Controls if any collision detection is done at all.

Controls if self collisions are detected.

Adds a ground plane to the simulation. The ground plane will be visualized with a blue grid.

Location of the center of the ground plane.

Number of collision detection passes to perform. These are interleaved between the constraint iterations. Since collision is expensive, it is best to minimize this. But frequent interleaving helps avoid tent-poling effects where a small collider is fighting with the no-stretch constraints. In practice we find 10 to be correct for most situations, and substeps often being a better solution to increase quality.

After all constraints are performed, a final round of collision detection is done. Collisions are often the most noticeable failure mode, and it is ideal if the next frame can start with non-intersecting geometry. Thus a final cleanup pass can achieve these requirements. We have found that “number of stacked layers + 2” is a good estimate for this number. This allows the effect of the underlying collider to ripple through the stacked layers fully.

In any collision pass, any colliding pair may not be fully resolved. This number of additional collider-pair passes will be run until they are resolved. Since these are only performed on active colliders (and no new collision search is done) this is very cheap.

The layer integer point attribute is used to flag point as belonging to different layers of cloth. Higher numbers refer to higher layers. Layer Shock will make lower layers this many times heavier during collision evaluation, ensuring the higher layers will move out of their way. The rest of the dynamics are unaffected by this, and the difference is fixed regardless of the number of layers between the two. This can be thought of as a way to dial between one-way layering of sims and fully coupled sims.

The method to use to compute wind shadow.

Scale for how much shadow to apply when the external geometry is hit.

Scale for how much shadow to apply when the geometry itself is hit.

The maximum distance to check for occlusions.

The distance to offset the ray from the particle. This is used to avoid self intersections if the node is used for self wind shadow.

The spread to send shadow rays. A value of 90 will sample the full hemisphere, and a value of 180 will send rays in all directions.

Number of samples to send randomly within the cone angle. If the cone angle is zero, only one sample will be taken.

This ramp maps the distance of the particle to the to the output value of the mask.

Apply blurring to the wind shadow mask and wind shadow velocity.

The number of times the input geometry is blurred. The higher the number, the more blurring.

Blurring relies on each point being able to see what its neighbors' attributes are. This parameter determines how points see their neighbors.

Determines how far points influence each other when Influence Type is set to proximity.

Determines the maximum number of neighbors each point is permitted to have. Large numbers of neighbors may reduce performance.

A more brute force approach to reducing dynamic velocity. The velocity is scaled directly by this amount, causing sudden movements to be quickly damped.

A threshold at which to apply full friction. When the ratio of the tangential velocity and the normal impulse is less than this, the tangential velocity will be fully eliminated through friction. This is roughly tan() of the slope angle that will allow sliding under gravity.

If the static threshold fails, this controls what percentage the tangential velocity will be reduced in the dynamic friction case.

A scale factor on the amount of friction effect to apply for collisions with external geometry.

A scale factor on the amount of friction effect to apply for collisions with self geometry.

A scale factor for the ground plane’s static friction. Useful to create frictionless grounds.

A scale factor for the ground plane’s dynamic friction. Useful to create frictionless grounds.

If this checkbox is turned on, all constraints in the specified Constraint Group will be solved in a separate, interleaved pass to the remaining constraints.

There are two main uses for this option. First, there are some expensive constraint types that do not need to be solved as frequently as the rest of the constraints, so enabling this option and choosing a low Solve Frequency can improve performance. For example, cloth bend constraints on high resolution cloth with low bend stiffness such a silk or cotton. You need to ensure that you're solving distance constraints every pass so the cloth doesn’t stretch. However, bend constraints are expensive and don’t need to be as strong, since silk and cotton have very low bend resistance and wrinkle easily. Solving the bend constraints as a secondary pass will give you strong performance gains. However, this method isn’t practical for stiffer material such as leather.

The second use case is constraints that cause frequent changes in the constraint topology, which causes the solver to re-order the constraint solve, possibly causing jittering in the remaining constraints. An example is organic tissue represented by tetrahedra and attached together with sliding Stitch constraints. As the sliding causes changes in the constraint topology the tetrahedra might jitter slightly as the constraint solve order changes. Moving the Stitch constraints to another pass by setting Constraint Group to @type=ptprim and increasing the Solve Frequency to 1 will retain the same stiffness but eliminate tetrahedral jittering due to changes in the solve order.

The constraint group(s) that should be solved in the interleaved secondary pass. This parameter accepts standard group syntax, so can contain explicit group names and ad-hoc groups, often specifying one or more constraint types with something like @type==bend. Any constraint not contained in these groups will be solved in the primary pass as usual.

The frequency at which to solve the secondary pass. A setting of 1 will solve the secondary pass as often as the primary pass, while a setting of 0.25 will solve once every four passes of the primary pass (one quarter of the time).

The multi-pass options allow a substep to be repeated until certain conditions are met. The current conditions are designed to fix issues that are caused by disabled points causing geometry to be caught and cause the undisabled points to generate stretching. Since collisions always pre-empt constraints, the result is cloth or hair stretching. Points adjacent to auto-disabled points will be themselves disabled if too much stretching is detected. The solve step will then be repeated in the hopes this frees up the geometry.

The maximum number of times to repeat the substep. If no new points need to be disabled, the process will stop immediately.

The amount of stretch at the end of a solve step that will trigger points to be eligible for disabling. This is to detect when failed collisions are pulling the Vellum object apart. By failing additional points the object usually can be released and result in a better result than continuing to stretch.

Various acceleration limiting options can be used to prevent the simulation from being over-eager to obey non-realistic forces. These can otherwise result in large energy spikes.

If Max Acceleration is exceeded for a point, assume it signals a sharp, discontinuous collision where the second order prediction will be wrong and add erroneous motion, often in the form of bouncing. In this case, fallback to first order integration for the affected point.

Cap the amount that velocity of a particle is allowed to change as a result of any of the dynamics. This is useful to prevent some instant motion being mis-identified as a massive force, and thus avoid fly-away particles.

During the collision resolution, if the collision correction moves a particle more than the acceleration amount, cap the effect. The hope is to fail more gracefully when a part of a model snags on the cloth and is pulling it in a surprising fashion.

When a weld breaks, the two new points start off next to each other. If the surfaces do not separate naturally due to whatever triggered the break, they may trigger collision detection and be pushed out, causing an explosive motion when breaking occurs. This option sets the disableself attribute when points are de-welded to avoid these self collisions. Note that this may cause layered cloth to self penetrate, however.

Normalize the stress computation over time, so that the computed values are more predictable as the Substeps parameter changes.

The method used for finding the next closest position on the target geometry when sliding Attach to Geometry or Stitch Points constraints. Closest Point simply chooses the closest point on the target geometry to the projected sliding position. This approach is fast but can improperly jump across concavities in the target geometry. Traverse Polygons starts from the current target primitive and successively walks outwards, finding the closest point on the surrounding primitives. This approach is more expensive but handles concave target geometry better. Traverse Triangles (Optimized) is similar to the previous option in its improved handling of concavities, but can be many times faster as it uses specialized triangle distance functions. However, it can only be used with target geometry consisting of triangles.

Particles that remain near-stationary for sufficient time will be put to sleep by setting their stopped attribute to 1.

You can use the Pinned visualization to visualize which points are sleeping.

How slowly particles need to be moving before they are eligible for sleeping. The amount of time they remain below this speed is accumulated in the deactivation_time attribute. If they exceed this speed, the deactivation_time is reset to zero.

How many seconds particles need to remain at or below the velocity threshold before they are put to sleep. See the POP Awaken DOP for methods to wake the particles back up again.

Potential intersection particles are any within this scaled distance of the average of the two particles pscale attribute. This is an overestimate because usually collisions are not updated during the constraint iterations, so it needs to record not just the currently colliding particles, but those that may start to collide due to the earlier iterations.

This also effects the range of the attraction force in clumping.

The maximum number of particles that will be considered when searching for potential collisions over the substep. Capping this is useful to avoid excessive computations, if too many particles are created at one spot. This parameter is ignored if OpenCL Neighbor Search is enabled, in which case all neighbors are considered within the radius determined by Search Scale.

The pscale attribute is used to determine the radius of each particle. If all particles have the same radius, faster acceleration structures can be used to find neighbors.

Ignore any neighbors that have the same non-empty name or non-negative piece point attribute. This option can be enabled to create separate clusters of grains that only interact with other clusters, often in conjunction with a Shape Match constraint to give rigid behavior.

A weighting for how much the particle collision forces are weighted. A value of zero will disable particle collision.

Scale with the repulsionweight point attribute.

A weighting for how much the particles will naturally stick together when close. A value of zero will disable particle clumping.

Scale with the attractionweight point attribute.

The radius of the kernel used for fluid calculations. In general the default value of 4 is recommended, but it can be increased or reduced slightly for different effects. In particular this value can effect the size of the drops created by surface tension, with larger values creating larger drops and fewer individual particles. Note that larger values will result in slower simulation and more memory usage for neighbor lists. Also values much below 3 or above 5 can lead to instability.

The viscosity of the fluid. Low values for viscosity help keep the simulation stable, while higher values can simulate liquids like honey. A per-particle viscosity attribute can be used to multiply this value for variable viscosity. Fluid particles will different phase values will be solved independently for viscosity, allowing multi-phase fluid behavior.

The viscosity of the fluid in contact with an SDF collision. This value is only used when the solver has a ground plane or a collision object created with the VDB Collider SOP.

The type of viscosity solve to perform. Explicit is fast but can be unstable for high particle counts, high viscosity, or low substeps. Implicit is slower but remains stable even for higher particle counts and viscosity values.

The tolerance for the viscosity solve when using the Implicit solve. Lower values are more accurate at the cost of more iterations.

The maximum number of iterations for the Implicit viscosity solve.

The surface tension of the fluid. Higher values for this setting reduce the curvature of the fluid and cause it to form blobs. A per-particle surfacetension attribute can be used to multiply this value for variable surface tension. Fluid particles will different phase values will be solved independently for surface tension, allowing multi-phase fluid behavior.

The adhesion of the fluid. Higher values for this setting cause the fluid to stick to the normal surface of collisions, without any friction effects. You can increase Collision Viscosity to have collisions' velocity affect the fluid in a friction-like way. A per-particle adhesion attribute can be used to multiply this value for variable adhesion. Fluid particles will different phase values will be solved independently for adhesion, allowing multi-phase fluid behavior.

Vellum fluids and grains perform many calculations involving nearby points. Generally performance is greatly improved by ensuring that points that are near other in 3D space are also near each other in memory. This option enables a spatial sort of the particles at the specified interval of frames.

When doing graph coloring, a fast parallel OpenCL algorithm is used. Unfortunately it may require 10× more memory than the rest of the solve on tetrahedral meshes. Thus, systems that might fit in memory for solving will not succeed the color pass. Disabling this forces all graph coloring to be done in a slower sequential manner, preserving RAM for the actual solve.

Perform any neighbor searches for grains and fluids using OpenCL, which is faster than the CPU but can use a bit more GPU memory.

Draw spheres for the pscale of the geometry. If the geometry has already solved it may have set the overlap_self attribute, if so, the actual effective thickness is drawn in cyan. This is useful to detect if the original thickness was too high - if so the extra thickness will be ignored by the simulation and just slow down the collisions.

Draw the thickness by offsetting the geometry in opposite directions rather than by using spheres.

Draw failed self collisions as red spheres. These points will no longer collide with their own geometry, nor will any edge or triangle attached to them.

Draw failed external collisions as orange spheres. These points will no longer collide with external geometry, nor will any edge or triangle attached to them.

Draw which points have been semantically fused together with the weld attribute. These can be broken during the solve using the auto break options, or by setting weld to -1.

Because the thickness may be small, the visualization spheres can sometimes be lost. This scales the visualization spheres allowing them to be readily seen.

Constraints are often added to different groups. This allows you to isolate one particular group to visualize.

Draw spheres over points pinned to the target animation.

Draw stitch constraints of points stitched together.

Draw attachment constraints for points attached to external geometry.

The constraint guides are not scaled by the thickness of the object, so this provides an absolute scale for their size.

Visualize properties of the constraints using a blue-to-red infrared coloring.

Toggles if the simulation geometry is false-colored in addition to the constraints.

Scales the maximum stretch stress to be the maximum coloration.

Scales the maximum bend stress to be the maximum coloration.

Sets the maximum stretch distance to be the maximum coloration.

Sets the maximum stretch ratio to be the maximum coloration.

Sets the maximum bend angle to be the maximum coloration.

Sets the maximum stretch plastic flow to be the maximum coloration.

Sets the maximum bend plastic flow to be the maximum coloration.

Scales the maximum volume stress to be the maximum coloration.

Scales the maximum volume distance to be the maximum coloration.

Scales the maximum volume ratio to be the maximum coloration.