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The Gravity Force DOP applies forces on objects as if they were inside a gravity field. The amount of force is proportional to the object’s mass. If an object has twice the mass, it receives twice the force. However, because an object of twice the mass requires twice the force to experience the same net acceleration, the Gravity Force can be thought of as a raw acceleration.
You can add noise to the force applied by this DOP by connecting a
Noise DOP to the second input of this node, which adds
the noise as subdata of the force data.
Using Gravity Force
-
Select the dynamic object to apply gravity force.
-
Click the
Gravity Force tool on the Drive Simulation tab.
Note
Gravity force is simply a predefined 
Force node whose force and direction can be altered in the parameter editor.
Parameters
Force
The amount of force to apply to a unit-massed object. Because the force is scaled by the mass, objects will undergo this acceleration.
If your units are meters, seconds, and kilograms, -9.81 is a good value for Earth’s gravity.
If your units are feet, seconds, and pounds, -32 is a good value for Earth’s gravity.
Sampling Mode
Indicates the preferred sampling level (point, circle, or sphere) to trade accuracy for efficiency of the computation.
Parameter Operations
Each data option parameter has an associated menu which specifies how that parameter operates.
Use Default
Use the value from the Default Operation menu.
Set Initial
Set the value of this parameter only when this data is created. On all subsequent timesteps, the value of this parameter is not altered. This is useful for setting up initial conditions like position and velocity.
Set Always
Always set the value of this parameter. This is useful when specific keyframed values are required over time. This could be used to keyframe the position of an object over time, or to cause the geometry from a SOP to be refetched at each timestep if the geometry is deforming.
You can also use this setting in
conjunction with the local variables for a parameter value to
modify a value over time. For example, in the X Position, an
expression like $tx + 0.1 would cause the object to
move 0.1 units to the right on each timestep.
Set Never
Do not ever set the value of this parameter. This option is most useful when using this node to modify an existing piece of data connected through the first input.
For example, an 
RBD State
DOP may want to animate just the mass of an
object, and nothing else. The Set Never option could be used
on all parameters except for Mass, which would use Set
Always.
Default Operation
For any parameters with their Operation menu set to Use Default, this parameter controls what operation is used.
This parameter has the same menu options and meanings as the Parameter Operations menus, but without the Use Default choice.
Data Sharing
Controls the way in which the data created by this node is shared among multiple objects in the simulation.
Data sharing can greatly reduce the memory footprint of a simulation, but at the expense of requiring all objects to have exactly the same data associated with them.
Do Not Share Data
No data sharing is used. Each object has its own copy of the data attached.
This is appropriate for situations where the data needs to be customized on a per-object basis, such as setting up initial positions and velocities for objects.
Share Data Across All Time
This node only creates a single piece of data for the whole simulation. This data is created the first time it is needed, so any expressions will be evaluated only for the first object.
All subsequent objects will have the data attached with the same values that were calculated from the expressions for the first object. It is important to note that expressions are not stored with the data, so they cannot be evaluated after the data is created.
Expressions are evaluated by the DOP node before
creating the data. Expressions involving time will also only be
evaluated when this single piece of data is created. This option
is appropriate for data that does not change over time, and is
the same for all objects, such as a Gravity
DOP.
Share Data In One Timestep
A new piece of data is created for each timestep in the simulation. Within a timestep though, all objects have the same data attached to them. So expressions involving time will cause this data to animate over time, but expressions involving the object will only evaluate for the first object to which the data is attached.
This option is appropriate for data that changes
over time, but is the same for all objects such as a 
Fan Force
DOP, where the fan may move or rotate over time.
Activation
Determines if this node should do anything on a given timestep and for a particular object. If this parameter is an expression, it is evaluated for each object (even if data sharing is turned on).
If it evaluates to a non-zero value, then the data is attached to that object. If it evaluates to zero, no data is attached, and data previously attached by this node is removed.
Group
When an object connector is attached to the first input of this node, this parameter can be used to choose a subset of those objects to be affected by this node.
Data Name
Indicates the name that should be used to attach the data to an object or other piece of data. If the Data Name contains a "/" (or several), that indicates traversing inside subdata.
For example, if the Fan Force DOP has the default Data Name
"Forces/Fan". This attaches the data with the name "Fan" to an
existing piece of data named "Forces". If no data named "Forces"
exists, a simple piece of container data is created to hold the
"Fan" subdata.
Different pieces of data have different requirements on what names should be used for them. Except in very rare situations, the default value should be used. Some exceptions are described with particular pieces of data or with solvers that make use of some particular type of data.
Unique Data Name
Turning on this parameter modifies the Data Name parameter value to ensure that the data created by this node is attached with a unique name so it will not overwrite any existing data.
With this parameter turned off, attaching two pieces of data with the same name will cause the second one to replace the first. There are situations where each type of behavior is desirable.
If an object
needs to have several Fan Forces blowing on it, it is
much easier to use the Unique Data Name feature to ensure that
each fan does not overwrite a previous fan rather than trying to
change the Data Name of each fan individually to avoid
conflicts.
On the other hand, if an object is known to have RBD
State data already attached to it, leaving this
option turned off will allow some new RBD State
data to overwrite the existing data.
Inputs
First Input
This optional input can be used to control which simulation objects are modified by this node. Any objects connected through this input and which match the Group parameter field will be modified.
If this input is not connected, this node can be used in conjunction with an Apply Data node, or can be used as an input to another data node.
All Other Inputs
If this node has more input connectors, other data nodes can be attached to act as modifiers for the data created by this node.
The specific types of subdata that are meaningful vary from node to
node. Click 
an input connector to see a list of available
data nodes that can be meaningfully attached.
Outputs
First Output
The operation of this output depends on what inputs are connected to this node. If an object stream is input to this node, the output is also an object stream containing the same objects as the input (but with the data from this node attached).
If no object stream is
connected to this node, the output is a data output. This data
output can be connected to an 
Apply Data DOP,
or connected directly to a data input of another data node, to
attach the data from this node to an object or another piece of
data.
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
The following examples include this node.
CountImpacts Example for Count channel node
DynamicLights Example for Dynamics channel node
DynamicPops Example for Dynamics channel node
ExtractTransforms Example for Dynamics channel node
AnimatedActiveState Example for Active Value dynamics node
AutoFreezeRBD Example for Active Value dynamics node
LookAt Example for Anchor: Align Axis dynamics node
ApplyRelationship Example for Apply Relationship dynamics node
BridgeCollapse Example for Apply Relationship dynamics node
ConstrainedTeapots Example for Apply Relationship dynamics node
MutualConstraints Example for Apply Relationship dynamics node
SimpleBlend Example for Blend Solver dynamics node
BuoyancyForce Example for Buoyancy Force dynamics node
AnimatedClothPatch Example for Cloth Object dynamics node
BendCloth Example for Cloth Object dynamics node
BendDamping Example for Cloth Object dynamics node
BlanketBall Example for Cloth Object dynamics node
ClothAttachedDynamic Example for Cloth Object dynamics node
ClothFriction Example for Cloth Object dynamics node
ClothUv Example for Cloth Object dynamics node
DragCloth Example for Cloth Object dynamics node
MultipleSphereClothCollisions Example for Cloth Object dynamics node
PanelledClothPrism Example for Cloth Object dynamics node
PanelledClothRuffles Example for Cloth Object dynamics node
AnchorPins Example for Constraint Network dynamics node
AngularMotorDenting Example for Constraint Network dynamics node
BreakingSprings Example for Constraint Network dynamics node
Chains Example for Constraint Network dynamics node
ControlledGlueBreaking Example for Constraint Network dynamics node
GlueConstraintNetwork Example for Constraint Network dynamics node
Hinges Example for Constraint Network dynamics node
PointAnchors Example for Constraint Network dynamics node
SpringToGlue Example for Constraint Network dynamics node
AutoFracturing Example for Copy Objects dynamics node
SimpleCopy Example for Copy Objects dynamics node
CrowdHeightField Example for Crowd Solver dynamics node
PartialRagdolls Example for Crowd Solver dynamics node
PinnedRagdolls Example for Crowd Solver dynamics node
TypesOfDrag Example for Drag Force dynamics node
fieldforce Example for Field Force dynamics node
CacheToDisk Example for File dynamics node
FEMSpheres Example for finiteelementsolver dynamics node
DensityViscosity Example for FLIP Solver dynamics node
FlipColorMix Example for FLIP Solver dynamics node
FlipColumn Example for FLIP Solver dynamics node
FlipFluidWire Example for FLIP Solver dynamics node
SpinningFlipCollision Example for FLIP Solver dynamics node
VariableViscosity Example for FLIP Solver dynamics node
FluidWireInteraction Example for Fluid Force dynamics node
BallInTank Example for Fluid Object dynamics node
FillGlass Example for Fluid Object dynamics node
FluidFeedback Example for Fluid Object dynamics node
PaintedGrog Example for Fluid Object dynamics node
RestartFluid Example for Fluid Object dynamics node
RiverBed Example for Fluid Object dynamics node
VariableDrag Example for Fluid Object dynamics node
HotBox Example for Gas Calculate dynamics node
DiffuseSmoke Example for Gas Diffuse dynamics node
CombinedSmoke Example for Gas Embed Fluid dynamics node
EqualizeFlip Example for Gas Equalize Volume dynamics node
EqualizeLiquid Example for Gas Equalize Volume dynamics node
dopexample_gasnetfetchdata Example for Gas Net Fetch Data dynamics node
UpresRetime Example for Gas Up Res dynamics node
GuidedWrinkling Example for Hybrid Object dynamics node
MagnetMetaballs Example for Magnet Force dynamics node
SimpleMultiple Example for Multiple Solver dynamics node
VolumeSource Example for Particle Fluid Emitter dynamics node
FluidGlass Example for Particle Fluid Solver dynamics node
PressureExample Example for Particle Fluid Solver dynamics node
ViscoelasticExample Example for Particle Fluid Solver dynamics node
ViscousFlow Example for Particle Fluid Solver dynamics node
WorkflowExample Example for Particle Fluid Solver dynamics node
AdvectByVolume Example for POP Advect by Volumes dynamics node
ParticlesAttract Example for POP Attract dynamics node
PointAttraction Example for POP Attract dynamics node
SphereAxisForce Example for POP Axis Force dynamics node
TorusAxisForce Example for POP Axis Force dynamics node
ParticleCollisions Example for POP Collision Detect dynamics node
CurveForce Example for POP Curve Force dynamics node
BaconDrop Example for POP Grains dynamics node
KeyframedGrains Example for POP Grains dynamics node
VaryingGrainSize Example for POP Grains dynamics node
SwarmBall Example for POP Interact dynamics node
LookatTarget Example for POP Lookat dynamics node
DragCenter Example for POP Property dynamics node
ProximateParticles Example for POP Proximity dynamics node
CrossTheStreams Example for POP Stream dynamics node
BillowyTurbine Example for Pyro Solver dynamics node
DampedHinge Example for RBD Angular Spring Constraint dynamics node
Stack Example for RBD Auto Freeze dynamics node
RagdollExample Example for Cone Twist Constraint dynamics node
ShatterDebris Example for RBD Fractured Object dynamics node
StackedBricks Example for RBD Fractured Object dynamics node
Pendulum Example for RBD Hinge Constraint dynamics node
SimpleKeyActive Example for RBD Keyframe Active dynamics node
DeformingRBD Example for RBD Object dynamics node
FrictionBalls Example for RBD Object dynamics node
RBDInitialState Example for RBD Object dynamics node
SimpleRBD Example for RBD Object dynamics node
ActivateObjects Example for RBD Packed Object dynamics node
AnimatedObjects Example for RBD Packed Object dynamics node
DeleteObjects Example for RBD Packed Object dynamics node
EmittingObjects Example for RBD Packed Object dynamics node
SpeedLimit Example for RBD Packed Object dynamics node
Chain Example for RBD Pin Constraint dynamics node
Chainlinks Example for RBD Pin Constraint dynamics node
Pendulum Example for RBD Pin Constraint dynamics node
GravitySlideExample Example for Slider Constraint dynamics node
PaddleWheel Example for RBD Solver dynamics node
Weights Example for RBD Spring Constraint dynamics node
InheritVelocity Example for RBD State dynamics node
Simple Example for RBD Visualization dynamics node
ReferenceFrameForce Example for Reference Frame Force dynamics node
RippleGrid Example for Ripple Solver dynamics node
Freeze Example for Script Solver dynamics node
ScalePieces Example for Script Solver dynamics node
SumImpacts Example for Script Solver dynamics node
DelayedSmokeHandoff Example for Smoke Object dynamics node
RBDtoSmokeHandoff Example for Smoke Object dynamics node
VolumePreservingSolid Example for Solid Object dynamics node
DentingWithPops Example for SOP Solver dynamics node
VisualizeImpacts Example for SOP Solver dynamics node
StaticBalls Example for Static Object dynamics node
FractureExamples Example for Voronoi Fracture Solver dynamics node
SimpleVortex Example for Vortex Force dynamics node
AnimatedSkin Example for Wire Glue Constraint dynamics node
CompressedSpring Example for Wire Object dynamics node
BeadCurtain Example for Wire Solver dynamics node
BendingTree Example for Wire Solver dynamics node
BreakWire Example for Wire Solver dynamics node
Pendulum Example for Wire Solver dynamics node
PackedFragments Example for Assemble geometry node
CaptureDeform Example for Cloth Deform geometry node
ConnectedBalls Example for Connectivity geometry node
LowHigh Example for Dop Import geometry node
ProxyGeometry Example for Dop Import geometry node
dopimportrecordsexample Example for DOP Import Records geometry node
ColourAdvect Example for Fluid Source geometry node
CoolLava Example for Fluid Source geometry node
FurBallWorkflow Example for Fur geometry node
glueclusterexample Example for Glue Cluster geometry node
PartitionBall Example for Partition geometry node
PlateBreak Example for TimeShift geometry node
TransformFracturedPieces Example for Transform Pieces geometry node
RampParameter Example for Parameter VOP node
