Directing Hair Motion on Tangled

Maryann Simmons Kelly Ward Hidetaka Yosumi Hubert Leo Xinmin Zhao

Walt Disney Animation Studios

(a) 2D Drawover. (b) Posing with hair rig. (c) Sim for settling/collision. (d) Final render.

1 IntroductionState-of-the-art simulation techniques can produce compelling andnatural hair motion. In creating Disney’s feature film Tangled,physically plausible motion was important but just one foundationalcomponent. The story contains an unprecedented amount of inter-action of the hair with the characters, as well as a high level ofart-direction. Through 2D “drawovers” the artistic vision was con-veyed in an often very detailed way - how the hair should move,and what poses and silhouettes it should hit. Simulation alone isnot sufficient when such a high degree of direction and interactionis required. We describe a hybrid approach leveraging the power ofcustom hair dynamics with the artistic control of key-framed anima-tion that was key to the success of directing hair motion on Tangled.

2 ApproachesAnimation rigs give direct control over the placement of the hairand are most suitable to use when motion is not physically-basedand/or difficult or impossible to hit through simulation. This ap-proach supports fast iterations, but does not scale well due to theamount of manual input required, making it only feasible for useon a small number of representative curves. In addition, throughrig controls alone, it is very time-consuming to produce animationthat faithfully preserves physical properties of the hair necessary toproduce natural, collision-free motion. In contrast, simulation canhandle independent and coherent motion of thousands of curves,producing natural and highly complex motion. Such dynamics,however only provide implicit control and therefore are difficult todirect. In addition it is a computationally expensive approach- lim-iting the number of iterations practical. Thus we introduce a systemof interleaving the best of both approaches on a shot-by-shot basis.

Animation Approach: Rig-based key-framed animationis used to produce hair mo-tion in shots calling fornon-physically-based motionand/or a high level of art-direction. The specializedhair rig contains controls formoving the hair as a whole as well as ten break-up controls thatallow sub-groups of hair to be pulled out and manipulated indepen-dently from the remaining hair volume. The rig contains IK, FK,twist, and pinch operations supported at the global or on the sub-group level; the number and distribution of these operation con-trollers can be altered per shot as necessary. This two-tiered hairrig is used for both posing the initial positions and for creating key-framed animation of the hair.

Simulation Approach: At the other end of the spectrum aresimulation-driven shots where physics primarily controls the mo-tion. By animating external forces (constraints, collision objects,

and force fields), the hair motion can be influenced to a desired out-come. Finer level control is provided through the manipulation ofsimulation properties via override sets and curve maps. These mapsprovide localized, per-vertex control over any constraint or propertyin the simulation, e.g. mass, stiffness, etc., as well as a mechanismto locally animate these properties within one simulation pass.

3 Interleaving Animation and SimulationHair shots are first categorized as passive, animation- or simulation-driven.

Passive shots do not require any art direction or special interac-tion. These shots are first batch processed using a default simulationsetup. The results of this pass are evaluated and the accepted pas-sive shots are sent to the downstream department. The remainingshots are divided into two sections. For those that share a commoncharacteristic (e.g. wet hair), a special case simulation setup is cre-ated and applied to each shot in that group through another roundof batch processing. Remaining ungrouped shots are re-classifiedas animation- or simulation-driven and passed on for shot-specificprocessing. This batch processing iterates until all shots in the ini-tial grouping are approved or re-classified.

The majority of non-passive highly-directed shots involve multi-ple interleaved iterations of animation and simulation, though onegenerally dominates. For example, shots where the hair undergoesextreme action as a whole are animation-driven, whereas if the hairmotion is organic, but still needs to hit a specific silhouette, the shotis simulation-driven.

Different portions of the hair may be classified and processed sepa-rately and the solutions blended, or combined using simulation tar-geting. Rig-animated shots undergo a simulation iteration by cre-ating targeting forces to pull the simulated curves towards the ani-mated ones. This simulation layer enhances the motion by addingfine-scale details difficult or impossible to achieve by hand. Theprocess of creating the key frame rig-driven poses itself is layeredwith simulation: the rig drives the curves directly- an interactiveinterface driven by the simulation in turn can be used to drive therig. In this way dynamics and explicit control are interleaved seam-lessly. For simulation-driven shots, the motion can be influencedpost-simulation by direct manipulation of the rig or other deform-ers, which ride along with the simulated hairs; this allows high-levelediting of the hair without re-running a time-consuming simulation.

The presented classification and interleaved animation/simulationworkflow made it possible to not only achieve the desired mo-tions with sufficient quality, but also to do it efficiently enough tomeet the practical reality of production scheduling – a significantachievement given the almost 500 art-directed hair shots.

All images are c©Disney Enterprises, Inc.