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This POP emits particles from the points of its Geometry Source. However, unlike the Source POP, this POP does not make use of birth rate parameters. Instead, it determines when to emit particles based on the velocity of emitted particles. The timing of emission is controlled in order to maintain a specified spacing between particles in the emitted stream. Currently, this POP is used for emission of fluid particles in the Particle Fluid Emitter DOP.
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.
A POP automatically adapts to the sop-space of the object it is cooking from, just as if it had a built-in Object Merge SOP. To over-ride this behavior, turn on the Ignore Transform Object parameter.
A detail attribute is added to record the last emission time. The name is the full path to this node, so will not interfere with other velocitystream nodes. However, if the same GDP will be reused for another simulation starting at another time, this detail attribute should be deleted with an Attribute SOP.
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.
Path to the SOP (when Geometry source is set to Use Parameter Values).
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.
Subset of all points/primitives in the source geometry to emit from.
Enables/disables particle emission from this node.
Controls the spacing between particles in the emitted stream. This is done by regulating the times at which particles are emitted. For example, if the velocity of emitted particles is set to 1 in the x direction, and the Stream Spacing parameter is set to 1, then particles will be emitted from each point in the Geometry Source every 1 second in order to maintain the desired spacing.
This parameter does not affect the behavior of particles once they are emitted, but rather affects the timing and direction of particle emission. It is used to regulate the timing of particle emission in the presence of external forces accelerating the particles. For example, if particles are emitted with no velocity, but are immediately affected by a downward gravity force, then the acceleration caused by that force could be specified here.
Stream Velocity Scale
This parameter is a multiplier which controls the velocity used to determine the timing of particle emissions. It does not affect the actual velocity of emitted particles, but rather can be used to internally scale the emission velocity to thicken or thin out the emitted particle stream.
Name of a group to put the new points into.
If the Birth group already exists, append the new particles to the group instead of replacing its contents.
How long the particle will live (in seconds).
Particles will live the number of seconds in Life expectancy, plus or minus this number of seconds. Use 0 for no variance.
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.
A list of names/patterns of attributes to inherit from the source geometry.
Create Local Variables
Create local variables corresponding to inherited attributes.
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.
Acceleration is inherited independently, if applicable.
(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.
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).
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.
Const. Birth Rate
Number of particles to emit per second (when Constant activation is on).
Set or add to velocity attribute.
Variance to velocity set above. The node will add +/- from 0 to this number along each axis to the Velocity parameter.
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.
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.
Value for the origin attribute (when Add origin attribute is on).
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.
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.
Number to use as the origin of emitted particles. This lets you distinguish different origins.
These variables refer to the source geometry’s properties.
BBX BBY BBZ
The point’s relative position in the bounding box.
Point or vertex alpha value.
CR CG CB
Diffuse point or vertex color.
Distance from particle to last collision.
Processing iteration number.
MAPU MAPV MAPW
Number of particles.
Total number of points.
Total number of primitives.
Total number of points in source group.
NX NY NZ
SCALEX SCALEY SCALEZ
Absolute speed of particle.
Elasticity of a point.
TX TY TZ
UPX UPY UPZ
Surface UV values.
VX VY VZ
Point spline weight.