Houdini 11 Nodes Particle nodes

Emits particles from geometry.

Tip

To set particle attributes on the points of the emitter geometry (so the emitted particles inherit them at birth), connect a Point SOP to the emitter SOP and use the attributes on the Particle and Force tabs.

Note

A POP automatically adapts to the AOP-space of the object it is cooking from, just as if it had a built-in Object Merge SOP. To override this behavior, turn on the Ignore Transform Object parameter.

Using Source from Geometry

  1. Create an object.

  2. Click the Source from Geometry tool on the Create Particles tab.

    The particle system will attach itself to the object you have selected.

  3. Click play to see the particles.

Tip

If you want the particles to inherit the velocity of the source geometry they are emitting from, the geometry must have a velocity attribute (v) on the points or primitives (depending on where the particles are emitted from).

You can use a Trail surface node to add the velocity attribute to the geometry, with the Compute Velocities option. You can copy the velocity attribute to a different level of the source geometry (for example, points to primitives) with the Attrib Promote surface node.

Parameters

Source

Emission Type

Where on source geometry to emit particles from.

Points (ordered)

Emit particles from each point in the source geometry, in order. Primitives (for example, primitive sphere) have only one point from which to emit particles.

Points (random)

Same as “Points (ordered)”, but emits particles from points in random order. This order does not change over time: the same randomized sequence repeats over and over.

Prim center (ordered)

Birth particles from the center of each primitive in the source geometry, in order. This is useful when used in combination with the Rotate to Template parameter in the Primitive SOP.

Prim center (random)

Same as “Prim center (ordered)”, but emits particles from primitive centers in random order. This order does not change over time: the same randomized sequence repeats over and over.

Prim center (attribute)

Use the attribute named in the Distribution Attribute parameter below as the probability that a primitive will emit a particle.

Primitives where the attributes is less than or equal to 0 will not emit particles.

For primitives where the attributes is over 0, for each particle to be emit, the probability that a primitive will emit it is equal to that primitive’s attribute value divided by the sum of the attribute values of all the primitives.

You use the Measure SOP to create an attribute on primitives that records the surface area, then use that attribute here so primitives with greater surface area emit particles more often.

Edges (ordered)

Emit particles from each spline/polygon edge in the source geometry, in order. The birth points are along continuous intervals on the curve, not just from CVs or points.

Edges (random)

Same as “Edges (ordered)”, but emits particles from edges in random order. This order does not change over time: the same randomized sequence repeats over and over.

Edges (attribute)

See “Prim center (attribute)” above.

Surfaces (ordered)

Emit particles from each UV spline surface or mesh type in the source geometry, in order. If there is no surface but only a spline curve, then emit particles from the curve. For polygons are used, only emits particles from the polygon edges.

Surfaces (random)

Same as “Surfaces (ordered)”, but emits particles from surfaces in random order. This order does not change over time: the same randomized sequence repeats over and over.

Surfaces (attribute)

See “Prim center (attribute)” above.

Volume

Emit particles from inside the volume of the source geometry. The behavior is undefined if the source geometry is not closed.

Metaballs

Emit particles from inside the volume of a metaball, according to the Density Threshold and Density Minimum parameters below.

Geometry Source

Specifies the SOP to use.

Use Parameter Values

Use the SOP specified in the SOP parameter below.

Use First Context Geometry

Use the SOP connected to the POP network’s first input.

Use Second Context Geometry

Use the SOP connected to the POP network’s second input.

Use Third Context Geometry

Use the SOP connected to the POP network’s third input.

Use Fourth Context Geometry

Use the SOP connected to the POP network’s fourth input.

SOP

Path to the SOP (when Geometry source is set to Use Parameter Values).

Source Group

Subset of all points/primitives in the source geometry to emit from.

Ignore Transform Object

Particles normally use the object space of the SOP being cooked. Turn this parameter on to not transform into the space of the cooking SOP.

Sample Threshold

Give up birthing when the sampling failures meet or exceed the specified threshold. Increase this value to permit the operator to try for longer before giving up.

Distribution Attribute

When Emission type is “Prim center (attribute)”, “Edges (attribute)”, or “Surfaces (attribute)”, the attribute to use as the probability of emitting a particle.

Density Threshold

When Emission type is “Metaballs”, continue sampling points until the summed density exceeds this threshold.

Density Minimum

When Emission type is “Metaballs”, points with densities less than or equal to this value will not emit particles.

Use Metaball Density

Use the metaball density function as the probability of emitting a particle, so denser areas emit particles more often.

Birth

This operator has two methods for emitting particles. You can use these methods together or separately:

  • Impulse creates a certain number of particles each time the node cooks.

  • Constant creates a certain number of particles per second.

Impulse Activation

Turns impulse emission on and off. Impulse emits the number of particles in the Impulse birth rate below each time the operator cooks. A value of 0 means off, any other value means on.

Impulse Birth Rate

Number of particles to emit each time the node cooks (when Impulse activation is on).

Constant Activation

Turns constant emission on and off. Impulse emits the number of particles in the Constant birth rate below each second. A value of 0 means off, any other value means on.

Constant Birth Rate

Number of particles to emit per second (when Constant activation is on).

Birth Probability

Number of particles to emit per source point.

Birth Group

Name of a group to put the new points into.

Preserve Group

If the Birth group already exists, append the new particles to the group instead of replacing its contents.

Life Expectancy

How long the particle will live (in seconds).

Life Variance

Particles will live the number of seconds in Life expectancy, plus or minus this number of seconds. Use 0 for no variance.

Accurate Births

Recook source geometry before emission of each particle.

This ensures particles are emitted from the correct location when the source geometry is animated. If the shape of the source geometry does not change, you should turn this option off.

Attributes

Inherit Attributes

A list of names/patterns of attributes to inherit from the source geometry.

Create Local Variables

Create local variables corresponding to inherited attributes.

Initial Velocity

How to set the initial velocity of the emitted particles.

Use inherited velocity

Use the inherited velocity attribute as the initial velocity of the particles.

Add to inherited velocity

Add the inherited velocity attribute to the values from the Velocity and Variance parameters below.

Set initial velocity

Set the initial value of the velocity attribute using the Velocity and Variance parameters below.

Note

Acceleration is inherited independently, if applicable.

Inherit Velocity

(When Initial velocity is Use inherited velocity or Add to inherited velocity) The proportion of the inherited velocity to use. Use 1 for the full inherited value, 0.5 to half the inherited value, and so on.

Velocity

Set or add to velocity attribute.

Variance

Variance to velocity set above. The node will add +/- from 0 to this number along each axis to the Velocity parameter.

Ellipsoid Distribution

By default, the variance (if any) is distributed in a box, the size of which is determined by the Variance parameter. When this option is on, the variance is distributed in an ellipsoid instead.

Add Attributes

The parameters on this tab let you control which and how attributes are initialized on the emitted particles. See the POP locals page for explanations of these variables.

Add ID Attributes

Add ID and parent attributes to the created particles.

Add Generation Attribute

Add the generation attribute to the created particles.

Add Origin Attribute

Add the origin attribute to the created particles. The origin attribute holds an arbitrary number that can help you identify the source of a particle.

Add Speed Attribute

Add speed attribute. In Houdini, the speed attribute is the length of the velocity vector.

Use as Origin

Controls what Houdini sets as the value of a particle’s origin attribute (the identity of the geometry that emitted the particle). This lets you distinguish the origin of a particle later.

index

Set the origin to the value of the Origin index parameter below.

index + geo num

Sets the hit ID to the value of the Origin index parameter, added to the number of the geometry operator the particle was emitted from.

Typically, you would use large steps for the Origin index parameter (hundreds or thousands) so you can still distinguish different origins after the geometry number is added.

For example, you could have two Source POPs with origin index settings of 100 and 200. When the Source POP adds the geometry number to the origin index, you would get origins like 101, 102, 205, 210, and so on. As long as there is no geometry number larger than 99, you can still distinguish the different POPs.

Origin Index

Number to use as the value of the origin attribute on emitted particles (when Add origin attribute is on). This lets you distinguish different origins.

Local variables

These variables refer to the source geometry's properties.
BBX BBY BBZ

The point’s relative position in the bounding box.

CA

Point or vertex alpha value.

CR CG CB

Diffuse point or vertex color.

DIST

Distance from particle to last collision.

DRAG

Point drag.

ITER

Processing iteration number.

MAPU MAPV MAPW

Texture coordinates.

MASS

Point mass.

NPART

Number of particles.

NPT

Total number of points.

NPRIM

Total number of primitives.

NGRP

Total number of points in source group.

NX NY NZ

Normal vector.

P

Source point.

PSCALE

Particle Scale.

SCALEX SCALEY SCALEZ

Non-uniform scale.

SPEED

Absolute speed of particle.

SPRINGK

Elasticity of a point.

TENSION

Spring tension.

TIMEINC

Time increment.

TX TY TZ

Point position.

UPX UPY UPZ

Up vector.

U V

Surface UV values.

VX VY VZ

Velocity direction.

WEIGHT

Point spline weight.

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