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The result is ready for use by the POP Solver.
Creation Frame Specifies Simulation Frame
Determines if the creation frame refers to global Houdini frames (
$F) or to simulation specific frames (
The latter is affected by the offset time and scale time at the DOP network level.
The frame number on which the object will be created. The object is created only when the current frame number is equal to this parameter value. This means the DOP Network must evaluate a timestep at the specified frame, or the object will not be created.
For example, if this value is set to 3.5, the Timestep parameter of the DOP Network must be changed to
1/(2*$FPS) to ensure the DOP Network has a timestep at frame 3.5.
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.
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 Object Transform
Specifies whether or not the transform of the object containing the Initial Geometry should be embedded in the Geometry data.
Enables the display of any
instancepath attribute as a guide geometry.
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.
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.
The elasticity of the object. 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.
The tangential elasticity of the object. If two objects of bounce forward 1.0 collide, their tangential motion will be affected only by friction. If two objects of bounce forward 0.0 collide, their tangential motion will be matched.
The coefficient of friction of the object. A value of 0 means the object is frictionless.
This governs how much the tangential velocity is affected by collisions and resting contacts.
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.
Temperature marks how warm or cool an object is. This is used in gas simulations for ignition points of fuel or for buoyancy computations.
Since this does not relate directly to any real world temperature scale, ambient temperature is usually considered 0.
The simulation object created by this node is sent through the single output.
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).
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.
This value is the inverse of the TIMESTEP value. It is the number of timesteps per second of simulation time.
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
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).
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).
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).
This string contains a space separated list of the unique object identifiers for every object being processed by the current node.
This string contains a space separated list of the names of every object being processed by the current node.
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 == $OBJCT should
always be used. This value will be zero if the node does not process
objects sequentially (such as the Group DOP).
This value is the simulation frame (see variable SF) at which the current object was created.
This value is equivalent to using the dopsttoframe expression on the OBJCT variable. This value will be zero if the node does not process objects sequentially (such as the Group DOP).
This is a string value containing the name of the object being processed.
Object names are not guaranteed to be unique within a simulation. However, if you name your objects carefully so that they are unique, the object name can be a much easier way to identify an object than the unique object identifier, OBJID.
The object name can
also be used to treat a number of similar objects (with the same
name) as a virtual group. If there are 20 objects named "myobject",
strcmp($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).
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.
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.
The following examples include this node.
Extracts the velocity field from a smoke simulation to use as a wind force on a POP simulation.
This example demonstrates how to use the FEM Solver to deform spheres when they collide with the ground plane. The spheres have particle based animation on them prior to collision with the ground and are swapped to the FEM solver on collision.
This example demonstrates how to use POP Advect by Filaments to advect particles using the velocity field of a set of vortex filaments.
This example demonstrates how to use POP Advect by Volumes to advect particles using the velocity from a smoke simulation.
This example demonstrates how to use the POP Attract node to get a group of particles to follow the motion of an animated sphere. POP Interact and POP Drag nodes are also used in the example to control the interaction between particles and their distance from the sphere.
This example demonstrates how to use the POP Attract node to get a particle sim to intercept and follow individual particles.
This example demonstrates how to use the POP Attract node with it’s type set to Point in order to control particle attraction on a per point basis.
This example shows three different ways in which the POP Axis Force node can be used with it’s type set to sphere to control your particle simulation.
This example demonstrates how to use the POP Axis Force node to cause a group of particles to billow upwards.
This example demonstrates the use of the POP Collision Detect node to simulate particles colliding with a rotating torus with animated deformations.
This example shows three different ways to use VEXpressions in your POP Color node to color your particles.
This example demonstrates the use of the POP Curve Force node to control the flow of a particle sim AND a flip fluid sim.
This example demonstrates how to control flocks of particles by using the POP Flock node.
This example demonstrates how to use the POP Force node to add curl noise to your particle simulation.
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.
This example demonstrates keyframing the internal grains of a solid pighead to create an animated puppet.
This example demonstrates attracting grain simulations to points on the surface of a model.
This example demonstrates interacting grain simulations of very different sizes.
This example demonstrates the use of the POP Interact node to control the distance between particles and create a ball shaped swarm.
This interactive example demonstrates the use of the POP Lookat node. Hit play and move the green target handle around in the viewport. The cone particles will orient themselves towards the target as you move it around.
This example shows how you can use the Drag Center parameter of the POP Property node to apply an off-center drag to falling objects.
This example demonstrates how to use POP Proximity node to find nearby particles and set attributes based on their proximity to one another.
This example shows how the POP Stream node can be used to define streams with different behaviours within your particle simulation.
This example demonstrates the how the shatter, RBD Fractured Object, and Debris shelf tools can be used to create debris emanating from fractured pieces of geometry.
First, the Shatter tool (from the Model tool shelf) is used on the glass to define the fractures. Then the RBD Fracture tool is used on the glass to create RBD objects out of the fractured pieces. Then the Debris tool is used on the RBD fractured objects to create debris.
This example combines a number of important DOPs concepts.
First, it uses both POP Solver and RBD Solver objects interacting with each other in a bidiretional manner. The RBD object affects the particles, and the particles affect the RBD object.
Second, the RBD object atually uses a multi-solver to combine an RBD Solver with a SOP Solver. The RBD Solver controls the motion of the overall object, while the SOP Solver performs the denting of the geometry.
Third, the SOP Solver extracts impact information from the RBD Solver to perform the denting. It extracts this information using DOP expression functions.
The end result is a simulation of a torus that is bombarded by a stream of particles. The particles bounce off the torus, and also cause the torus to move. In addition, each particle collision causes a slight denting of the torus.