Realistic Hair Simulation: Animation and Rendering

International audienceThe last five years have seen a profusion of innovative solutions to one of the most challenging tasks in character synthesis: hair simulation. This class covers both recent and novel research ideas in hair animation and rendering, and presents time tested industrial practices that resulted in spectacular imagery

Super Space Clothoids

Special Issue: SIGGRAPH 2013 ConferenceInternational audienceThin elastic filaments in real world such as vine tendrils, hair ringlets or curled ribbons often depict a very smooth, curved shape that low-order rod models -- e.g., segment-based rods -- fail to reproduce accurately and compactly. In this paper, we push forward the investigation of high-order models for thin, inextensible elastic rods by building the dynamics of a G2-continuous piecewise 3D clothoid: a smooth space curve with piecewise affine curvature. With the aim of precisely integrating the rod kinematic problem, for which no closed-form solution exists, we introduce a dedicated integration scheme based on power series expansions. It turns out that our algorithm reaches machine precision orders of magnitude faster compared to classical numerical integrators. This property, nicely preserved under simple algebraic and differential operations, allows us to compute all spatial terms of the rod kinematics and dynamics in both an efficient and accurate way. Combined with a semi-implicit time-stepping scheme, our method leads to the efficient and robust simulation of arbitrary curly filaments that exhibit rich, visually pleasing configurations and motion. Our approach was successfully applied to generate various scenarios such as the unwinding of a curled ribbon as well as the aesthetic animation of spiral-like hair or the fascinating growth of twining plants

N-ary implicit blends with topology control

International audienceConstructive implicit surfaces are attractive for modeling and animation because they seamlessly handle shapes with complex and dynamic topology. However, the way they merge shapes is difficult to control. This paper introduces a solution: an improved blend operator that provides control over how topology changes are handled. It is based on a correction applied to the standard blending operator: the sum. Building on summation preserves the n-ary nature of the blend, providing the simplicity of arbitrary (e.g. flat) construction trees and segmentation invariance. The correction is based on projection to a reference case in the variation-space defined by the field and the norm of its gradient. It provides a single parameter, allowing for tuning behavior to achieve effects ranging from avoiding topological combination, through merging only during overlap, to merging at a distance. Dynamic adjustment of the parameter allows for context-dependent effects. Applications range from skeleton-based modeling, where shapes keep the topology of their skeleton, to objects that change topology during animation, with controllable merging. We illustrate the latter with Manga-style hair, where merging depends on the angle between hair wisps

Art Directed Fire-Hair Simulation

Fire simulation and hair simulation can be used to create stylized characters and character animation in movies. In this research a system was created whereby fire simulation was guided by hair simulation, which this thesis refers to as Fire-Hair. This simulation system was built inside Houdini, a professional software package widely used in the visual effects industry. The goal of this research was to develop a workflow that utilized velocity field generated by the hair simulation to drive the fire simulation, and to let simulated fire represent the shape and animation of hair strands. This simulation approach is packaged as a digital asset for future use, with all requisite modifiable parameters exposed to artists. About 20 hair strands were simulated to drive the fire simulation. Hair strand shapes were defined by curves created by the artist; these shapes remain modifiable after creation. Velocity fields which follow hair motion are used as a control field to affect the fire simulation. The final result shows both the physical appearance of fire as well as the shape and motion of hair. The approach was applied to several animated characters to verify reliability and ensure it was visually convincing and robust. The simulated results were rendered using the Houdini built-in render tool, Mantra

Hairstyle modelling based on a single image.

Hair is an important feature to form character appearance in both film and video game industry. Hair grooming and combing for virtual characters was traditionally an exclusive task for professional designers because of its requirements for both technical manipulation and artistic inspiration. However, this manual process is time-consuming and further limits the flexibility of customised hairstyle modelling. In addition, it is hard to manipulate virtual hairstyle due to intrinsic hair shape. The fast development of related industrial applications demand an intuitive tool for efficiently creating realistic hairstyle for non-professional users. Recently, image-based hair modelling has been investigated for generating realistic hairstyle. This thesis demonstrates a framework Struct2Hair that robustly captures a hairstyle from a single portrait input. Specifically, the 2D hair strands are traced from the input with the help of image processing enhancement first. Then the 2D hair sketch of a hairstyle on a coarse level is extracted from generated 2D hair strands by clustering. To solve the inherently ill-posed single-view reconstruction problem, a critical hair shape database has been built by analysing an existing hairstyle model database. The critical hair shapes is a group of hair strands which possess similar shape appearance and close space location. Once the prior shape knowledge is prepared, the hair shape descriptor (HSD) is introduced to encode the structure of the target hairstyle. The HSD is constructed by retrieving and matching corresponding critical hair shape centres in the database. The full-head hairstyle is reconstructed by uniformly diffusing the hair strands on the scalp surface under the guidance of extracted HSD. The produced results are evaluated and compared with the state-of-the-art image based hair modelling methods. The findings of this thesis lead to some promising applications such as blending hairstyles to populate novel hair model, editing hairstyle (adding fringe hair, curling and cutting/extending hairstyle) and a case study of Bas-relief hair modelling on pre-processed hair images

Physical validation of simulators in Computer Graphics: A new framework dedicated to slender elastic structures and frictional contact

International audienceWe introduce a selected set of protocols inspired from the Soft Matter Physics community in order to validate Computer Graphics simulators of slender elastic structures possibly subject to dry frictional contact. Although these simulators were primarily intended for feature film animation and visual effects, they are more and more used as virtual design tools for predicting the shape and deformation of real objects; hence the need for a careful, quantitative validation. Our tests, experimentally verified, are designed to evaluate carefully the predictability of these simulators on various aspects, such as bending elasticity, bend-twist coupling, and frictional contact. We have passed a number of popular codes of Computer Graphics through our benchmarks by defining a rigorous, consistent, and as fair as possible methodology. Our results show that while some popular simulators for plates/shells and frictional contact fail even on the simplest scenarios, more recent ones, as well as well-known codes for rods, generally perform well and sometimes even better than some reference commercial tools of Mechanical Engineering. To make our validation protocols easily applicable to any simulator, we provide an extensive description of our methodology, and we shall distribute all the necessary model data to be compared against

Inverse Elastic Cloth Design with Contact and Friction

Physically based cloth modeling is classically achieved through a trial and error process. The rest (undeformed) configuration of the cloth, often represented as a 2D pattern assembly, is edited geometrically and adjusted iteratively depending on the feedback provided by a static cloth simulator, which predicts the deformed 3D shape under gravity and contacts. Matching a reference 3D shape while keeping the time of the modeling process reasonable is thus difficult , unless the user possesses advanced skills in real cloth tailoring. In contrast, in this paper we investigate a new, inverse strategy for modeling realistic cloth intuitively. Our goal is to take as input a target (deformed) 3D shape, and to interpret this configuration automatically as a stable equilibrium of a cloth simulator, by retrieving the unknown rest shape. In the presence of gravity and frictional contact, such an inverse problem formulates as an ill-posed nonlinear system subject to nonsmooth constraints. To select and compute a plausible solution, we design an iterative two-step solving process. In a first step, contacts are reduced to frictionless bilateral constraints, and starting from an as-flat-as possible pose, a unique rest pose is retrieved using the adjoint method on a regularized energy. The second step modifies this rest pose so as to project bilateral forces onto the admissible Coulomb friction cone, for each contact. We show that our method converges well in most cases towards a plausible rest configuration, and demonstrate practical inversion results on various cloth geometries modeled by an artist

3D hair design and key frame animation in real time

Ankara : The Department of Computer Engineering and the Institute of Engineering and Science of Bilkent University, 2008.Thesis (Master's) -- Bilkent University, 2008.Includes bibliographical references leaves 54-57.Computer generated animations of humans, animals and all other kinds of objects have been studied extensively during the last two decades. The key for creating good animations has been to correctly imitate the behaviors of real objects and reflect these into computer generated images. With the rapid development of computer technology, creating realistic simulations has become possible, and the most striking components of these realistic animations happen to be the most dynamic (moving) parts; hair, in the case of human animations. With the development of high quality hair animations, the concern is not only creating physically correct animations, but also controlling these animations. An implementation of a key frame hair animation creation system, supported by a hair design tool, helping to model and animate hair easily, and provide these functionalities in real time is the aim of the proposed system. This work reviews several hair animation and sketching techniques, and proposes a system that provides a complete level of control (capable of controlling even the individual hair strands) of key frame animation and hair design in real time.Başarankut, BarkınM.S