float

dragnormal, dragtangent

(Point) How much to scale wind effects when the face of the cloth is heading straight into the wind versus sliding parallel to it. For hair, normal is cross-wise to the hair and tangent is along the hair direction.

float

inertia

(Point) The resistance of a particle to rotational hair constraints. If this is zero, the particle will not rotate.

integer

weld, branchweld, collisionweld

(Point) Default -1.

A value of 1 or a reference to itself means not welded, but something might be welded to this point. The value is a point number, unless there is an @id attribute. Then it is a point id.

@weld is the user accessible option. @branchweld is built by the hair constraints when it is forced to split points for hair simulation. @collisionweld is generated on demand to provide a single weld to the detangle algorithm.

float

friction, dynamicfriction

(Point, Detail) How much to scale the static friction (friction) and dynamic friction scale (dynamicfriction) settings by.

float

pressurescale

(Point) How much to scale the effect of a Pressure constraint on a per-point basis, from 0 to 1.

vector

P, pprevious, plast, v, vprevious, vlast

(Point) For 2nd order we need the previous frames position and velocity (pprevious, vprevious), and the frame before that, two frames ago (plast, vlast).

point, vector

orient, orientprevious, orientlast, w, wprevious, wlast

(Point) Hair requires point orientations and the corresponding angular velocity (w). Same previous/last.

integer

stopped

(Point) This is a standard POP attribute, but it is used and respected by Vellum. Auto-sleep and awaken will manipulate the stopped attribute. Pins that are to be released can be controlled using stopped rather than setting the mass to zero.

Stopped controls both orientation and position updates via bits 0 and 1.

• 0 - Particle is free.

• 1 - Particle can rotate, but is fixed in space.

• 2 - Particle can’t rotate, but is free to move in space.

• 3 - Particle can’t rotate or move.

integer

dead

(Point) This is a standard POP attribute, but it is used and respected by Vellum. If set to 1 the point and any associated constraints will be removed from the simulation. This attribute can be set easily with the POP Kill DOP.

float

pscale

(Point) Radius at each point. Triangles are fattened at their corners by spheres of the corner pscales, tapered tubes on edges, and sloped planes in center.

integer

layer

(Point) Controls soft priority of collisions. Only greater/less matters, not magnitude of difference.

integer

disableself, disableexternal

(Point) A value of 0 means this point will use self collisions or external collisions.

If any points in an edge or triangle are disabled (not 0), the whole edge/tri is fully disabled.

Auto-enable/disable does bitwise tricks with this value:

• When Houdini auto-disables points:

  • If disableself was 0 Houdini will change it to 2 (disables collisions).

  • If it was 1 Houdini will change it to 3 (still disabled).

  • If it was 2, it will not be changed.

  (Technically, Houdini bitwise ORs the value with 2).

• When Houdini auto-enables points:

  • If disableself was 0 or 1 it doesn’t change.

  • If it was 2 Houdini changes it to 0 (enables collisions).

  • If it was 3 Houdini changes it to 1 (still enabled).

  (Technically, Houdini turns the second bit off.)

This trick means that to disable a point independent of auto-detection, you should set the value to 1. However, if you want to re-enable something but aren’t sure if it is still tangled, you can set to 2 rather than 0 to delay enabling collisions until it is detected clean.

float

overlap_self, overlap_external

(Point) The default value is 10000.

Stores how much of the original pscale is overlapped. So 0 means that the pscale is fully used. This avoids first-frame explosion by effectively reducing the effective pscale by overlap_foo for collisions, so points don’t start overlapping. Visualizing thickness will draw cyan rather than green for points modified this way. As points separate, this will decrease to grab any exposed room. Thus a pscale that is too high will become unsteady as points grow and then fight distance constraints.

string

collisionignore

(Point) The default value is “”.

Stores a pattern for the objects and collision groups to not collide with. This means the default empty string will enable all collisions. To disable all collisions, use the string *. This supports the common Houdini matching rules, so foo* will collide against the objects whose name doesn’t start with foo. Exclusion parameters such as * ^foo are not currently supported.

The currently solving Vellum object always uses the object name self. The Vellum Solver SOP has the default object names of groundplane for any ground plane added, and external for any collisions in the third input. If packed primitives are in the third input, and they have a primitive attribute objname, that is used for those primitives implied object name.

Note that this will match against both the object name and the collision group. The latter is specified with the string collisiongroup point attribute. To match a specific object/collision group pair, use a slash. For example, external/bar will match only the collision group bar of the external object.

string

collisiongroup

(Point) The default value is “”.

Gives the collision group that this point belongs to. If this matches a collision ignore label, it will not engage in collision detection.

The default empty string will only match against the * pattern.

string, integer

target_path, target_pt, pintoanimation, gluetoanimation

(Point) The target path and point number for any pins when the Target parameter is set in Vellum Source. If pintoanimation is 1, the pinned points' position will be updated to match the specified target point on the target path geometry. If gluetoanimation is 1, both the position and orientation will be updated.

float

targetweight

(Point) Affect the strength of the pinned points using a 0..1 weighting value.

float, string

breakthreshold, breaktype

(Point) The threshold for breaking welds and branch welds, measured against the breaktype, which can be one of the following: stretchstress, bendstress, stretchdistance, stretchratio, or bendangle. If breakthreshold is less than zero, no breaking will occur.

vector, integer

pressuregradient, volumepts, volume

(Point) When a point is part of a Pressure constraint, these attributes hold values computed during constraint evaluation. The pressuregradient is a vector pointing outwards along the direction of greatest volume gain, usually very close to a point normal. The volumepts array contains the points needed to compute the volume attribute for each point, which are added together to compare against the Pressure constraint’s restlength.

float

dP, dPw

(Point) Constraint displacements/weights. Likely of last iteration, so out of date.

string

patchname

(Point) Identifies each generated patch in a simulation so it can be updated/replaced.

float

dampingratio

(Prim) 0..1 damping ratio.

float

restlength, restlengthorig

(Prim) The initial “distance” of the constraint. The units are variable. Distance constraints use distance, bend constraints use angle in degrees, volume and pressure constraints use volume.

The actual dynamics use only restlength. restlengthorig is for convenience so you can animate scaling effects without extra math.

point

restvector

(Prim) Orientation constraints need a rest orientation, so store a quaternion here. This vector is also used to store target positions for pin constraints.

vector

rest

(Point) The original point position in rest space, used by shapematch constraints.

float

stiffness

(Prim) 0..1e10. How strongly to enforce the constraint. High values may not be achieved with a given iteration count, but low values should stop getting stronger at a certain iteration level.

float

compressionstiffness

(Prim) 0..1e10. How strongly to enforce the constraint when it is being compressed. High values may not be achieved with a given iteration count, but low values should stop getting stronger at a certain iteration level.

float

stiffnessdropoff stiffnessscaled stiffnessdropoffmin

(Prim) The distance from the rest state at which the stiffness of the constraint drops off to zero, or increases from zero to full stiffness, depending on the direction of the dropoff. A negative values indicates a decreasing dropoff, where the stiffness starts at full strength and decreases to zero at the Dropoff distance from the rest state. A positive value indicates an increasing dropoff, where the stiffness starts at zero and increases to full stiffness at the specified distance from rest. For stretch constraints this value is in length units; for bend constraints it is stored in degrees. The effective stiffness value used in constraint solving will be stored in stiffnessscaled when stiffnessdropoff is in effect. If the stiffnessdropoffmin attribute exists and is greater than zero, its value will be used as the minimum stiffness when calculating the final stiffness value due to dropoff.

string

type

(Prim) Type of the constraint.

float

stress

The amount of stress applied by the constraint.