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Physically-based Sticky Lips

A physically-based model of the sticky-lip effect for facial animation, using total Lagrangian explicit dynamics FEM with a new breaking element for the saliva, so the lips stick and separate realistically as the mouth opens.

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Abstract

A novel solution for the sticky lip problem in computer facial animation, recreating the way the lips stick together when drawn apart in speech or in the formation of facial expressions. Where traditional approaches rely on an artist estimating the behaviour, this presents a physically-based model: the mouth is modelled with the total Lagrangian explicit dynamics finite element method, with a new breaking element modelling the saliva between the lips. Subtle yet complex behaviours are recreated implicitly, reproducing varying degrees of stickiness between the lips as well as asymmetric effects.

How to read this

Category: Method: a physically-based facial animation model (sticky lips)
Contributions: A physically-based model of the sticky-lip effect, replacing artist-estimated behaviour
Models the mouth with total Lagrangian explicit dynamics FEM
Introduces a new breaking element representing saliva so lips stick and separate realistically
Context: Sits within physically-based facial animation and FEM soft-tissue simulation, contrasting with traditional hand-keyed or blendshape estimates of lip stickiness.
Correctness: Validated qualitatively by recreating subtle behaviours such as varying stickiness and asymmetric separation; a reader should note the effect rests on the breaking-element model of saliva and on the explicit-dynamics FEM parameterization rather than measured material data.
Clarity: Accessible in motivation but technical in the FEM detail; a first pass conveys the idea, a second pass is needed for the breaking-element formulation.
How to read it: First pass for the problem and the breaking-element idea; do a second pass on the total Lagrangian explicit dynamics setup and the breaking criterion if you want to reproduce the stickiness behaviour.

Related work

Realtime Performance-Driven Physical Simulation for Facial Animation 2019 / CGF
Direct Manipulation Blendshapes 2010 / TOG
Realtime Performance-Based Facial Animation 2011 / SIGGRAPH
High Fidelity Facial Animation Capture and Retargeting with Contours 2013 / SCA

Keywords

facial animation sticky lips lip contacts physically based simulation finite element method total lagrangian explicit dynamics soft tissue breaking element saliva visual speech facial expressions mouth simulation contact modelling

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