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Stabilizing Impossible Collisions with Loki

Xiao Zhai, Eston Schweickart, Jefri Haryono, Nikolay Ilinov, Andrea Merlo

SIGGRAPHWeta FX0 citesCFX

Stabilizes cloth and hair collisions in impossible upstream animation configurations without modifying the original animation, integrated into Weta's Loki framework.

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Abstract

We present a suite of techniques from our in-house simulation framework, Loki, addressing the pervasive challenge of collision instabilities in character effects, particularly in cases where nonphysical pinching prevents collision resolution. We introduce a proximity-tolerant mode for contact projection that trades collision residual for stability, a compliant kinematic mechanism for on-demand gap expansion, and contact-aware strain limiting to prevent penetrations while enforcing target edge lengths. Additionally, we showcase our tools for collider management, including hierarchical collision exclusion, one-sided collision handling, and paintable collision thickness maps. These techniques collectively demonstrate a robust and intuitive workflow for combining physically based collisions with challenging production animations.

How to read this

Category: Method: production collision-stability techniques for character FX
Contributions: A proximity-tolerant contact-projection mode trading collision residual for stability under nonphysical pinching
A compliant kinematic mechanism for on-demand gap expansion plus contact-aware strain limiting that prevents penetration while enforcing target edge lengths
Collider-management tools: hierarchical collision exclusion, one-sided collision handling, and paintable thickness maps
Context: A suite within Weta's in-house Loki multiphysics framework (2022), addressing collision instability when upstream animation presents impossible, pinching configurations.Builds on: Loki: A Unified Multiphysics Simulation Framework for Production
Correctness: These are pragmatic stabilization techniques aimed at robustness in impossible inputs; the proximity-tolerant mode explicitly trades physical accuracy (collision residual) for stability, so expect controlled non-physicality rather than exact contact resolution.
Clarity: Reads as a techniques digest from a production framework; accessible per-technique, with a first pass conveying each tool's intent.
How to read it: Read technique-by-technique for the contact-projection and strain-limiting ideas; a second pass on the proximity-tolerant projection is worth it if you face pinching artifacts in cloth or hair sims.

Builds on

Loki: A Unified Multiphysics Simulation Framework for Production 2022

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Related work

Implicit Multibody Penalty-based Distributed Contact 2014 / TVCG
Clean Cloth Inputs: Removing Character Self-Intersections with Volume Simulation 2018 / SIGGRAPH
Fast Corotated FEM using Operator Splitting 2018 / CGF
Hair and Fur in an Evolving Pipeline 2020 / SIGGRAPH

Keywords

loki collision handling contact projection proximity-tolerant mode compliant kinematics constitutive strain limiting collider management hierarchical collision exclusion one-sided collisions paintable collision thickness contact mechanics physically based animation character effects gap expansion costume simulation codimensional incremental potential contact self-contact de-penetration

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