Illustration by Alvin Shi. It's several curves with gradient coloring. Alvin has black hair and glasses in real life.

Alvin Shi

orcid/0009-0008-0006-6479

github/alvf

alvf.itch.io

[email protected]

About

Hello! I'm Alvin, a Ph.D. student working at the Yale Computer Graphics group under Dr. Theodore Kim. My main research interests are in deformable simulation and geometry processing, specifically for hair.

Publications

A rendering of NYT-bestseller Carvell Wallace with a medium-length natural fade.
Curly-Cue: Geometric Methods for Highly Coiled Hair
Haomiao Wu*, Alvin Shi*, A.M. Darke, and T. Kim (* joint 1st authors)
Proceedings of SIGGRAPH Asia 2024
We present geometric methods for generating shapes that are characteristic of highly coiled hair. Different features become visually relevant when hairs are well-approximated by high-frequency helices instead of low-frequency curves, so we present algorithms for three such phenomena. First, a Fourier-based method for phase locking, the process by which disparate helices near the scalp coalesce into a single curl. Second, a method for period skipping which models individual helices deviating from the coalesced curl. Third, a non-linear optimization that directly generates the shapes of switchbacks, a.k.a. helical perversions, which heretofore could only be produced through direct physical simulation. By applying all three methods in tandem, we show that we can achieve richly detailed depictions of highly coiled hair.
A still-frame of 8000 guide strands in a hairball simulation.
Lifted Curls: A Model for Tightly Coiled Hair Simulation (doi: 10.1145/3606920)
Alvin Shi*, Haomiao Wu*, Jarred Parr, A.M. Darke, and T. Kim (* joint 1st authors) (Best Paper Award)
Proceedings of Symposium on Computer Animation (SCA) 2023
We present an isotropic, hyperelastic model specifically designed for the efficient simulation of tightly coiled hairs whose curl radii approach 5 mm. Our model is robust to large bends and torsions, even when they appear at the scale of the strand discretization. The terms of our model are consistently quadratic with respect to their primary variables, do not require per-edge frames or any parallel transport operators, and can efficiently take large timesteps on the order of 1/30 of a second. Additionally, we show that it is possible to obtain fast, closed-form eigensystems for all the terms in the energy. Our eigenanalysis is sufficiently generic that it generalizes to other models. Our entirely vertex-based formulation integrates naturally with existing finite element codes, and we demonstrate its efficiency and robustness in a variety of scenarios.
A still-frame of two rainbow strands stably stretching against eachother in collision.
A Unified Analysis of Penalty-Based Collision Energies (doi: 10.1145/3606934)
Alvin Shi and T. Kim
Proceedings of Symposium on Computer Animation (SCA) 2023
We analyze a wide class of penalty energies used for contact response through the lens of a reduced frame. Applying our analysis to both spring-based and barrier-based energies, we show that we can obtain closed-form, analytic eigensystems that can be used to guarantee positive semidefiniteness in implicit solvers. Our approach is both faster than direct numerical methods, and more robust than approximate methods such as Gauss-Newton. Over the course of our analysis, we investigate physical interpretations for two separate notions of length. Finally, we showcase the stability of our analysis on challenging strand, cloth, and volume scenarios with large timesteps on the order of 1/40 s.