Houdini 12 Particles

You can direct particle motion with forces. Forces can emanate from geometry in the scene (for example, you can create a sphere and have it attract or repel particles), or you can create a general directional force (such as gravity).

Note

Forces can be scaled and modified by attributes such as mass, charge, and drag.

How to direct particle motion with forces

To...Do this
Make geometry attract or repel particles

  1. Click the Attractor button from the Drive Particles tab of the shelf.

  2. Click LMB the particle system you want to be attracted or repelled and press Enter to confirm your selection.

  3. Click LMB the object you want the particles to be attracted to or repelled from and press Enter to confirm your selection.

The degree of attraction/repulsion can be modified in the parameter editor.

For specific parameter help see the Attractor node help.

Create a general directional force

  1. Click the Force button on the Drive Particles tab of the shelf.

  2. Click LMB the particle system you want to be affected and press Enter to confirm your selection.

  3. Specify the node settings in the operation controls toolbar or parameter editor.

You must specify the amount of force and direction of force in the operation controls toolbar or parameter editor.

For specific parameter help see the Force node help.

Note

You can also use the Fan or Wind tools on the shelf depending on how you want the particles to react to the directional force.

Make the particles affected by gravity

  1. Click the Gravity button on the Drive Particles tab.

  2. Click LMB the particle system you want to be affected and press Enter to confirm your selection.

  3. Specify the node settings in the operation controls toolbar or parameter editor.

Note

The Gravity POP is simply predefined force node whose force and direction can be altered in the parameter editor.

Add random force to particles

  1. Click the Noise button on the Drive Particles tab.

  2. Click LMB the particle system you want to be affected and press Enter to confirm your selection.

  3. Specify the node settings in the operation controls toolbar or parameter editor.

For specific parameter help see the Noise node help.

Mass, drag, and charge

The mass and charge attributes change how some POPs interact with a particle. You can set the mass and/or charge of particles with the Property POP.

Drag is a resistance against a particle’s velocity. The resistance is proportional to the particle’s speed and the value of its drag attribute, which represents the particle’s drag coefficient, or how “un-aerodynamic” the particle is. So faster particles and particles with high drag coefficient have their speed retarded more by drag. You can set the drag of particles with the Drag POP.

Changing the mass of a particle scales forces on the particle according to the following formula:

F = M x A

...where F is force, M is mass, and A is acceleration. So, particles with high mass require more force (F) to change their motion.

The most obvious application of the charge attribute is in the Interact POP, where like subatomic particles, particles with like charges repel while particles with opposite charges attract.

Some operators use the charge to multiply the computed force. So if the resulting force is positive, the force will act in its intended direction. If the resulting force is negative, the force will act in the opposite direction.

For example, the Attractor POP creates a force that attracts the particle to the attractor. A positive force will move the particle towards the attractor. If the particle has a negative charge, however, the positive force multiplied by the negative charge would result in a negative force that would repel the particle away from the attractor.

Operators that use the mass and charge attributes, such as Attractor, will often have Ignore mass and Ignore charge options to prevent the attributes from affecting the normal operation of the node.