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Holding the shape in hair simulation

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.