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You can use this operator to set an initial up vector for new particles, or to manually change the up vector of existing particles.
When Houdini calculates a particle’s frame of reference, the velocity defines the Z direction, the up vector defines the Y direction and the cross product of the two defines the X direction. Whereas the particle’s rotation indicates the bank/roll of the particle as it moves forward, the frame of reference can point in a different direction from where the particle is headed.
The frame of reference orients geometry instanced on the particle.
A default rule applied to all particles transforms the Up Vector according to the particle’s directional change from frame to frame. To turn off the default rule, use the Suppress Rule POP.
Because the default rule needs to know what the previous velocity of the particle is, this operator also adds the previous velocity attribute. If the Up Vector is inherited from source geometry, the previous velocity attribute will automatically be added.
The up vector is evaluated either in component form or as a vector.
This operator modifies the following attributes:
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.
To all points
Rotate all input particles.
To moving points
Only rotate particles with velocity greater than 0.
Up vector to use.
Standard POP local variables
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.
Point is dead.
Processing iteration number.
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.
Percent of total life used (from 0 to 1).
Expected lifetime of particle.
MAPU MAPV MAPW
Point or vertex texture coordinates.
Total number of points.
Total number of points in source group.
NX NY NZ
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.
The sliding state of the particle.
Elasticity of a point.
Point is stopped.
1 if particle is stuck to a collision object.
TX TY TZ
Surface UV values.
VX VY VZ
Point spline weight.
Distance from particle to last collision.
HCR HCG HCB
Diffuse color at the collision point on the surface the particle collided with.
ID for last collision. You can control how this attribute is set in the Collision or Limit POP to help distinguish types of collisions.
The time at which the last collision occurred.
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.
The UV coordinates for the surface location where the last collision occurred.
Number of times the particle has collided.
Added by Color POP
Point or vertex alpha value.
CR CG CB
Diffuse point or vertex color.
Added by Property POP
Charge of the particle.
Point is clinging to geometry.
Leader to follow.
SCALEX SCALEY SCALEZ
Added by Proximity POP
Either the point number or id of the particle nearest to this one.
The distance to the nearest particle.
The number of particles within a specified proximity to this particle.
Added by Rotation POP
ROTX ROTY ROTZ
Added by Source POP
ID number, which always remains constant.
Original Source point was birthed from.
Parent’s ID Number.
Added by Speed Limit POP
Added by Sprite POP
Sprite rotation around view axis (in degrees).
Texture coordinate of sprite’s lower-left corner.
Size of sprite in texture space.
Controlled by Suppress Rule POP
Suppress default position rule.
Suppress default velocity rule.
Suppress default up-vector rule.
Suppress default aging rule.
1 if particle is suppressing its default rotation rule.
1 if particle is suppressing its default angular velocity rule.
Added by Up Vector POP
PVX PVY PVZ
UPX UPY UPZ
Absolute speed of particle.