Volume Preserving Simulation of Soft Tissue with Skin

Simulation of human soft tissues in contact with their environment is essential in many fields, including visual effects and apparel design. Biological tissues are nearly incompressible. However, standard methods employ compressible elasticity models and achieve incompressibility indirectly by setting Poisson's ratio to be close to 0.5. This approach can produce results that are plausible qualitatively but inaccurate quantatively. This approach also causes numerical instabilities and locking in coarse discretizations or otherwise poses a prohibitive restriction on the size of the time step. We propose a novel approach to alleviate these issues by replacing indirect volume preservation using Poisson's ratios with direct enforcement of zonal volume constraints, while controlling fine-scale volumetric deformation through a cell-wise compression penalty. To increase realism, we propose an epidermis model to mimic the dramatically higher surface stiffness on real skinned bodies. We demonstrate that our method produces stable realistic deformations with precise volume preservation but without locking artifacts. Due to the volume preservation not being tied to mesh discretization, our method also allows a resolution consistent simulation of incompressible materials. Our method improves the stability of the standard neo-Hookean model and the general compression recovery in the Stable neo-Hookean model.

Locking-Proof Tetrahedra

The simulation of incompressible materials suffers from locking when using the standard finite element method (FEM) and coarse linear tetrahedral meshes. Locking increases as the Poisson ratio gets close to 0.5 and often lower Poisson ratio values are used to reduce locking, affecting volume preservation. We propose a novel mixed FEM approach to simulating incompressible solids that alleviates the locking problem for tetrahedra. Our method uses linear shape functions for both displacements and pressure, and adds one scalar per node. It can accommodate nonlinear isotropic materials described by a Young’s modulus and any Poisson ratio value by enforcing a volumetric constitutive law. The most realistic such material is Neo-Hookean, and we focus on adapting it to our method. For , we can obtain full volume preservation up to any desired numerical accuracy. We show that standard Neo-Hookean simulations using tetrahedra are often locking, which, in turn, affects accuracy. We show that our method gives better results and that our Newton solver is more robust. As an alternative, we propose a dual ascent solver that is simple and has a good convergence rate. We validate these results using numerical experiments and quantitative analysis.

Current Upper Blepharoplasty and Ptosis Management Practice Patterns Among The Aesthetic Society Members

Background Patients presenting for upper blepharoplasty can exhibit different aging patterns and we have anecdotally observed wide variability in upper blepharoplasty approaches among surgeons. However, upper blepharoplasty practice patterns have not been systematically analyzed among The Aesthetic Society members. Objectives The aim of this study is to report upper blepharoplasty practice patterns, the recognition of different patient presenting features, and to assess the reported incidence and management of ptosis among The Aesthetic Society members. Methods A 29-item electronic questionnaire was distributed to 1729 Aesthetic Society members with available email addresses. Results Two-hundred fourteen Aesthetic Society members submitted the questionnaire, for a response rate of 12.4%. There was a significantly increased rate of volume preservation among surgeons with greater experience (≥ 10 years in practice) and a high-volume (≥100 cases in past 12 months) of upper blepharoplasty cases. Furthermore, high-volume upper blepharoplasty surgeons were significantly more likely to perform concomitant upper lid fat grafting (p =0.03) browlift (p = 0.02) and ptosis repair (p = 0.01). Ninety-five percent of respondents reported a mild/moderate ptosis (MRD1 2 to <4mm) incidence of <25%. Among surgeons who perform ptosis repair, 97.4% utilize levator advancement or plication as their most commonly used technique. Conclusions High-volume upper blepharoplasty surgeons are more likely to preserve upper lid volume and perform concomitant browlift and ptosis repair. Our data suggests that different upper eyelid aging patterns and mild/moderate ptosis are underrecognized.

A Time-independent Deformer for Elastic-rigid Contacts

We introduce a new tool that assists artists in deforming an elastic object when it comes in intersection with a rigid one. As opposed to methods that rely on time-resolved simulations, our approach is entirely based on time-independent geometric operators. It thus restarts from scratch at every frame from a pair of objects in intersection and works in two stages: the intersected regions are first matched and a contact region is identified on the rigid object; the elastic object is then deformed to match the contact while producing plausible bulge effects with controllable volume preservation. Our direct deformation approach brings several advantages to 3D animators: it provides instant feedback, permits non-linear editing, allows for the replicability of the deformation in different settings, and grants control over exaggerated or stylized bulging effects.