Readability
and legibility are often confused. Readability is most often and more
properly used to describe the ease with which written language is read
and understood – it concerns the difficulty of the language itself, not
its appearance. Factors that affect readability include sentence and
word length, and the frequency of uncommon words.
In contrast, legibility describes how easily or comfortably a
typeset text can be read. It is not connected with content or language,
but rather with the size and appearance of the printed or displayed
text.
(full 3D hemispherical scattering measurements ->)
1. The primary specular highlight continues all the way around the
hair, while the secondary highlight is confined to the side of the hair
toward the source.
2. A pair of large out-of-plane peaks, or glints, are present, and as
the incidence angle increases the peaks move closer to the incidence
plane, eventually merging and disappearing.
3. The scattering distribution depends on the angle of rotation of the
hair fiber about its axis. (Because hair fibers are not generally
circular in cross section.)
4. Three trasport modes are derived: surfacce reflection, transmission, and internal reflection.
2 Fibers 2.1 Hair fibers and fiber scattering
The fiber is modeled as a dielectric cylinder covered with tilted
sacles (the cuticle) and with a pigmented interior (the cortex).
The cones of the R and TRT components shift in opposite directions,
causing them to separate into two visually distinguishable highlights.
(The R highlight is white and the TRT highlight is colored.)
2.2 Scattering
The bidirectional scattering function S for a fiber (different from the bidirectional reflection distribution function f_r for a surface):
The scattering integral (the curve intensity scattered from an infinitesimal length of fiber):
The presence D in this equation (1) indicates that
a thick fiber intercepts more light, and therefore appears brighter from a distance, than a thin fiber.
3 Scattering measurements 3.1 Incidence plane
As the scattering angle increases, the secondary highlight fades out, while the primary highlight maintains more constant amplitude. Both peaks maintain approximately constant width.
The equal-angle peak
3.2 Normal plane
The hair has a 180 degree rotational symmetry and is bilaterally symmetric in cross section.
The evolution of the peaks as the fiber roatates appears similar to the internal reflection from a transparent elliptical cylinder.
3.3 3D hemispherical measurements 3.3.1 Changes in glints with angle of incidence
The azimuth at which the glints occur changes as a function of incidence angle, with the glints moving toward the incidence plane as the incidence moves from normal to grazing.
3.3.2 Hemispherical scattering
3.4 Summary
4 Theory of scattering from fibers 4.1 Scattering from cylinders
Walt Disney Features Animation EVASION-INRIA, Grenoble, France Rhythm & Hues Studios Cornell University EVASION/INRIA & INP Grenoble, France University of North Carolina at Chapel Hill
I. A. Hair Modeling Overview
hair modeling - hairstyling / hair simulation / hair rendering hairstyling: modeling the shape of the hair - geometry, density, distribution, orientation hair simulation: dynamic motion of hair - collision, mutual interactions hair rendering: visual depiction of hair - color, shadows, light scattering, transparency, anti-aliasing
ref. Magnenat-Thalmann
- geometric complexity and thin nature of an individual strand coupled with the complex collisions and shadows that occur among the hairs
GPU
I. B. Applications and Remaining Problems
cosmetic product entertainment industry (feature animation) interactive systems (virtual environments, videogames)
II. HAIRSTYLING II. A. Hair Structural and Geometric Properties
hair types - Asian / African / Caucasian - Asian: smooth, regular, a circular corss-section - African: irregular, an elliptical cross-section - Caucasian: ranged from smooth to curly
Visual styling techniques are for applications that desire a visually-prausible solution to match the final results with the appearance of real-world hair.
II. B. Attaching Hair to the Scalp
1) 2D Placement : spherical mapping of 2D map to the 3D contour of the scalp * TY Kim - 2D space defined by the two parametric coordinates of the patch wrapped interactively over the head model * Bando - harmonic mapping, compensation based on a Poisson disc distribution
2) 3D Placement : selecting triangles defined the scalp interactively
3) Distribution of Hair Strands on the Scalp : uniform distribution over the scalp * the user's painting local hair density as color levels and further characteristics such as length or curliness
ref. B. Hernandez & I. Rudomin, <Hair Paint>, Computer Graphics International (CGI), June 2004, pp. 578-581
II. C. Global Hair Shape Generation
1) Geometry-Based Hairstyling : a parametric representation of hair in the form of trigonal prisms or generalized cylinders a) Parametric Surface: A patch of a parametric surface such as a NURBS surface("hair strips") are given a location on the scalp, an orientation, and weighting for knots to define a desired hair shape. * U-shape * Thin Shell Volume (TSV) * key hair curves generated along the isocurves of the NURBS volume b) Wisps and Generalized Cylinders: The positioning of one general space curve serves as the center of a radius function defining the cross-section of a generalized cylinder ("a hair cluster"). c) Multi-resolution Editing: A hierarchy of generalized cylinders allows users to select a desired level of control in shape modeling.
2) Physically-based Hairstyling a) The Cantilever Beam: A cantilever beam is defined as a straight beam embedded in a fixed support at one end only. Gravity and extra-forces need to be applied to the strand. b) Fluid Flow: The idea that static hair shapes resemble snapshots of fluid flow around obstacles. c) Styling Vector and Motion Fields: Given a global field generated by superimposing procedurally defined vector field primitives, hair strands are extracted by tracing the field lines of the vector field. Hair deformation is computed by using the previous algorithm applied on the modified vector field. (Three types of constraints: point / trajectory / direction)
3) Generation of Hairstyles from Images : the automatic reconstruction of hair from imgaes a) Hair Generation from Photographs: Building a 3D hair volume from various viewpoints of the subject's hair is for simple hairstyles. b) Hair Generation from Sketches: In a sketch-based system dedicated to modeling cartoon hairstyles, Curves representing clusters of hair are generated between silhouette surface and the scalp.
4) Evaluation
II. D. Managing Finer Hair Properties
1) Details of Curls and Waves - a class of trigonometric offset functions with rabdin terms - a breakaway behaviour from the fluid flow based on a probability function - the degree of similarity among the master strands controlled by a length distribution, a deviation radius function and a fuziness value * a Markov chain process * a Gibbs distribution * the Kirchhoff model (: a mechanically accurate model for static elastic rods)
2) Producing Hair Volume - for hair self-collisions, hair can be viewed as a continuous medium - the idea that hair strands with pores at higher latituds on the head cover strands with lower pores - a hair-head collision detection & response algorithm * a quasi-static head
3) Modeling Styling Products and Water Effects - A styling force is used to enable hairstyle recovery as the hair moves due to external force or head movement. (1)The desire is to retain the deformed hairstyle rather than returning to the initial style. (2)Breakable static links or dynamic bonds can be used to capture hairstyle recovery by applying extra spring forces between nearby sections of hair (to mimic the extra clumping of hair created by styling products). - Using a dual-skeleton model for simulating the stiffness of hair motion, separated spring forces can be used to control the bending of hair strands versus the stretching of curls. - " As water is absorbed into hair the mass of the hair increases uip to 45%, while its elasticity modulus decreased by a factor of 10." And the volume of the hair decreases due to the bonding nature of water. * Young's modulus of each fiber - An interactive virtual hairstyling system introduced by Ward et al.
ref. K. Ward, N. Galoppo, M. C. Lin <Modeling hair influenced by water and styling products>, International Conference on Computer Animation and Social Agents (CASA), May 2004, pp. 207-214 K. Ward, N. Galoppo, M. Lin <Interactive virtual hair salon>, PRESENCE: Teleoperators & Virtual Environments (to appear), 2007
III. HAIR SIMULATION III. A. The Mechanics of Hair
- The irregular surface of individual hair strands causes anisotropic friction inside hair, with an amplitude (that strongly depends on the orientation of the scales and of the direction of motion). - Hair-hair friction results in triboelectricity. - The more intricate the hair's geometry is, the less degrees of freedom it has during motion.
III. B. Dynamics of Individual Hair Strands
1) Mass-Spring Systems : A single hair strand is modeled as a set of particles connected with stiff springs and hinges.
2) One Dimensional Projective Equations : The statics of a cantlever beam is simulated to get an initial plausible configuration of each hair strand. Then, each hair strand is considered as a chain of rigid sticks.
3) Rigid multi-body serial chain : Each hair strand can be represented as a serial, rigid, multi-body open chain using the reduced or spatial coordinates formulation, in order to keep only the bending and twisting degrees of freedom of the chain. * Articulated-Body Method * DOF * multi-pass forward dynamics algorithm
4) Dynamic Super-Helices * Kirchhoff theory for elastic rods : The curvatures and the twist of the rod are assumed to remain constant over each predefined piece of the rod. As a result, the shape of the hair strand is a piecewise helix, with a finite number of degrees of freedom. This model is then animated using the princiles of Lagrangian mechanics, accounting for the typical nonlinear behavior of hair, as well as for its bending and twisting deformation modes.
5) Handling external forces
ref. D. W. Lee & H. S. Ko <Natural hairstyle modeling and animation>, Graphical Models, vol. 63, no. 2, pp. 67-85, March 2001
[39]
6) Evaluation (Table II 삽입!)
III. C. Simulating the Dynamics of a Full Hairstyle
detection and resonse => computing hair contacts and collisions
1) Hair as a Continuous Medium : hair as an anisotropic continuous medium a) Animating Hair Using Fluid Dynamics: Interaction dynamics, including hair-hair, hair-body, and hair-air interactions, are modeled using fluid dynamics. Individual hair strands are kinematically linked to fluid particles in their vicinity. The density of the hair medium is defined as the mass of hair per unit of volume occupied. The pressure and viciosity represent all of the forces due to interactions to hair strands. Hair-body interactions are modeled by creating boundary fluid particles around solid objects. A fluid particle, or Smooth Particle Hydrodynamics (SPH), exerts a force on the neighboring fluid particles basedd on its normal direction. The viscous pressure of the fluid, which is dependent on the hair density, accounts for the frictional interactions between hair strands. b) Loosely Connected Particles: Each particle represents a certain amount of hair material which has a local orientation (the orientation of a particle being the mean orientation of every hair strand covered by the particle). Initially, connected chains are setteld between neighboring particles being aligned with local hair orientation: two neighboring particles having similar directions and being aligned with this direction are linked. -> breakable links between close particles c) Interpolation between Guide Hair Strands: Using multiple guide hair strands for the interpolation of a strand alleviates local clustering of strands. A collision among hair strands is detected by checking for intersections between two hair segments and between a hair vertex and a triangular face. d) Free Form Deformation (FFD): A mechanical model lis defined for a lattice surrounding the head. The lattice is then deformed as a particle system and hair strands follow the deformation by interpolation. * metaball
2) Hair as Disjoint Groups : to capture local discontinuities observed inside long hair during fast motion a) Real-time Simulation of Hair Strips: The projective angular dynamics method is applied to the control point mesh of the NURBS surface. The strips of texture-mapped hair are simulated using a mass-spring model and 3D morphing. b) Simulation of Wisps: During motion, the shape of a wisp is approximated by parabolic trajectories of fictive particles initially located near the root of each wisp.
III. D. Multi-resolution Methods
1) Level-of-Detail Representations : Three different levels of detail (LODs) for modeling hair - individual strands, clusters and strips represented by subdivision curves, subdivision swept volumes, and subdivision patches. The family of swept shpere volumes (SSVs) as bounding volumes encapsulates the hair.
2) Adaptive Clustering : The Adaptive Wisp Tree (AWT) represents at each time step the wisps segments of the hierarchy that are actually simulated (called active segments). The AWT dynamically splits or merges hair wisps while always preserving a tree-like structure.
IV. HAIR RENDERING IV. A. Representing Hair for Rendering
explicit models - line or triangle-based renderers volumetric models - volume renderers, or rendering algorithms
1) Explicit Representation - curved cylinders / trigonal prisms with three sides / ribbon-like connected triangle strips / tessellating a curved hair geometry into polygons
2) Implicit Representation - volumetric textures (texels) / the cluster hair model
IV. B. Light Scattering in Hair
: The first requirement for any hair rendering system is a model for the scattering of light by individual fibers of hair.
1) Hair Optical Properties - A hair fiber is composed of three structures: the cortex, the cuticle, and the medulla. - A hair is composed of amorphous proteins that act as a transparent medium with an index of refraction (refractive index=1.55). - The cortex and medulla contain pigments that absorb light, often in a wavelength-dependent way; these pigments are the cause of the color of hair.
2) Notation and Radiometry of Fiber Reflection Because fibers are usually treated as one-dimensional entities, light reflection from fibers needs to be described somewhat differently from the more familiar surface reflection.
Light scattering at a surface is conventionally described using the bidirectional reflectance distribution function (BRDF). The BRDF gives the density with respect to the projected solid angle of the scattered flux that results from a narrow incident beam. It is defined as the ration of surface radiance (intensitiy per unit projected area) exiting the surface in direction w_r to surface irradicance (flux per unit area) falling on the surface from a differential solid angle in the direction w_i:
The scattering function f_s for a fiber is "the ratio of curve radiance (intensity per unit projected length) exiting the curve in direction w_r to curve irradiance (flux per unit length) falling on the curve from a differential solid angle in the direction w_r."
The curve radiance due to illumination from an incoming radiance distribution
ref. S. Marschner, H. W. Jensen, M. Cammarano, S. Worley, and P. Hanrahan <Light scattering from human hair fibers>, ACM Transactions on Graphics, vol. 22, no. 3, pp. 780-791, July 2003, proceedings of ACM SIGGRAPH 2003
Curve irradiance measures the radiant power intercepted per unit length of fiber and therefore increases with the fiber's width.
=> Given two fibers with identical properties but different widths, the wider fiber will produce a brighter curve in a rendered image because the wider fiber intercepts more incident light. This definition is consistent with the behavior of real fibers: very fine hairs do appear fainter when viewed in isolation.
Light transmitted through a smooth cylinder will emit on the same cone as the surface reflection, no matter what sequence of refractions and internal reflections it may have taken.
4) Measurements of Hair Scattering There are two specular peaks, one on either side of the specular direction, and there is a sharp ture specular peak that emerges at grazing anles.
5) Models for Hair Scattering Fermat's Principle: A light ray, in going between two points, must
traverse as optical path length which is stationary with respect to variations of the path.
Fresnel factor Fresnel diffraction or near-field diffraction is a process of diffraction which occurs when a wave passes through an aperture
and diffracts in the near field, causing any diffraction pattern
observed to differ in size and shape, relative to the distance. It
occurs due to the short distance in which the diffracted waves propagate.
6) Light Scattering on Wet Hair When objects become wet thet typically appear darker and shinier. As hair becomes wet, a thin film of water is formed around the fibers, forming a smooth, mirror-like surface on the hair. This smoother surface creates a shinier appearance of the hair due to increased specular reflections. Light rays are subject to total internal reflection inside the film of water around the hair strands, contributing to the darker appearance wet hair has over dry hair. Water is absorbed into the hair fiber, increasing the opacity value of each strand leading to more aggressive self-shadowing.
IV. C. Hair Self-Shadowing and Multiple Scattering
Self-shadowing creates crucial visual patterns that distinguish one hairstyle from another.
1) Ray-casting through a Volumetric Representation
2) Shadow Maps The shadow map is a depth image of hair rendered from the light's point of view. Each point of be shadowed is projected onto the light's camera and the point's depth is checked against the depth in the shadow map.
The transmittance function accounts for two important properties of hair. Fractional Visibility: If more hair fibers are seen along the path from the light, the light gets more attenuated (occluded), resulting in less illumination (shadow). Translucency
2) Interactive Volumetric Rendering > hair modeling as a set of connected particles (<= fast cloud rendering techniques) > accumulating transmittance values through a light-oriented voxel grid (-> interactive results for animated hair)
3) Graphics Hardware Graphics processor units (GPUs) languages such as Cg
V.
> physically-based realism (for cosmetic prototyping) > visual realism with a high user control (for feature films) > computations acceleration (for virtual environments and videogaes)
V. A. Hairstyling
- haptic techniques for 3D user input
V. B. Animation
V. C. Rendering
- simulating accurate models for both the scattering of individual hair fibers - the computations of self-shadows at interactive rates
fidelity (n) 원래 것과 똑같음, 박진성(迫眞性) erratic (adj) 산만한 on the fly: 작동/활동 중에 intricacy (n) 복잡함 -> (adj) intricate scalp (n) 두피 bangs (n) (단발의) 앞머리 braids (n) 땋은 머리 wisp (n) 한 줌 cantilever (n) 외팔보 pore (n) 털구멍 vortex (n) 소용돌이 heuristic (n) 발견적 방법 cumbersome (adj) 방해가 되는 facilitate (vt) (사물이 일을) 용이하게 하다 bun (n) (여성 뒷머리의) 쪽 perturb (vt) 교란하다 elongation (n) 연장(선) triboelectric (adj) [전기] 마찰 전기의 <- (n) triboelectricity articulate (v) (뼈를) 관절로 잇다 order of magnitude: 어떤 수치에서 그 10배까지의 범위 vicinity (n) 근처 voxel (n) [컴퓨터] (입체 화상을 구성하는) 3D 화소(畵素) auxiliary (adj) 보조의 dampen (v) 축축하게 하다 fictive (adj) 상상의 dissipative (adj) [물리] 에너지가 흩어지는 empiric = empirical (adj) 경험주의의 occlude (v) (구멍, 틈새를) 막다 tessellate (vt) (방바닥, 포장도로를) 바둑판 모양으로 만들다 cortex (n) 피층(皮層), 피질(皮質) medulla (n) 모수(毛髓) amorphous = formless pigment (n) [생물] 색소(色素) corollary (n) <문어> 추론, [수학] 계(系) graze (v) 가볍게 닿으며 지나가다 lobe (n) 둥근 돌출부 preliminary (adj) 예비적인 caustic (adj) [광학] 화선(화면)의 attenuate (v) 희박해지다 occlude (v) (구멍, 틈새를) 막다 haptic (adj) 촉각의
<A Self-Shadow Algorithm for Dynamic Hair using Density Clustering>
Tom Metens, Philippe Bekaert, Frank Van Reeth (Expertise Centre for Digital Media, Limburgs Universitair Centrum) Jan Kautz (CSAIL, Massachusetts Institute of Technology)
<A Simple, Efficient Method for Realistic Animation of Clouds>
Yoshinori Dobashi & Tsuyoshi Okita, Hiroshima City University Kazufumi Kaneda & Hideo Yamashita, Hiroshima University Tomoyuki Nishita, University of Tokyo
<An Integrated System for Modeling, Animating and Rendering Hair>
Agnes Daldegan & Nadia Magnenat Thalkmann, MIRALab, University of Geneva Tsuneya Kurihara, Central Research Laboratory, Hitachi, Ltd. Daniel Thalmann, Computer Graphics Lab, Swiss Federal Institute of Technology
Hair is a filamentous outgrowth of dead cells from skin, found only on mammals.
cf. Trichocytes : the specialized epithelial cells from which the highly mechanically resilient tissues hair and nail are formed. Epithelium : In biology and dermatology, a tissue composed of a layer of cells.
cf. A pigment is a material that changes the color of light it reflects as the result of selective color absorption. This physical process differs from fluorescence, phosphorescence, and other forms of luminescence, in which the material itself emits light. Many materials selectively absorb certain wavelengths
of light. Materials that humans have chosen and developed for use as
pigments usually have special properties that make them ideal for
coloring other materials. (...) Pigments, unlike fluorescent or phosphorescent substances, can only subtract wavelengths from the source light, never add new ones.
stereo viewing with left and right versions of the front and back bufferes
cf. stereo vision Stereoscopic depth is the sensation of depth that emerges from the
fusion of the two slightly different projections of the world on the
two retinas.
Advanced Graphics Programming Techniques Using OpenGL In August, this course was presented at the 1999 SIGGRAPH Conference in Los Angeles, California. The course demonstrated advanced techniques and concepts for programming with OpenGL.
Readership: Professional game and game engine developers; real-time graphics and simulation developers; creators of movie special effects; computer graphics, geometric modeling, CAGD, animation, and visualization programmers and researchers pertaining to all applications of practical, artistic, entertainment, medical, military, manufacturing etc. products.
Series: The Morgan Kaufmann Series in Computer Graphics
ISBN: 978-1-55860-848-1
ISBN10: 1-55860-848-6 Book/Hardback
Measurements: 7 3/8 X 9 1/4 in
Pages: 712 Imprint: Morgan Kaufmann
Publication Date: 2 December 2002
Price: £83.95
Because it was important to achieve a photorealistic look for various types of fur (e.g. long, short, wet) and at extreme close-ups, we opted to model each strand of hair using a simple spline-based geometric model that incorporates specialized lighting and self-shadowing methods.
A flattened
representation of the self-shadowing model we use.
Stephen Jay Kline
Woodard Professor of Science, Technology, and Society,
and of Mechanical Engineering, Emeritus
Stanford University, Stanford CA 94305 USA
Stephen Jay Kline
Woodard Professor of Science, Technology, and Society,
and of Mechanical Engineering, Emeritus
Stanford University, Stanford CA 94305 USA
In philosophy, systems theory and the sciences, emergence refers to the way complex systems and patterns, such as those that form a hurricane, arise out of a multiplicity of relatively simple interactions. Like intelligence in the field of AI, or agents in distributed artificial intelligence, emergence is central to the physics of complex systems and yet very controversial.
"Perhaps the most elaborate recent definition of emergence was provided by Jeffrey Goldstein in the inaugural issue of Emergence.(Goldstein 1999) To Goldstein, emergence refers to "the arising of novel and coherent structures, patterns and properties during the process of self-organization in complex systems." The common characteristics are: (1) radical novelty (features not previously observed in systems); (2) coherence or correlation (meaning integrated wholes that maintain themselves over some period of time); (3) A global or macro "level" (i.e. there is some property of "wholeness"); (4) it is the product of a dynamical process (it evolves); and (5) it is "ostensive" - it can be perceived. For good measure, Goldstein throws in supervenience -- downward causation." (Corning 2002)
The term "emergent" was coined by the pioneer psychologist G. H. Lewes who wrote:
"Every resultant is either a sum or a difference of the co-operant forces; their sum, when their directions are the same -- their difference, when their directions are contrary. Further, every resultant is clearly traceable in its components, because these are homogeneous and commensurable. It is otherwise with emergents, when, instead of adding measurable motion to measurable motion, or things of one kind to other individuals of their kind, there is a co-operation of things of unlike kinds. The emergent is unlike its components insofar as these are incommensurable, and it cannot be reduced to their sum or their difference." (Lewes 1875, p. 412)(Blitz 1992)
However, the concept behind the term has been in use since at least the time of Aristotle.[1] John Stuart Mill[2] and Julian Huxley[3] are just some of the historic luminaries who have written on the concept.
In mathematics and computer science, graph theory is the study of graphs, mathematical structures used to model pairwise relations between objects from a certain collection. A "graph" in this context refers to a collection of vertices and a collection of edges that connect pairs of vertices. A graph may be undirected, meaning that there is no distinction between the two vertices associated with each edge, or its edges may be directed from one vertex to another
* The Seven Bridges of Königsberg is a famous solved mathematics problem inspired by an actual place and situation. The city of Königsberg, Prussia (now Kaliningrad, Russia) is set on the Pregel River, and included two large islands which were connected to each other and the mainland by seven bridges. The question is whether it is possible to walk with a route that crosses each bridge exactly once.
In 1736, Leonhard Euler proved that it was not possible. In proving the result, Euler formulated the problem in terms of graph theory, by abstracting the case of Königsberg
* An Eulerian path, Eulerian trail or Euler walk in an undirected graph is a path that uses each edge exactly once. If such a path exists, the graph is called traversable.
An Eulerian cycle, Eulerian circuit or Euler tour in an undirected graph is a cycle that uses each edge exactly once. If such a cycle exists, the graph is called Eulerian or unicursal.
* In algebraic topology, the Euler characteristic is a topological invariant, a number that describes one aspect of a topological space's shape or structure. It is commonly denoted by χ (Greek letter chi).
The Euler characteristicχ was classically defined for polyhedra, according to the formula
where V, E, and F are respectively the numbers of vertices (corners), edges and faces in the given polyhedron.
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