Houdini 16.5 Nodes Particle nodes

Attribute Transfer POP node

Transfers point or vertex attributes from reference geometry onto particles.

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This operator transfers attributes (such as normal direction) from some referenced geometry to the input particles.

Vertex attributes are changed to point attributes for the particles if necessary. All vertices referencing a particular point are considered a single sample and the value of the first one is used as the sample value.

You can choose which attributes are transferred with patterns in the Points and Vertices parameters on the Attributes tab.

This operator can use two different methods for controlling when and how attributes are transferred, controlled in the Conditions tab.

Parameters

Activation

Turns this node on and off. The node is only active if this value is greater than 0. This is useful to control the effect of this node with an expression.

Note that this is activation of the node as a whole. You can’t use this parameter to deactivate the node for certain particles.

Source Group

Only affect a group of points (created with, for example, a Group POP or Collision POP) out of all the points in the input.

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).

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.

SOP Group

Subset of the referenced geometry to use. Either a group pattern or a named group (created by a Group SOP) can be used.

SOP Group Type

How to interpret the SOP Group parameter

Primitives

Interpret the string as a primitive group.

Points

Interpret the string as a point group.

Attributes

Choose which attributes are transferred with patterns in the Points and Vertices parameters.

  • To transfer all available attributes, leave the field blank or set it to a single asterisk (*).

  • To exclude attributes, prefix an attribute name with the caret character (^).

  • If the first attribute is preceded by a caret (^), all attributes are assumed to be transferred except where explicitly excluded. Otherwise, no attributes are assumed except where explicitly included.

Points

List of names/patterns of point attributes to transfer. Turn off the checkbox to avoid transferring any point attributes.

Vertices

List of names/patterns of vertex attributes to transfer. Turn off the checkbox to avoid transferring any vertex attributes.

Conditions

This tab lets you choose between two methods (Weighted average and Smoothed particle sampling) to control when and how attributes are transferred.

This tab displays different parameters depending on which method you choose.

Weighted average

Destination attributes are interpolated from the source attributes using a weighted proximity technique. Attribute calculations use the distance between the destination point and the candidate source points.

Smoothed particle sampling

This mode is based on the SPH (Smoothed Particle Hydrodynamics) model. This is a specialized model of attribute transfer useful in certain technical situations.

Each attribute is sampled using the formula:

A(r) = Sum(over all j) (m\[j] * A\[j]/p\[j] * Wh(r-r\[j])

…where r is a position in space, Wh is a normalized smoothing kernel over a radius h, and mj, Aj, pj, rj are the mass, attribute value, density, and position of sample j.

You might want to use the SPH model when you need to sample some physical quantity (say, temperature) at an arbitrary location in a space (generally a gas, liquid, or solid volume) and you have a discrete sampling of that quantity.

You would not use it when doing things such as grabbing colors from geometry, but you might want to use it if, for example, you are estimating a force vector within a wind tunnel at an arbitrary point given some spatial sampling of the force field.

Weighted average

Kernel Function

Interpolating metaball kernel . The kernels are parameterized by distance and typically give more weight to samples that are closer.

Kernel Radius

Radial extent of the kernel function. Specifies the drop-off radius for the kernel function. As the kernel radius parameter approaches zero, the filter resembles nearest neighbor interpolation. As the kernel radius parameter increases, the filter approaches uniform weighting.

Max Sample Count

Number of closest samples to interpolate from. specifies the maximum number of samples to interpolate from. A maximum sample count of 1 gives nearest neighbor interpolation.

Distance Threshold

Radial extent of the filter domain where the final attributes are determined from the source selection only. Specifies the maximum distance source points must be to be considered. Turning off this option indicates that no points/vertices will be excluded based on distance.

Initialize Attributes to Zero

Zero attributes when there are no samples. Attributes will be set to zero if no samples are found, otherwise, the attribute will maintain the previous value.

Smoothed particle sampling

SPH Kernel Function

Normalized smoothing function. Specifies the normalized smoothing kernel to use.

SPH Kernel Radius

The extent over which the samples are smoothed. Samples outside this radius do not contribute. specifies the maximum distance over which the kernel acts. The actual radius h used above is computed based on this value for each kernel.

Coefficient Attribute

An attribute that is used as a coefficient of the term for a sample in the sum. Specifies the name of the point attribute on the sample geometry to use as the m[j]/p[j] factor in the SPH formula.

Locals

Standard POP local variables

AGE

The seconds a particle in the template has been alive.

AX AY AZ

Acceleration of the particle.

BBX BBY BBZ

The point’s relative position in the bounding box.

DEAD

Point is dead.

ITER

Processing iteration number.

JUSTHIT

A collision for this particle was detected (for example, by the Collision POP) during the processing of this timestep (that is, this iteration of the particle simulation). This variable is cleared at the beginning of each timestep. Note that the collision POP actually detects any collisions which would have occurred the during the previous frame.

LIFE

Percent of total life used (from 0 to 1).

LIFESPAN

Expected lifetime of particle.

MAPU MAPV MAPW

Point or vertex texture coordinates.

NPT

Total number of points.

NGRP

Total number of points in source group.

NX NY NZ

Normal vector.

PT

The point number of the currently processed point. The PT is not constant like ID; it changes based on the number of points.

RESTX RESTY RESTZ

The rest position.

SLIDING

The sliding state of the particle.

SPRINGK

Elasticity of a point.

STOPPED

Point is stopped.

STUCK

1 if particle is stuck to a collision object.

TENSION

Spring tension.

TIMEINC

Time increment.

TX TY TZ

Point position.

U V

Surface UV values.

VX VY VZ

Velocity direction.

WEIGHT

Point spline weight.

Added by Collision POP/Limit POP

DIST

Distance from particle to last collision.

HCR HCG HCB

Diffuse color at the collision point on the surface the particle collided with.

HITID

ID for last collision. You can control how this attribute is set in the Collision or Limit POP to help distinguish types of collisions.

HITTIME

The time at which the last collision occurred.

HMAPU HMAPV

The texture map UV coordinates for the surface location where the last collision occurred.

HNX HNY HNZ

The normal at the surface location where the last collision occurred.

HTX HTY HTZ

World space position of the last collision.

HU HV

The UV coordinates for the surface location where the last collision occurred.

NUMHIT

Number of times the particle has collided.

Added by Color POP

CA

Point or vertex alpha value.

CR CG CB

Diffuse point or vertex color.

Added by Property POP

ATTRACT

Attractor point.

CHARGE

Charge of the particle.

CLING

Point is clinging to geometry.

DRAG

Point drag.

FOLLOW

Leader to follow.

MASS

Point mass.

PSCALE

Particle Scale.

SCALEX SCALEY SCALEZ

Non-uniform scale.

Added by Proximity POP

NEAREST

Either the point number or id of the particle nearest to this one.

NEARESTDIST

The distance to the nearest particle.

NUMPROXIMITY

The number of particles within a specified proximity to this particle.

Added by Rotation POP

ROTA

Rotation angle.

ROTX ROTY ROTZ

Rotation axis.

Added by Source POP

GEN

Generation.

ID

ID number, which always remains constant.

ORIGIN

Original Source point was birthed from.

PARENT

Parent’s ID Number.

Added by Speed Limit POP

SPEEDMAX

Maximum speed.

SPEEDMIN

Minimum speed.

Added by Sprite POP

SROT

Sprite rotation around view axis (in degrees).

STEXU STEXV

Texture coordinate of sprite’s lower-left corner.

STEXW STEXH

Size of sprite in texture space.

SX SY

Sprite scale.

Controlled by Suppress Rule POP

SUPPPOS

Suppress default position rule.

SUPPVEL

Suppress default velocity rule.

SUPPUP

Suppress default up-vector rule.

SUPPAGE

Suppress default aging rule.

SUPPROT

1 if particle is suppressing its default rotation rule.

SUPPANGVEL

1 if particle is suppressing its default angular velocity rule.

Added by Up Vector POP

PVX PVY PVZ

Previous velocity.

UPX UPY UPZ

Up vector.

Added by Location, Source, Softbody, Split POPs

SPEED

Absolute speed of particle.

See also

Particle nodes