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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 AOPspace of the object it is cooking from, just as if it had a builtin Object Merge SOP. To override this behavior, turn on the Ignore Transform Object parameter.
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.
Locals
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
Nonuniform 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.
Examples
The following examples include this node.
DynamicPops Example for Dynamics channel node
MountainSplash Example for Attribute Transfer geometry node
ChopSoftBody Example for Channel geometry node
VelocityStamp Example for Copy Stamp geometry node
CurvesectRods Example for Curvesect geometry node
IntersectGrid Example for Intersect VOP node
See also 