Houdini 17.0 Nodes Dynamics nodes

Output dynamics node

Serves as the end-point of the simulation network. Has controls for writing out sim files.

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The Output DOP is usually used to mark the end of a DOP simulation chain. It should normally always have the Output flag set on itself.

It also provides the capabilities of the Dynamics ROP.

Output from this node can be played back using DOP Network Playback

Parameters

Save to Disk

Saves the simulation to disk as a sequence of .sim files.

Save to Disk in Background

Starts another copy of Houdini in the background and instructs that copy to save out the simulation as a sequence of .sim files. This allows one to continue working and load the .sim files as they complete.

Start/End/Inc

Specifies the range of frames to render (start frame, end frame, and increment). All values may be floating point values. The range is inclusive.

These parameters determine the values of the local variables for the output driver.

$NRENDER

The number of frames to be rendered by the output driver.

$N

The current frame being rendered (starting at 1 and going to $NRENDER).

Render with Take

Uses the settings in a particular take while rendering. Choose Current to use the current take when rendering.

Output File

The file to save the simulation state to. Make sure to include $SF in the filename to write out separate files for each frame.

Output Every Sim Frame Using $SF

Every single simulation frame will be output, rather than just the frames hit by the step rate of the frame range. In this mode one just has to set the entire range without worrying about how sub-stepping will be set up.

$SF should be used instead of $F in the file name in these cases.

Initialize Simulation OPs

Force all simulation OPs to be reset. This includes DOP Networks, POP SOPs, and other OPs that cache their results.

This is the safest way to render out a simulation, because it starts the simulation from scratch and discards any partial simulations you might have done with different parameters. However, throwing away an already-cooked simulation can be expensive, especially for relatively slow solvers such as fluids.

Alfred Style Progress

A percentage complete value is printed out as files are written. This is in the style expected by Pixar’s Alfred render queue.

Inputs

All

All the objects connected to the input of this node are fed out through the single output.

Outputs

First

All the objects or data connected to the input of this node are fed out through the single output.

Locals

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

The following examples include this node.

ClipLayerTrigger Example for Agent Clip Layer dynamics node

This example demonstrates how to use the Agent Clip Layer DOP to apply a clip to the upper body of an agent. The clip is activated when the agent is inside a bounding box.

BreakingSprings Example for Constraint Network dynamics node

This example shows how to use a SOP Solver to break spring constraints in a constraint network that have stretched too far.

ControlledGlueBreaking Example for Constraint Network dynamics node

This example shows how to gradually remove glue bonds from a constraint network and control the crumbling of a building.

Hinges Example for Constraint Network dynamics node

This example demonstrates how to use pin constraints to create hinges between objects.

SoftConstraintNetwork Example for Constraint Network dynamics node

This example shows how to create a simple network of soft constraints, which are used to allow an object to bend before breaking.

AnimatedStaticAgents Example for Crowd Solver dynamics node

This example file demonstrates how to set up "animated static" agents for the crowd solver. These agents follow SOP-level animation and can be used for avoidance or turned into ragdolls.

CrowdHeightField Example for Crowd Solver dynamics node

This example demonstrates using heightfields for terrain adaptation in the crowd solver, and for collisions against ragdolls in the Bullet solver.

FollowTerrain Example for Crowd Solver dynamics node

This example demonstrates how to set up a crowd simulation where agents are oriented to follow the terrain normal.

FootLocking Example for Crowd Solver dynamics node

This example demonstrates how to set up foot locking for an agent.

PartialRagdolls Example for Crowd Solver dynamics node

This example demonstrates how to set up a partial ragdoll, where a subset of the agent’s joints are simulated as active objects by the Bullet solver and the remaining joints are animated.

PinnedRagdolls Example for Crowd Solver dynamics node

This example demonstrates how to set up constraints to attach a ragdoll to an external object, and how to use motors to drive an active ragdoll with an animation clip.

Stadium Crowd Example Example for Crowd Solver dynamics node

Crowd example showing a stadium setup

The setup creates a stadium crowd. The rotating cheer_bbox object is used as a bounding box for the agents. When they are inside it it will trigger a transition from a sitting to a cheering state. After a few seconds the cheering crowd sits back down by transitioning into a sitting state.

Note

The animation clips need to be baked out before playing the scene. This should happen automatically if example is created from Crowds shelf. Otherwise save scene file to a location of your choice and click Render on '/obj/bake_cycles' ropnet to write out the files. The default path for the files is ${HIP}/agents.

Tip

To only see a section of the crowd for quicker preview there’s a switch node in /obj/crowdsource/switch_all_subsection. When 0 it will show all agents, when set to 1 will only show a small section.

Street Crowd Example Example for Crowd Solver dynamics node

Crowd example showing a street setup with two agent groups

The setup creates two groups of agents. The yellow agents are zombies which follow a path of the street. The blue agents are living pedestrians that wander around until they come into proximity of the zombies and then they swtich into a running state.

Triggers to change agent states are setup in the crowd_sim dopnet. The zombies group uses proximity to the stoplights and the color of the light to transition into a standing state when lights are red. The living group transition into a running state when they get close to the zombie agents.

Note

The animation clips need to be baked out before playing the scene. This should happen automatically if example is created from Crowds shelf. Otherwise save scene file to a location of your choice and click Render on '/obj/bake_cycles' ropnet to write out the files. The default path for the files is ${HIP}/agents.

ClipTransitionGraph Example for Crowd Transition dynamics node

This example demonstrates how to use a clip transition graph to provide transition clips for state transitions.

CrowdTriggers Example for Crowd Trigger dynamics node

This example file demonstrates how the built-in trigger types for the Crowd Trigger DOP can be used.

GuidedWrinkling Example for FEM Hybrid Object dynamics node

This is a setup for guided wrinkling using the hybrid object. The first sim creates a detailed mesh consisting of both tets and triangles that doesn’t have any wrinkles yet. The second sim is targeted to the animation creates by the first sim and this adds in the wrinkles.

BaconDrop Example for POP Grains dynamics node

This example demonstrates dropping slices of bacon onto a torus. It shows how to extract a 2d object from a texture map and how to repeatedly add the same grain-sheet object to DOPs.

KeyframedGrains Example for POP Grains dynamics node

This example demonstrates keyframing the internal grains of a solid pighead to create an animated puppet.

TargetSand Example for POP Grains dynamics node

This example demonstrates attracting grain simulations to points on the surface of a model.

VaryingGrainSize Example for POP Grains dynamics node

This example demonstrates interacting grain simulations of very different sizes.

FrictionBalls Example for RBD Object dynamics node

This example demonstrates the friction parameter on an RBD Object.

RBDInitialState Example for RBD Object dynamics node

This example demonstrates the use of the Initial State parameter of an RBD object.

AnimatedObjects Example for RBD Packed Object dynamics node

This example shows how to use animated packed primitives in an RBD Packed Object and set up a transition to active objects later in the simulation.

DeleteObjects Example for RBD Packed Object dynamics node

This example shows how to remove objects from the simulation that are inside a bounding box.

SpeedLimit Example for RBD Packed Object dynamics node

This example shows how to limit the speed of specific objects in the simulation.

StaticBalls Example for Static Object dynamics node

This example uses static object nodes in an RBD simulation of a grid falling and bouncing off three spheres before it hits the ground.

CrowdPov Example for Agent Cam object node

This example demonstrates how the agent cam can be assigned to a crowd agent to give you the point of view from someone in a crowd simulation.

AgentRelationshipBasic Example for Agent Relationship geometry node

This example demonstrates how to create a simple parent-child agent setup.

MountainSplash Example for Attribute Transfer geometry node

This example demonstrates how to transfer attributes from the points on one geometry to the points on another, using the AttribTransfer SOP.

A line is crept along the surface of a deformed grid. A section of the grid is painted red using the Paint SOP. Using the AttribTransfer SOP, the animated line inherits the attributes from the points on the grid.

Particles are then birthed along the line based on the color attribute (Cd). As the inherited color nears red, particles are born. The particles also use the velocity inherited by the points on the line.

Please press play to see the animation.

ExtractAnimatedTransform Example for Extract Transform geometry node

This example shows how to create packed primitives with animated transforms from deforming geometry that represents rigid motion. The result is ideal for colliders in a rigid body simulation.

CoolLava

This example demonstrates how to cool Lava using the Cool Within Object shelf tool.

TransformFracturedPieces Example for Transform Pieces geometry node

This example demonstrates using the Transform Pieces SOP to transform high-resolution geometry from the results a DOPs rigid-body fracture simulation that used low-resolution geometry.

Fuzzy Logic Obstacle Avoidance Example Example for Fuzzy Defuzz VOP node

This example shows agent obstacle avoidance and path following implemented using a fuzzy logic controller.

See also

Dynamics nodes