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Holding the Shape in Hair Simulation
Practical force and constraint techniques for art-directing hair simulation shape across Brave, Inside Out, Good Dinosaur, Coco, and Incredibles 2.
Abstract
Hair simulation models are based on physics, but require additional controls to achieve certain looks or art directions. A common simulation control is to use hard or soft constraints on the kinematic points provided by the articulation of the scalp or explicit rigging of the hair [Kaur et al. 2018; Soares et al. 2012]. While following the rigged points adds explicit control during shot work, we want to author information during the setup phase to better follow the groomed shape automatically during simulation (Figure 1). We have found that there is no single approach that satisfies every artistic requirement, and have instead developed several practical force-and constraint-based techniques over the course of the making of Brave, Inside Out, The Good Dinosaur, Coco, Incredibles 2, and Toy Story 4. We have also discovered that kinematic constraints can sometimes be adversely affected by mesh deformation and discuss how to mitigate this effect for both articulated and simulated hair.
How to read this
- Category
- Production-grounded technique paper (art-directable hair simulation)
- Contributions
- Several practical force- and constraint-based techniques to make hair simulation follow a groomed shape automatically during setup, beyond hard/soft constraints on rigged points
- Methods to author shape-holding information at setup time so simulation better follows the groom in shot work
- Mitigations for cases where kinematic constraints are adversely affected by mesh deformation, for both articulated and simulated hair
- Context
- Extends the studio's curly-hair simulation line (Iben et al.) and relates to prior kinematic-constraint hair control, developed across several feature productions.Builds on: Artistic Simulation of Curly Hair
- Correctness
- Explicitly notes there is no single approach satisfying every artistic requirement, so these are pragmatic, production-validated techniques rather than a unified theory; their suitability is art-direction dependent and demonstrated through film examples.
- Clarity
- Accessible and practitioner-oriented; a first pass conveys the techniques, with constraint and force details rewarding a second pass.
- How to read it
- First pass for the catalog of shape-holding controls and when each applies; second pass on the constraint formulations and the mesh-deformation mitigation if implementing in a hair pipeline.
Builds on
Built upon by
Related work
- Artistic Simulation of Curly Hair 2013 / SCA
- Gravity Preloading for Maintaining Hair Shape Using the Simulator as a Closed-Box Function 2022 / SIGGRAPH
- Scriptable Character FX Solution 2019 / DigiPro
- Hair Emoting with Style Guides in Turning Red 2022 / SIGGRAPH
Keywords
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