object, you can use the dopfield expression function. dopfield("/obj/AutoDopNetwork", $impactobject, "Position", "Options", 0, "mass")

Parameters

The parameters let you filter out impact points, for example ignoring all but one impact that happen very close together in space and time.

Display Geometry

Controls if the impact points are displayed in the viewport.

Time Threshold

The minimum amount of time (in seconds) between recorded impact points. Higher values give fewer impact points and bigger gaps in time.

Impact Threshold

The minimum amount of impact force for recorded impact points. Higher values only create points for stronger impacts and ignore weaker impacts.

Distance Threshold

The minimum distance between recorded impact points. Higher values give fewer impact points and bigger gaps in space.

Force Impact Object

Only create impact points for collisions with this DOP object.

Outputs

The input objects are passed through to the 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.