parameters

air-density

Default Value	0.001 (float)
Name	Air Density
Description	Air density for drag and lift force computation.

air-friction

Default Value	0.2 (float)
Name	Air Tangental Friction
Recommended Range	0 to 1
Description	When an object is moving in the air, both drag and lift forces are computed. This value controls the ratio of the tangential friction to the normal friction.

area-density

Default Value	1000 (float)
Name	Shell Density
Description	Material density for the shell.

area-poiss-rat

Default Value	0.25 (float)
Name	Poisson’s Ratio for Shells
Allowed Range	0.0 to 0.4999
Description	Poisson’s ratio for the shell material, encoding how much the material responds to compression. 0.0 means the material does not respond to any compression, while 0.5 means completely incompressible. 0.5 leads to numerical instability, so it must be less than 0.5.

area-young-mod

Default Value	100 (float)
Name	Young’s Modulus for Shells
Recommended Range	100 to 1e4
Description	This value is not the actual Young’s modulus but the one divided by the density of the material.

barrier

Default Value	“cubic” (str)
Name	Barrier Model for Contact
Choices	cubic, quad, log
Description	Contact barrier potential model.

bend

Default Value	1 (float)
Name	Bend Stiffness for Shells
Recommended Range	0.0 to 1e2
Description	This bending stiffness not scaled by the density. If you change the density, you need to adjust this value accordingly. This behavior also change by the material thickness.

bvh-alloc-factor

Default Value	2 (int)
Name	Extra Memory Allocation Factor for BVH on the GPU
Recommended Range	2 to 3
Description	We pre-allocate the memory for the BVH on the GPU to avoid dynamic memory allocation. BVH size can dynamically grow during the simulation. This number is multiplied to the initial BVH size to allocate the memory.

ccd-max-iters

Default Value	1024 (int)
Name	ACCD Maximum Iterations
Recommended Range	1024 to 2048
Description	ACCD seeks for the maximal time of impact (TOI) by iteratively advancing the time. If the maximal TOI is not found within this number of iterations, the seek is terminated.

ccd-reduction

Default Value	0.01 (float)
Name	ACCD Reduction Factor
Recommended Range	1e-3 to 1e-2
Description	ACCD needs some small number to determine that the gap distance is close enough to the surface. This factor is multiplied to the initial gap to set this threshold.

cg-max-iter

Default Value	10000 (int)
Name	Maximum Number of PCG Iterations
Description	When PCG iterations exceed this value, the solver is regarded as diverged. If enable_retry is enabled, the step size is reduced and the step is retried. If not, simulation is terminated.

cg-tol

Default Value	0.001 (float)
Name	Relative Tolerance for PCG
Recommended Range	1e-4 to 1e-3
Description	The relative tolerance for the PCG solver. The solver is terminated when the relative residual reaches below this value.

constraint-ghat

Default Value	0.001 (float)
Name	Gap Distance for Boundary Conditions
Reommended Range	1e-3 to 1e-2
Description	For boundary conditions such as pinned vertices and wall conditions, this value is used to determine the gap distance to activate the barrier to enforce the boundary conditions.

contact-ghat

Default Value	0.001 (float)
Name	Contact Gap
Recommended Range	0.25e-3 to 1e-2
Description	This value dictates the maximal gap distance when nearly touching objects are considered in contact. A contact barrier is then activated to prevent the objects from penetrating. Since we employ single precision floating point, we recommend setting this value to be larger than 1e-4.

contact-offset

Default Value	0 (float)
Name	Shell Contact Offset
Recommended Range	0 to 1e-2
Description	This value is used to offset the contact to give a visible thickness to shell contact surfaces.

csrmat-max-nnz

Default Value	70000000 (int)
Name	Maximal Matrix Entries for Contact Matrix Entries on the GPU
Recommended Range	1e6 to 1e9
Description	We pre-alocate the contact matrix entries to avoid cost arising from dynamic memory allocation. This value is manually chosen by the user. If this value is too large, the GPU runs out of memory. On the other hand, if this value is too small, the simulation may fail due to the lack of memory for contact matrix assembly.

disable-strain-limit

Default Value	False (bool)
Name	Flag to Disable Strain Limit
Description	When this flag is enabled, the strain limit is disabled regardless of parameters strain_limit_tau and strain_limit_eps.

dt

Default Value	0.001 (float)
Name	Step Size
Recommended Range	1e-3 to 1e-2
Description	Step size for the simulation. Small step size will make the simulation more accurate. Large step size may make the simulation faster but can introduce noticeable damping. We recommend to use 1e-3 for most cases. Setting a larger value can results in PCG solver divergence. You may turn on enable_retry option to retry the step with a smaller step size when either the PCG solver diverges or line search fails.

dt-decrease-factor

Default Value	0.75 (float)
Name	Step Size Reduction Factor
Description	When PCG solver diverges or line search fails, the step size is reduced by this factor and the step is retried if enable_retry is enabled.

eiganalysis-eps

Default Value	0.01 (float)
Name	Epsilon for Eigenvalue Analysis
Recommended Range	1e-3 to 1e-2
Description	When two singular values are close to each other, our analysis can be unstable as it involves division by the difference of the two singular values. In such cases, we switch to a stable approximation.

enable-retry

Default Value	False (bool)
Name	Flag to Enable Retry when PCG Diverges
Description	When this flag is enabled, the simulation will retry the step with a smaller step size when the PCG solver diverges or line search fails. This is possible when the step size is set large.

fake-crash-frame

Default Value	-1 (int)
Name	Fake Crash Frame
Description	Frame number at which to intentionally crash the simulation for testing purposes. Set to -1 to disable fake crashing.

fitting-dt

Default Value	0.001 (float)
Name	Step Size When Fitting
Recommended Range	1e-3 to 1e-2
Description	When simulation undergoes fitting, this step size is used instead of the regular step size.

fix-xz

Default Value	0 (float)
Name	Whether to fix xz positions
Description	For falling objects if their y position is higher than this value, their xz positions are fixed.

fps

Default Value	60 (float)
Name	Frame Per Second for Video Frames
Description	The time interval for writing meshes are determined by this value so that when a video is generated, the frame rate corresponds to this value.

frames

Default Value	300 (int)
Name	Maximal Frame Count to Simulate
Description	This number dictates the maximal number of frames to simulate.

friction

Default Value	0.5 (float)
Name	Friction Coefficient
Allowed Range	0 to 1
Description	Friction coefficient for the contact. 0.0 means no friction and 1.0 means full friction.

friction-eps

Default Value	1e-05 (float)
Name	Epsilon for Friction
Recommended Range	1e-5 to 1e-4
Description	We employ a quadratic friction model to approximate Coulomb friction. In doing so, we need to set a small value to avoid division by zero. This occurs when the motion undergoes static friction.

gravity

Default Value	-9.8 (float)
Name	Gravity Coefficient

include-face-mass

Default Value	False (bool)
Name	Flag to Include Shell Mass for Volume Solids
Description	The mass of volume solids is computed by their tetrahedral elements. If this option is enabled, the mass of the shell is included for the surface elements of the volume solids.

isotropic-air-friction

Default Value	0 (float)
Name	Air Dragging Coefficient
Description	Per-vertex air dragging coefficient.

line-search-max-t

Default Value	1.25 (float)
Name	Extended Line Search Maximum Time
Recommended Range	1.25 to 1.75
Description	Continuous Collision Detection (CCD) is used to determine the time of impact (TOI), but if we advance the time to the actual TOI, contact gaps can be nearly zero. Such a small gap can cause the solver to diverge, so we extend the time to the TOI by this factor, and the actual TOI is recaled by dividing by this factor. For example, if the actual TOI is 1.0 and this value is 1.25, the actual TOI is 1.0/1.25.

min-newton-steps

Default Value	0 (int)
Name	Lower Bound of Newton Steps
Recommended Range	0 to 32
Description	Minimal Newton’s steps that must be consumed to advance a step. The solver tries to determine the actual Newton’s step count, but this number assumes that static friction is not present. If static friction is present, we recommend setting this value to 32. Also, the step size should be somewhat large (e.g., 1e-2) to more accurately account for static friction.

model-shell

Default Value	“baraffwitkin” (str)
Name	Constitutive Model for Shells
Choices	baraffwitkin, shhk (Stable Neo-Hookean), arap (As-Rigid-As-Possible), stvk (St. Venant-Kirchhoff)
Description	Physical model for the Finite Element shell.

model-tet

Default Value	“snhk” (str)
Name	Constitutive Model for Volumetric Solids
Choices	snhk (Stable Neo-Hookean), arap (As-Rigid-As-Possible), stvk (St. Venant-Kirchhoff)
Description	Physical model for the Finite Element volumetric solid.

rod-bend

Default Value	0.001 (float)
Name	Bend Stiffness for Rods
Recommended Range	0.0 to 10
Description	Rod bending stiffness. The actual force is amplified by the rod mass, which includes rod density.

rod-density

Default Value	1000 (float)
Name	Rod Density
Description	Material density for the rod.

rod-offset

Default Value	0.005 (float)
Name	Rod Contact Offset
Recommended Range	0 to 1e-2
Description	This value is used to offset the contact to give a visible thickness to rod contact surfaces.

rod-young-mod

Default Value	10000 (float)
Name	Young’s Modulus for Rods
Recommended Range	1e3 to 1e6
Description	Unlike shells or volumetric solids, this value is the actual Young’s modulus.

stitch-stiffness

Default Value	1 (float)
Name	Stiffness Factor for Stitches
Recommended Range	0.5 to 2.0
Description	Stiffness factor for the stitches.

strain-limit-eps

Default Value	0.025 (float)
Name	Strain Limit Epsilon
Recommended Range	0 to 0.05
Description	After the strain limit is activated, this value is used to control the maximal stretch beyond the strain_limit_tau. For example, if this value is 0.025 and strain_limit_tau is 0.025, the maximal stretch ratio is 5%.

strain-limit-tau

Default Value	0.025 (float)
Name	Strain Limit Trigger Threshold
Recommended Range	0 to 0.05
Description	When the strain limit is enabled, this value allocates some margin before the strain limit is activated. For example, if this value is 0.025, the strain limit is activated when the stretch ratio exceeds 2.5%.

target-toi

Default Value	0.25 (float)
Name	Target Accumulated Time of Impact (TOI)
Recommended Range	0.1 to 0.5
Description	At each inner Newton’s step, the minimal TOI for continuous collision detection is computed. This TOI is used to rewind back the time to the TOI and a new Newton’s step is computed. We accumulate this TOI, and if the accumulated TOI exceeds this value, we terminate the Newton’s loop and compute the actual step size advanced.

volume-density

Default Value	1000 (float)
Name	Volume Density
Description	Material density for the volumetric solid.

volume-poiss-rat

Default Value	0.35 (float)
Name	Poisson’s Ratio for Volume Solids
Allowed Range	0.0 to 0.4999
Description	Poisson’s ratio for the volumetric solid material, encoding how much the material responds to compression. 0.0 means the material does not respond to any compression, while 0.5 means completely incompressible. 0.5 leads to numerical instability, so it must be less than 0.5.

volume-young-mod

Default Value	500 (float)
Name	Young’s Modulus for Volumetric Solids
Recommended Range	100 to 1e4
Description	This value is not the actual Young’s modulus but the one divided by the density of the material.

wind

Default Value	0 (float)
Name	Wind Coefficient
Description	The wind strength.

wind-dim

Default Value	0 (int)
Name	Wind Direction
Description	The wind direction.