houdini attributes cheatsheet

houdini_attributes.md

https://mrkunz.com/blog/08_22_2018_VEX_Wrangle_Cheat_Sheet.html

f@Frame	The current floating frame number, equivalent to the $FF Hscript variable
f@Time	The current time in seconds, equivalent to the $T Hscript variable
i@SimFrame	The integer simulation timestep number ($SF), only present in DOP contexts
f@SimTime	The simulation time in seconds ($ST), only present in DOP contexts
f@TimeInc	The timestep currently being used for simulation or playback
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Attribute Wrangle
v@P	The position of the current element
i@ptnum	The point number attached to the currently processed element
i@vtxnum	The linear number of the currently processed vertex
i@primnum	The primitive number attached to the currently processed element
i@elemnum	The index number of the currently processed element
i@numpt	The total number of points in the geometry
i@numvtx	The number of vertices in the primitive of the currently processed element
i@numprim	The total number of primitives in the geometry
i@numelem	The total number of elements being processed
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Volume Wrangle
v@P	The position of the current voxel
f@density	The value of the density field at the current voxel location
v@center	The center of the current volume
v@dPdx, v@dPdy, v@dPdz	These vectors store the change in P that occurs in the x, y, and z voxel indices
i@ix, i@iy, i@iz	Voxel indices For dense volumes (non-VDB) these range from 0 to resolution-1
i@resx, i@resy, i@resz	The resolution of the current volume
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Common Geometry Attributes
i@id	A unique indexing number that remains the same throughout time Used to match elements between frames
f@pscale	Particle radius size Uniform scale Set display particles as 'Discs' to visualize
f@width	Thickness of curves Enable 'Shade Open Curves In Viewport' on the object node to visualize
f@Alpha	Alpha transparency override The viewport uses this to set the alpha of OpenGL geometry
f@Pw	Spline weight Mostly depreciated at this point
v@P	Point position Used this to lay out points in 3D space
v@Cd	Diffuse color override The viewport uses this to color OpenGL geometry
v@N	Surface or curve normal Houdini will compute the normal if this attribute does not exist
v@scale	Vector scale Allows directional scaling or stretching (in one direction)
v@rest	Used by procedural patterns and textures to stick on deforming and animated surfaces
v@up	Up vector The up direction for local space, typically (0, 1, 0)
v@uv	UV texture coordinates for this point/vertex
v@v	Point velocity The direction and speed of movement in units per second
p@orient	The local orientation of the point (represented as a quaternion)
p@rot	Additional rotation to be applied after orient, N, and up attributes
s@name	A unique name identifying which primitives belong to which piece Also used to label volumes
s@instance	Path of an object node to be instanced at render time
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DOP Particle Attributes
f@age	Time in seconds since the particle was born
f@life	Time in seconds the particle is allowed to live When f@age>f@life, i@dead will be set to 1
f@nage	Normalized age, f@age divided by f@life Implicit attribute, you cannot write to this
i@dead	Whether a particle is living (0) or dead (1) A dead particle is deleted in the Reaping stage
i@id	A unique id for the particle that remains the same throughout a single simulation
i@stopped	Whether a particle is moving (0) or stopped (1)
i@stuck	Whether a particle is free (0) or stuck (1)
i@sliding	Whether a particle is free (0) or sliding along a surface (1)
f@cling	Force applied to sliding paritcles inwards (according to the collision's surface normal)
s@pospath	The path to the object that the particle is colliding with
i@posprim	Which collision primitive in the path geometry whose position we wish to refer to
v@posuv	Parametric uv on the collision primitive
i@hittotal	The cumulative total of all hits for the particle (only incremented once per timestep)
i@has_pprevious	This is set to 1 if v@pprevious contains valid values
v@pprevious	Stores the position of the particle on the previous frame Used for collision detection
i@hitnum	The number of times the particle collided in the last POP Collision Detect
s@hitpath	The path to the object that was hit A path to a file on disk or an op: path
i@hitprim	The primitive hit Could be -1 if it the collision detector couldn’t figure out which prim
v@hituv	The parametric UV space on the primitive
v@hitpos	Where the hit actually occurred Useful if the colliding object was moving
v@hitnml	The normal of the surface at the time of the collision
v@hitv	The velocity of the surface at the time of the collision
f@hittime	When the collision occurred, that could be within a frame
f@hitimpulse	Records how much of an impulse was needed for the collision resolution varies with timestep
f@bounce	When particles bounce off another object, this controls how much energy they keep
f@bounceforward	Controls how much energy they keep in the tangential direction
f@friction	When particles bounce, they are slowed down proportional to how hard they hit
s@collisionignore	Objects that match this pattern will not be collided
f@force	Forces on the particle for this frame
f@mass	Inertia of the particle
v@spinshape	This is multiplied by f@pscale to determine the shape of the particle for rotational inertia
f@drag	How much the particle is effected by any wind effects
f@dragexp	Ranges from 1 to 2, default is set on the solver Used for both angular and linear drag
v@dragshape	How much the particle is dragged in each of its local axes
v@dragcenter	If specified, drag forces will also generate torques on the particle
v@targetv	The local wind speed Thought of as the goal, or target, velocity for the particle
f@airresist	How important it is to match the wind speed Thickness of the air
f@speedmin	Minumum speed, in units per second, that a particle can move
f@speedmax	Maximum speed, in units per second, that a particle can move
p@orient	Orientation of the particle Used for figuring out 'local' forces
v@w	Angular speed of the particle A vector giving the rotation axis
v@torque	The equivalent of force for spins No inertial tensor (the equivalent of mass) is supported
v@targetw	The goal spin direction and speed for this particle
f@spinresist	How important it is to match the targetw
f@spinmin	Minumum speed in radians per second that a particle can spin
f@spinmax	Maximum speed in radians per second that a particle can spin
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DOP Grains Attributes
i@ispbd	A value of 1 causes the particle to behave as grains
f@pscale	Used to determine the radius of each particle
f@repulsionweight	How much the particle collision forces are weighted
f@repulsionstiffness	How strongly particles are kept apart Higher values result in less bouncy repulsion
f@attractionweight	How much the particles will naturally stick together when close
f@attractionstiffness	How strongly nearby particles stick to each other
v@targetP	Particles are constrained to this location
f@targetweight	The weight of the v@targetP constraints
f@targetstiffness	The stiffness with which particles are fixed to their v@targetP attribute
f@restlength	Particles connected by polylines will be forced to maintain this distance (prim attribute)
f@constraintweight	Scale, on a per-particle basis of the constraint force
f@constraintstiffness	This controls the stiffness on a per-particle basis
f@strain	This primitive attribute records how much the constraint is stretched
f@strength	If f@strain exceeds this primitive attribute, the constraint will be removed
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DOP Packed RBD Attributes
i@active	Specifies whether the object is able to react to other objects
i@animated	Specifies whether the transform should be updated from its SOP geometry at each timestep
i@deforming	Specifies whether the collision shape should be rebuilt from its SOP geometry each timestep
f@bounce	The elasticity of the object
i@bullet_add_impact	Impacts that occur during the sim will be recorded in the Impacts or Feedback data
i@bullet_ignore	Specifies whether the object should be completely ignored by the Bullet solver
f@bullet_angular_sleep_threshold	The sleeping threshold for the object’s angular velocity
f@bullet_linear_sleep_threshold	The sleeping threshold for the object’s linear velocity
i@bullet_want_deactivate	Disables simulation of a non-moving object until the object moves again
i@computecom	Specifies whether the center of mass should be computed from the collision shape
i@computemass	Specifies whether the mass should be computedfrom the collision shape and density
f@creationtime	Stores the simulation time at which the object was created
i@dead	Specifies whether the object should be deleted during the next solve
f@density	The mass of an object is its volume times its density
f@friction	The coefficient of friction of the object
f@inertialtensorstiffness	Rotational stiffness A scale factor applied to the inertial tensor
i@inheritvelocity	v and w point attributes from the SOP geometry will override the initial velocity
f@mass	The mass of the object
s@name	A unique name for the object Used by Constraint Networks
p@orient	The orientation of the object
v@P	The current position of the object’s center of mass
v@pivot	The pivot that the orientation applies to If i@computecom is non-zero, this is auto-computed
v@v	Linear velocity of the object
v@w	Angular velocity of the object, in radians per second
i@bullet_adjust_geometry	Shrinks the collision geometry
i@bullet_autofit	Use the bounds of the object for Box, Capsule, Cylinder, Sphere, or Plane
f@bullet_collision_margin	Padding distance between collision shapes
s@bullet_georep	Can be convexhull, concave, box, capsule, cylinder, compound, sphere, or plane
i@bullet_groupconnected	Create convex hull per set of connected primitives
f@bullet_length	The length of the Capsule or Cylinder collision shape in the Y direction
v@bullet_primR	Orientation of the Box, Capsule, Cylinder, or Plane collision shape
v@bullet_primS	Size of the Box collision shape
v@bullet_primT	Position of the Box, Sphere, Capsule, Cylinder, or Plane collision shape
f@bullet_radius	Radius of the Sphere, Capsule, or Cylinder collision shape
f@bullet_shrink_amount	Specifies the amount of resizing done by Shrink Collision Geometry
s@activationignore	Won't be activated by collisions with any objects that match this pattern
s@collisiongroup	Specifies the name of a collision group that this object belongs to
s@collisionignore	The object will not collide against any objects that match this pattern
f@min_activation_impulse	Minimum impulse that will cause the object to switch from inactive to active
f@speedmin	Minumum speed, in units per second, that a particle can move
f@speedmax	Maximum speed, in units per second, that a particle can move
f@spinmin	Minumum speed in radians per second that a particle can spin
f@spinmax	Maximum speed in radians per second that a particle can spin
f@accelmax	Limits the change in the object’s speed that is caused by enforcing constraints
f@angaccelmax	Limits the change in the object’s angular speed that is caused by enforcing constraints
f@airresist	Specifies how important it is to match the target velocity (v@targetv)
f@drag	How much the the v@targetv and f@airresist attributes effect the object
f@dragexp	Ranges from 1 to 2, default is set on the solver Used for both angular and linear drag
v@force	Specifies a force that will be applied to the center of mass of the object
f@spinresist	Specifies how important it is to match the target angular velocity (v@targetw)
v@targetv	Target velocity for the object Used in combination with the f@airresist attribute
v@targetw	Target angular velocity for the object Used in combination with the f@spinresist attribute
v@torque	Specifies a torque that will be applied to the object
i@bullet_autofit_valid	Stores whether the solver has already computed collision shape attributes
i@bullet_sleeping	Tracks whether the object has been put to sleep by the solver
f@deactivation_time	Amount of time the speed has been below the Linear Threshold or Angular Threshold
i@found_overlap	Used by the solver to determine whether it has performed the overlap test
i@id	A unique identifier for the object
i@nextid	Stores the i@id the solver will assign to the next new object
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DOP Constraint Network Attributes
f@width	Width of each edge
f@density	Density of each point
p@orient	Initial orientation of each point This value is stored as a quaternion
v@v	Initial velocity of each point
v@w	Initial angular velocity of each point measured in radians per second
f@friction	Friction of each point
f@klinear	Defines how strongly the wire resists stretching
f@damplinear	Defines how strongly the wire resists oscillation due to stretching forces
f@kangular	Defines how strongly the wire resists bending
f@dampangular	Defines how strongly the wire resists oscillation due to bending forces
f@targetstiffness	Defines how strongly the wire resists deforming from the animated position
f@targetdamping	Defines how strongly the wire resists oscillation due to stretch forces
f@normaldrag	The component of drag in the directions normal to the wire
f@tangentdrag	The component of drag in the direction tangent to the wire
i@nocollide	Collision detection for the edge is disabled (Only used if Collision Handling is SDF)
v@restP	Rest position of each point
p@restorient	Rest orientation of each point
i@gluetoanimation	Causes a point’s position and orientation to be constrained to the input geometry
i@pintoanimation	Causes a point’s position to be constrained to the input geometry
v@animationP	Target position of each point
p@animationorient	Target orientation of each point
v@animationv	Target velocity of each point
v@animationw	Target angular velocity of each point
i@independentcollisionallowed	Toggle external collisions (Only non-SDF Geometric Collision)
i@independentcollisionresolved	Unresolved external collisions (Only non-SDF Geometric Collision)
i@codependentcollisionallowed	Toggle soft body collisions (Only non-SDF Geometric Collision)
i@codependentcollisionresolved	Unresolved toggle soft body collisions (Only non-SDF Geometric Collision)
i@selfcollisionallowed	Toggle self collisions (Only non-SDF Geometric Collision)
i@selfcollisionresolved	Unresolved toggle self collisions (Only non-SDF Geometric Collision)
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DOP FLIP Attributes
f@pscale	Particle scale
v@v	Particle velocity
f@viscosity	The "thickness" of a fluid
f@density	The mass per unit volume
f@temperature	The temperature of the fluid
f@vorticity	Measures the amount of circulation in the fluid
f@divergence	Positive values cause particles to spread out, negative cause them to clump together
v@rest	Used to track the position of the fluid over time
v@rest2	Used for blending dual rest attributes, avoids stretching
f@droplet	Identifies particles that separate from the main body of fluid
f@underresolved	Particles that haven't fully resolved on the grid
i@ballistic	Specifies particles which will be ignored by the fluid solve
v@Lx	Angular momentum X axis
v@Ly	Angular momentum Y axis
v@Lz	Angular momentum Z axis