Houdini 16.5 Nodes Dynamics nodes

POP Collision Detect dynamics node

A POP node that detects and reacts to collisions.

On this page

Overview

The POP Collision Detect node finds collisions between particles and geometry. It stores the resulting collision information in a set of hit attributes:

i@hittotal

The cumulative total of particle hits.

i@hitnum

The number of times the particle hit in this detection node.

s@hitpath

A path to the object it hit. This is an op: path so it can be used directly by VEX.

i@hitprim

The primitive the object hit. Can be -1 if the specific primitive could not be resolved.

v@hituv

The parameteric location on the primitive of the hit. This is not texture uvs. The primuv() VEX function can be used to recover information about the hit location.

v@hitpos

The location in space of the collision. Often does not correspond to the current location of the particle as it usually passes through the collision. Also may not correspond to the evaluated location of the hitpath since intra-frame collisions use swept geometry.

v@hitnml

The normal of geometry at the time of the collision.

v@hitv

The velocity of the geometry at the time and position of the collision.

@hittime

When, in seconds, the collision occurred.

You can read the hit attributes directly, or use the POP Collision Behavior node to transform them in some common ways.

Tips and notes

  • For simple RBD collisions, you can use the controls on the Collision Behavior tab of the POP Solver.

  • This node detects collisions of all particles in the Group field, even if they already collided. So if you're sticking particles, this node will update the hittime of stuck particles at each time step.

    To avoid this, you can specify the Group as @stuck=0, so the node only affects particles that aren’t stuck.

  • In SOP mode, this node only supports collisions with triangles and quads. You can detect collisions with SDFs using the Relationship or DOP Objects and pointing to Static Objects.

  • This does not handle dynamic collision response, such as bouncing. Instead, static or RBD objects should be added to the system.

This operator modifies the hitnum, hitpath, hitprim, hituv, hitv, hitpos, hitnml, hittime, P, Cd, stopped, stuck, sliding, pospath, posuv, and posprim attributes.

For more information see Particle Collisions.

Using Collision Detect

  1. Create a particle system along the curve using the Location or Source shelf tools.

  2. Create an object for the particle system to collide with.

  3. Click the Collision Detect tool on the Particles tab.

  4. Select the object for collision detection and press Enter.

    For specific parameter help, see the Collision Detect node help. You can choose what happens to the particles once they hit using the options on the Behavior tab.

    Note

    Turn on Move to Hit if you want to trigger an effect from a particle’s collision. For example, if you want to birth more particles, you want the particle at the hit location, not where it ended up at the end of the frame. This will move the particle back to it’s hit location.

    In the following example, particles that hit the box are sticking to it and are colored red.

Parameters

Activation

Turns this node on and off. The node is only active if this value is greater than 0. This is useful to control the effect of this node with an expression.

Note

This is activation of the node as a whole. You can’t use this parameter to deactivate the node for certain particles.

Group

Only affect a group of points (created with, for example, a Group POP or Collision Detection POP) out of all the points in the current stream.

Guide

Controls if the guide geometry for this node should be shown.

Note

Even if it is enabled here, it can also be disabled by using the Hidden flag on the DOP Node.

Collision

Collision Target

What geometry to do collision detection with. This must be a quad and tri mesh. The mesh can be deforming, in which case its connectivity must remain constant.

Relationship

All DOP Objects with this relationship (as usually defined by Merge DOPs) to this object will be tested.

DOP Objects

A specific list of DOP objects within this simulation will be tested.

SOP

Use specific SOP.

Use Xth Context Geometry

Use one of the SOPs wired into this DOP network.

Relationship

The type of relationship to do collision detection with.

DOP Objects

A list of DOP Objects. Patterns like * can be used to match multiple objects. These refer to objects inside of this simulation.

SOP Path

The path to a SOP to fetch geometry from.

Use Deforming Geometry

The geometry at the beginning and ending of the particle’s motion will be fetched and swept to allow proper collision during the geometry’s motion. However, if the geometry has changing point counts this cannot be done, and instead only a single frame should be used.

Default Particle Size

To allow robust collision detection the particles are treated as finite-sized spheres. By default pscale is used, but if the pscale attribute is missing then this size is used.

Behavior

Accumulate Hits

Normally the hit attribute, @numhit, will be zeroed out before collision detection is performed. If this is set, it will not be cleared, allowing you to count the total number of hits over a lifetime of a particle. However, this will cause the other options, such as Group, to continue to see the particle as hitting every frame since they use this variable to detect if any hits occurred. Another option is to use a POP Wrangle with i@totalhit += i@numhit; to create a total hit attribute that accumulates.

Group Name

All particles that just hit, ie, @numhit>0, will be added to this group.

Preserve Group

If the group isn’t preserved, it is cleared out first so the only particles in the group will be those that just hit. If it is preserved, the group will accumulate all particles that ever hit.

Color Hits

Particles that are just hit will have their Cd attribute set to this value. This is useful for quick visualization of hits.

Add Hit Total

Adds to the integer hittotal attribute any hits that occurred due to this collision detection.

Move to Hit

Often if you want to trigger an effect off a particle’s collision, such as birthing more particles, you want the particle at its hit location, not where it ended up at the end of the frame. This will move the particle back to its hit location. This consists of @P = v@hitpos;

Response

What happens to particles that collide

Die

Particles that hit will set the dead attribute to 1, causing them to be deleted during the reaping pass.

Stop

This sets the stopped attribute to 1. Particles that stop will no longer integrate their velocity, position, orientation, or angular velocity. They can still be moved directly. For example, by the Look At POP in instantaneous mode.

Stick

Particles that hit will have the stuck attribute set to 1. The pospath, posprim, and posuv attributes will be setup to point to the hit location, causing the integrator to keep moving the particles to their stuck location every frame. Usually you also want to turn on Move to Hit with this.

This node will continue to consider stuck particles as "colliding" and update the attributes at each time step. To avoid this, you can specify the Group as @stuck=0, so the node only affects particles that aren’t stuck.

Slide

Particles that hit will have the sliding attribute set to 1. The pospath, posprim, and posuv attributes will be setup to point to the hit location, causing the integrator to try to slide the particles along the surface.

You can also use the Cling attribute on the POP Property node to set how much the particles will cling to the object they are sliding on.

Attributes

Add Hit Total Attribute

Adds the integer attribute hittotal that stores the cumulative total number of times the particle has collided with anything.

Add Hit Num Attribute

Adds the integer attribute hitnum that stores the number of times the particle has collided in this particular node.

Add Hit Pos Attribute

Adds the vector attribute hitpos that stores the position that the particle collided.

Add Hit Normal Attribute

Adds the vector attribute hitnml that stores the normal of the geometry at the time of the collision.

Add Hit Velocity Attribute

Adds the vector attribute hitv that stores the velocity of the geometry at the time of the collision. For SDF collisions this will use the point velocity, so either point numbers should be consistent or point velocity attributes present.

Add Hit Time Attribute

Adds the float attribute hittime that stores the time in seconds of the collision.

Add Hit Path Attribute

Adds the string attribute hitpath that stores the object that the particle collided. This is an op: path usable in VEX.

Add Hit Prim Attribute

Adds the integer attribute hitprim that stores the primitive hit by the particle. -1 if the primitive cannot be determined.

Add Hit UV Attribute

Adds the vector attribute hituv that stores the parametric coordinates of where the primitive was hit. This is not texture UVs.

Bindings

Geometry

The name of the simulation data to apply the POP node to. This commonly is Geometry, but POP Networks can be designed to apply to different geometry if desired.

Evaluation Node Path

For nodes with local expressions, this controls where ch() style expressions in VEX are evaluated with respect to. By making this ., you can ensure relative references work. It is important to promote this if you are embedding a node inside an HDA you are also exporting the local expressions.

Inputs

First Input

This optional input has two purposes.

First, if it is wired to other POP nodes, they will be executed prior to this node executing. The chain of nodes will be processed in a top-down manner.

Second, if the input chain has a stream generator (such as POP Location, POP Source, or POP Stream), this node will only operate on the particles in that stream.

Outputs

First Output

The output of this node should be wired into a solver chain.

Merge nodes can be used to combine multiple solver chains.

The final wiring should go into one of the purple inputs of a full-solver, such as POP Solver or FLIP Solver.

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

ParticleCollisions Example for POP Collision Detect dynamics node

This example demonstrates the use of the POP Collision Detect node to simulate particles colliding with a rotating torus with animated deformations.

The following examples include this node.

ParticleCollisions Example for POP Collision Detect dynamics node

This example demonstrates the use of the POP Collision Detect node to simulate particles colliding with a rotating torus with animated deformations.

See also

Dynamics nodes