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PhysX Force Operator

Path: Particle View > Add or select PhysX Force.

The PhysX Force operator is similar to the standard Force operator but is better suited to simulation usage. The regular Force operator adjusts particle speed. This is not convenient if you use PhysX World to run the simulation, because you need to use the PhysX Switch operator to relay the adjusted speed information to simulation by turning on Match Position or Speed > Speed. Even with that, the constant direct manipulation of speed data can distort the simulation workflow. PhysX Force defines actual forces that influence particles, and the force field information is passed to the PhysX simulation.

As with the standard Force operator, you can use Script Wiring with the PhysX Force operator to define the Influence setting. An additional option is the ability to wire the Influence parameter with a data channel in a Data Operator from Box#3. For that you need to choose the From Data Operator option, and also use Output PhysX sub-operator to be set to the Force Influence type for output.

Note: In general, place all PhysX operators above a PhysX World operator in an event, so the PhysX properties defined by the operators are known to the simulation engine before the simulation cycle starts.

See also PhysX Drag Operator.

Interface

Because the PhysX Force operator is designed for simulation usage, it has a few more options than the standard Force operator.

Force Space Warps - Lists space warps currently in the simulation.

To add a space warp to the simulation, click Add and then select the space warp, or, to add several at one time, use By List. To remove a space warp, highlight its name in the list and click Remove.

=> - To select a space warp assigned in PhysX Force, highlight it in the list and then click this button. This selects the space warp in the scene and makes it easy to access the space warp parameters.

Force Type - Defines the force magnitude, as affected by particle properties. The standard Force operator applies the same force value to a particle; the force magnitude is affected only by particle speed and position. With the standard Force operator, if two particles have the same position and speed then the same force is applied to these particles.

The PhysX Force operator can also take into consideration other particle properties — their mass and shape — by using the appropriate Force Type option:

  • Gravity - The force magnitude is adjusted by particle mass, in direct proportion — the higher the mass, the higher the force — in a way somewhat similar to the gravity effect. The option most closely resembles the standard Force operator. Because force is adjusted proportionally to particle mass, two particles, even with different masses, will travel with the same speed, as in the fabled experiment in which Galileo dropped two different weights off the Leaning Tower of Pisa.

  • Surface - The force magnitude is adjusted by particle surface area visible from the direction of the force. This type simulates the effect of wind force.

  • Uniform - The force magnitude is applied as is, without any adjustment by particle mass or size. This is like a finite force from an external force. With this force type, if two particles of different sizes have different masses, then the larger particle would accelerate less.

Force Variation Threshold - When on, the force applied to a particle is calculated as a cumulative force applied to all vertices of a particle shape. Thus, a non-uniform force can create a torque moment.

The standard Force operator uses particle position and speed to calculate the force applied. It does not take a particle's dimensional properties into account; a particle is considered as a point.

The PhysX Force operator can take into account particle size while dealing with non-uniform forces — forces whose value in space varies. If a particle has significant size, a force applied to one end of a particle can differ from the force applied to the other end. The difference in force can also create a torque force applied to a particle.

Shape Size - The minimum particle size upon which a force applied is calculated as a cumulative force applied to all vertices of a particle shape. With this option you can speed up calculations by specifying that force for sufficiently small particles can be calculated as applied to a particle point. Available only when Force Variation Threshold is on.

Force Field Overlapping - Determines how multiple forces that occupy the same volume of space affect the particles. With Additive, the forces are combined according to their relative strengths. With Maximum, only the force with the greatest strength affects the particles.

Influence % - Specifies the strength with which the force or forces are applied to the particles as a percentage. Default=1000.0.

Exponent (10**N) - This setting can be useful when dealing with different force types. The overall effect of a force applied can be affected by particle mass/size, so you might need to apply large force values when dealing with small particles. To simplify the process, you don't need to increase the Influence % value by an order of magnitude; instead, you can just increase Exponent spinner by 1.

Offset Influence - Choose the time frame for applying animated parameters. This works the same way as in the standard Force operator. The options are:

  • Absolute Time - Any keys set for parameters are applied at the actual frames for which they're set.

  • Particle Age - Any keys set for parameters are applied at the corresponding frames of each particle's existence.

  • Event Duration - Any keys set for parameters are applied to each particle starting when it first enters the event.

Impulse On Event Entry - Applies the force only once, when a particle enters the event. The value of the force applied is calculated as if the force is applied constantly for the period of time specified by the Time Warp value.

Important: When PhysX Shape is set to define mass by density, and PhysX Force > Force Type is set to Gravity (see preceding), do not use Impulse On Event Entry, as this can lead to simulation errors.

When off, forces are applied on a constant basis while particles are present in the event containing the PhysX Force operator. Impulse On Event Entry is useful when you need to apply force only when a particle enters the event, perhaps as a "jolt" or "impulse" to be applied to a particle to change its speed or spin.

  • Time Warp - The duration over which the force is calculated when Impulse On Event Entry is on; this derived force is then applied instantaneously to each particle as it enters the event.