의상 시뮬레이션을 위한 수렴 보장 풀림 방법

학위논문(박사) -- 서울대학교대학원 : 공과대학 전기·정보공학부, 2022.2. 고형석.수십 년 동안 그래픽스 필드에선 의상 시뮬레이션 중에 발생하는 자가 충돌 처리 실패(엉킴)를 해결하기 위한 여러가지 방법이 제안되었다. 그러나 제안된 방법들은 간단한 의상(티셔츠, 바지)에 대해서만 동작하고, 실제 가상 피팅이나 에니메이션 제작에 등장하는 복잡한 의상에선 대다수가 엉킴 해결에 실패하였다. 본 논문에서는 엉킴을 두 그룹으로 나누고, 각각에 대한 새로운 이산 충돌 처리 방법을 제안하며, 엉킴이 있는 복잡한 의상에 적용하는 실험을 통해 제안된 방법의 효용성을 입증한다. 첫번째 그룹, BLI를 제외한 6가지 엉킴에 대해서는 ESEF(변-압축 / 입실론-압출)를 제안하였다. 6가지 엉킴은 잘못된 영역이 확정적으로 정의됨을 이용하며, 가장 바깥부분부터 서서히 해결하는 아웃투인 방식으로 엉킴을 점진적으로 해결하였다. 이를 위해 매 타임 스텝마다 의상 메쉬의 엉킴 분석을 수행하고, 그 결과를 정점, 변, 삼각형을 채색하는 형태로 저장하였다. 이후 채색을 참조하여 메쉬의 필요한 영역에 두가지 기법 삼각형-수축과 정점-당기기를 가하였고, 최종적으로 모든 엉킴이 없어질때까지 이를 반복적으로 적용하였다. 삼각형-수축과 정점-당기기는 연속 충돌 처리에서 통상적으론 반올림 오류를 보정하기 위해 사용되어 왔던 입실론 값의 의미를 재해석하였다. 입실론의 효용을 반올림 오류 방어로 한정하지 않고, 더 나아가서 이산 충돌 처리에 응용하여 특정 조건에서 유한한 타임 스텝안에 엉킴 해결을 보장할 수 있게 되었다. 두번째 그룹, BLI에 대해서는 BLI-Resolver를 제안하였다. 먼저 BLI의 특징과 어떤 상황에서 발생하는지 분석하고, 이를 통해 원하는 엉킴 해결의 형태(스타일)가 의상의 분류 또는 특정 영역에 따라 달라져야 함을 보였다. 따라서 각각의 스타일에 대응하기 위해 BLI를 해결할 세 가지 알고리즘, 메쉬-찢기, 영역-교차, 접힘-교차를 제안하였다. 메쉬-찢기는 의상 메쉬를 필요에 따라 임시로 몇몇 삼각형들을 누락 후 재구성하여 엉킴 해결에 유리한 메쉬로 변경하였다. 영역-교차, 접힘-교차는 BLI를 직접적으로 해결하지 않고, 다른 6가지 엉킴으로 변환하여, ESEF가 해결할 수 있게 해주었다. 제안된 두가지 방법(ESEF, BLI-Resolver)을 통합하여 엉킴의 스펙트럼을 모두 다룰 수 있게 되어, 의상 시뮬레이션 속의 이산충돌처리의 마침표를 찍게 되었다. 이 방법들은 기존의 연속 충돌 처리가 구현되어 있는 시뮬레이터에 쉽게 통합이 가능하며, 시뮬레이터의 종류에 영향을 받지 않는 특징이 있다. 또한 의상의 복잡도나 종류에 구애받지 않고 유한한 타임스텝 내로 엉킴이 풀림을 보장할 수 있으며, 의상의 디자인에 대한 정보가 제공된 경우 엉킴이 디자인에 적합한 방향으로 해결된다. 최종적으로 실험을 통해 이전의 방법으로 해결할 수 없었던 다양하고 실용적인 의복에서의 엉킴이 해결됨을 보였다.For decades, methods have been proposed to solve the failure of self-collision (intersection) that occurs during clothing simulation. But when applied in reality, they report failure in various cases. In this paper, we divide these intersections into two groups and propose a new discrete collision handling (DCH) method for each to solve them properly in real situations. The first method, Edge-Shortening / Epsilon-Finessing (ESEF), is a method that guarantees convergence of six among seven intersection classifications except for BLI. It performs intersection analysis of the clothing mesh at every time step, and stores the result in the form of coloring the vertices, edges, and triangles. Referring to the coloring, the method resolves the tanglements in an out-to-in manner by applying the proposed operations, triangle shrinkage and vertex pull. The operations reinterpret the traditional use of tolerance value in continuous collision handling (CCH) methods, which were normally used for defending round-off errors. It gives a second thought to that tolerance value, and proposes a new DCH method that uses the tolerance value for the resolution purpose. Under certain conditions, ESEF turns out to guarantee the resolution of the tanglements in a finite number of time steps. The second method, BLI-Resolver, specifically targets BLI only. We analyze how BLIs occur, and realize that the desired form of resolution (i.e., resolution style) can vary depending on the type or particular region of the garment. Therefore, we identify the need for three resolution algorithms for BLI, namely, Mesh-Tearing, Regional-Flip, Crease-Flip, in order to cover the resolution styles. BLI-Resolver is the first to (1) identify the need for the resolution styles for the case of BLI, (2) propose the actual algorithms to cover each resolution style, and (3) demonstrate that the proposed resolution styles and algorithms work stably for BLIs. With the two methods, we can now cover the full spectrum of intersections. Intersections are guaranteed to resolve, in a design-appropriate direction when sufficient information of the clothing is given. Experiments report success in various and practical clothing where previous methods failed to resolve.1 Introduction 1 2 Related Works 5 2.1 Cloth Untangling: General 5 2.2 Cloth Untangling: Multi-Garment 7 2.3 Summary and Limitations 8 2.4 Contribution of Proposed Work 12 3 Preliminary 17 3.1 Edge-Shortening / Epsilon-finessing 17 3.2 Boundary-Loop-Interior Resolver 20 3.2.1 Repulsive-ICM on BLI 20 3.2.2 New Approach for BLI 23 4 Edge Shortening / Epsilon Finessing 25 4.1 Overview 25 4.2 Modifications to Conventional Simulator 28 4.2.1 UV-Space Mesh Update 28 4.2.2 CCH vs. m-CCH 33 4.2.3 Resolution of Elementary Tanglements over Simulation Loop 35 4.2.4 Working of ε_CCD-Finesses in a Cloth Mesh 36 4.2.5 Possible Scenario of Edge Shortening Hindrance 39 4.3 Scheduling the Operations 40 4.3.1 Possible Scenarios when No Fan is TIT-Passable 44 4.4 Soundness in Intrinsically Planar Cases 44 4.5 Extensions to Process Clothing 48 5 Boundary-Loop-Interior Resolver 51 5.1 Overview 51 5.2 Modifications to Conventional Simulator 52 5.3 Mesh-Tearing 52 5.3.1 L-to-B Propagation 55 5.3.2 Revived Triangles 56 5.4 Regional-Flip 59 5.4.1 Crease-Flip 60 5.5 Selecting Resolution Style/Algorithm 62 6 Experiment Results 65 6.1 Overview 65 6.2 Rudimentary Cases 69 6.3 Exploded Handkerchief 69 6.4 Clothes 73 6.5 Round Folds 78 6.6 Sharp Folds 78 6.7 User Interactions 78 6.8 Exploded Handkerchief 80 7 Conclusion 85 A Edge Shortening When Intersection Path Exists Across Multiple Panels 89 B Edge Shortening When Intersection Path Exists Across the Dart Opening 95 C Convexification 97 D Discussion on the Values of ε_RG and γ 99 E Details of BLI Coupling for Regional-Flip 103 Bibliography 105 초록 119박

Fast GPU-Based Two-Way Continuous Collision Handling

Step-and-project is a popular way to simulate non-penetrated deformable bodies in physically-based animation. First integrating the system in time regardless of contacts and post resolving potential intersections practically strike a good balance between plausibility and efficiency. However, existing methods could be defective and unsafe when the time step is large, taking risks of failures or demands of repetitive collision testing and resolving that severely degrade performance. In this paper, we propose a novel two-way method for fast and reliable continuous collision handling. Our method launches the optimization at both ends of the intermediate time-integrated state and the previous intersection-free state, progressively generating a piecewise-linear path and finally reaching a feasible solution for the next time step. Technically, our method interleaves between a forward step and a backward step at a low cost, until the result is conditionally converged. Due to a set of unified volume-based contact constraints, our method can flexibly and reliably handle a variety of codimensional deformable bodies, including volumetric bodies, cloth, hair and sand. The experiments show that our method is safe, robust, physically faithful and numerically efficient, especially suitable for large deformations or large time steps

Data-driven robotic manipulation of cloth-like deformable objects : the present, challenges and future prospects

Manipulating cloth-like deformable objects (CDOs) is a long-standing problem in the robotics community. CDOs are flexible (non-rigid) objects that do not show a detectable level of compression strength while two points on the article are pushed towards each other and include objects such as ropes (1D), fabrics (2D) and bags (3D). In general, CDOs’ many degrees of freedom (DoF) introduce severe self-occlusion and complex state–action dynamics as significant obstacles to perception and manipulation systems. These challenges exacerbate existing issues of modern robotic control methods such as imitation learning (IL) and reinforcement learning (RL). This review focuses on the application details of data-driven control methods on four major task families in this domain: cloth shaping, knot tying/untying, dressing and bag manipulation. Furthermore, we identify specific inductive biases in these four domains that present challenges for more general IL and RL algorithms.Publisher PDFPeer reviewe

Robust, Efficient, and Accurate Contact Algorithms

Robust, efficient, and accurate contact response remains a challenging problem in the simulation of deformable materials. Contact models should robustly handle contact between geometry by preventing interpenetrations. This should be accomplished while respecting natural laws in order to maintain physical correctness. We simultaneously desire to achieve these criteria as efficiently as possible to minimize simulation runtimes. Many methods exist that partially achieve these properties, but none yet fully attain all three. This thesis investigates existing methodologies with respect to these attributes, and proposes a novel algorithm for the simulation of deformable materials that demonstrate them all. This new method is analyzed and optimized, paving the way for future work in this simplified but powerful manner of simulation

Surface-Only Simulation of Fluids

Surface-only simulation methods for fluid dynamics are those that perform computation only on a surface representation, without relying on any volumetric discretization. Such methods have superior asymptotic complexity in time and memory than the traditional volumetric discretization approaches, and thus are more tractable for simulation of complex fluid phenomena. Although for most computer graphics applications and many engineering applications, the interior flow inside the fluid phases is typically not of interest, the vast majority of existing numerical techniques still rely on discretization of the volumetric domain. My research first tackles the mesh-based surface tracking problem in the multimaterial setting, and then proposes surface-only simulation solutions for two scenarios: the soap-films and bubbles, and the general 3D liquids. Throughout these simulation approaches, all computation takes place on the surface, and volumetric discretization is entirely eliminated

Realistic Visualization of Animated Virtual Cloth

Photo-realistic rendering of real-world objects is a broad research area with applications in various different areas, such as computer generated films, entertainment, e-commerce and so on. Within photo-realistic rendering, the rendering of cloth is a subarea which involves many important aspects, ranging from material surface reflection properties and macroscopic self-shadowing to animation sequence generation and compression. In this thesis, besides an introduction to the topic plus a broad overview of related work, different methods to handle major aspects of cloth rendering are described. Material surface reflection properties play an important part to reproduce the look & feel of materials, that is, to identify a material only by looking at it. The BTF (bidirectional texture function), as a function of viewing and illumination direction, is an appropriate representation of reflection properties. It captures effects caused by the mesostructure of a surface, like roughness, self-shadowing, occlusion, inter-reflections, subsurface scattering and color bleeding. Unfortunately a BTF data set of a material consists of hundreds to thousands of images, which exceeds current memory size of personal computers by far. This work describes the first usable method to efficiently compress and decompress a BTF data for rendering at interactive to real-time frame rates. It is based on PCA (principal component analysis) of the BTF data set. While preserving the important visual aspects of the BTF, the achieved compression rates allow the storage of several different data sets in main memory of consumer hardware, while maintaining a high rendering quality. Correct handling of complex illumination conditions plays another key role for the realistic appearance of cloth. Therefore, an upgrade of the BTF compression and rendering algorithm is described, which allows the support of distant direct HDR (high-dynamic-range) illumination stored in environment maps. To further enhance the appearance, macroscopic self-shadowing has to be taken into account. For the visualization of folds and the life-like 3D impression, these kind of shadows are absolutely necessary. This work describes two methods to compute these shadows. The first is seamlessly integrated into the illumination part of the rendering algorithm and optimized for static meshes. Furthermore, another method is proposed, which allows the handling of dynamic objects. It uses hardware-accelerated occlusion queries for the visibility determination. In contrast to other algorithms, the presented algorithm, despite its simplicity, is fast and produces less artifacts than other methods. As a plus, it incorporates changeable distant direct high-dynamic-range illumination. The human perception system is the main target of any computer graphics application and can also be treated as part of the rendering pipeline. Therefore, optimization of the rendering itself can be achieved by analyzing human perception of certain visual aspects in the image. As a part of this thesis, an experiment is introduced that evaluates human shadow perception to speedup shadow rendering and provides optimization approaches. Another subarea of cloth visualization in computer graphics is the animation of the cloth and avatars for presentations. This work also describes two new methods for automatic generation and compression of animation sequences. The first method to generate completely new, customizable animation sequences, is based on the concept of finding similarities in animation frames of a given basis sequence. Identifying these similarities allows jumps within the basis sequence to generate endless new sequences. Transmission of any animated 3D data over bandwidth-limited channels, like extended networks or to less powerful clients requires efficient compression schemes. The second method included in this thesis in the animation field is a geometry data compression scheme. Similar to the BTF compression, it uses PCA in combination with clustering algorithms to segment similar moving parts of the animated objects to achieve high compression rates in combination with a very exact reconstruction quality.Realistische Visualisierung von animierter virtueller Kleidung Das photorealistisches Rendering realer Gegenstände ist ein weites Forschungsfeld und hat Anwendungen in vielen Bereichen. Dazu zählen Computer generierte Filme (CGI), die Unterhaltungsindustrie und E-Commerce. Innerhalb dieses Forschungsbereiches ist das Rendern von photorealistischer Kleidung ein wichtiger Bestandteil. Hier reichen die wichtigen Aspekte, die es zu berücksichtigen gilt, von optischen Materialeigenschaften über makroskopische Selbstabschattung bis zur Animationsgenerierung und -kompression. In dieser Arbeit wird, neben der Einführung in das Thema, ein weiter Überblick über ähnlich gelagerte Arbeiten gegeben. Der Schwerpunkt der Arbeit liegt auf den wichtigen Aspekten der virtuellen Kleidungsvisualisierung, die oben beschrieben wurden. Die optischen Reflektionseigenschaften von Materialoberflächen spielen eine wichtige Rolle, um das so genannte look & feel von Materialien zu charakterisieren. Hierbei kann ein Material vom Nutzer identifiziert werden, ohne dass er es direkt anfassen muss. Die BTF (bidirektionale Texturfunktion)ist eine Funktion die abhängig von der Blick- und Beleuchtungsrichtung ist. Daher ist sie eine angemessene Repräsentation von Reflektionseigenschaften. Sie enthält Effekte wie Rauheit, Selbstabschattungen, Verdeckungen, Interreflektionen, Streuung und Farbbluten, die durch die Mesostruktur der Oberfläche hervorgerufen werden. Leider besteht ein BTF Datensatz eines Materials aus hunderten oder tausenden von Bildern und sprengt damit herkömmliche Hauptspeicher in Computern bei weitem. Diese Arbeit beschreibt die erste praktikable Methode, um BTF Daten effizient zu komprimieren, zu speichern und für Echtzeitanwendungen zum Visualisieren wieder zu dekomprimieren. Die Methode basiert auf der Principal Component Analysis (PCA), die Daten nach Signifikanz ordnet. Während die PCA die entscheidenen visuellen Aspekte der BTF erhält, können mit ihrer Hilfe Kompressionsraten erzielt werden, die es erlauben mehrere BTF Materialien im Hauptspeicher eines Consumer PC zu verwalten. Dies erlaubt ein High-Quality Rendering. Korrektes Verwenden von komplexen Beleuchtungssituationen spielt eine weitere, wichtige Rolle, um Kleidung realistisch erscheinen zu lassen. Daher wird zudem eine Erweiterung des BTF Kompressions- und Renderingalgorithmuses erläutert, die den Einsatz von High-Dynamic Range (HDR) Beleuchtung erlaubt, die in environment maps gespeichert wird. Um die realistische Erscheinung der Kleidung weiter zu unterstützen, muss die makroskopische Selbstabschattung integriert werden. Für die Visualisierung von Falten und den lebensechten 3D Eindruck ist diese Art von Schatten absolut notwendig. Diese Arbeit beschreibt daher auch zwei Methoden, diese Schatten schnell und effizient zu berechnen. Die erste ist nahtlos in den Beleuchtungspart des obigen BTF Renderingalgorithmuses integriert und für statische Geometrien optimiert. Die zweite Methode behandelt dynamische Objekte. Dazu werden hardwarebeschleunigte Occlusion Queries verwendet, um die Sichtbarkeitsberechnung durchzuführen. Diese Methode ist einerseits simpel und leicht zu implementieren, anderseits ist sie schnell und produziert weniger Artefakte, als vergleichbare Methoden. Zusätzlich ist die Verwendung von veränderbarer, entfernter HDR Beleuchtung integriert. Das menschliche Wahrnehmungssystem ist das eigentliche Ziel jeglicher Anwendung in der Computergrafik und kann daher selbst als Teil einer erweiterten Rendering Pipeline gesehen werden. Daher kann das Rendering selbst optimiert werden, wenn man die menschliche Wahrnehmung verschiedener visueller Aspekte der berechneten Bilder analysiert. Teil der vorliegenden Arbeit ist die Beschreibung eines Experimentes, das menschliche Schattenwahrnehmung untersucht, um das Rendern der Schatten zu beschleunigen. Ein weiteres Teilgebiet der Kleidungsvisualisierung in der Computergrafik ist die Animation der Kleidung und von Avataren für Präsentationen. Diese Arbeit beschreibt zwei neue Methoden auf diesem Teilgebiet. Einmal ein Algorithmus, der für die automatische Generierung neuer Animationssequenzen verwendet werden kann und zum anderen einen Kompressionsalgorithmus für eben diese Sequenzen. Die automatische Generierung von völlig neuen, anpassbaren Animationen basiert auf dem Konzept der Ähnlichkeitssuche. Hierbei werden die einzelnen Schritte von gegebenen Basisanimationen auf Ähnlichkeiten hin untersucht, die zum Beispiel die Geschwindigkeiten einzelner Objektteile sein können. Die Identifizierung dieser Ähnlichkeiten erlaubt dann Sprünge innerhalb der Basissequenz, die dazu benutzt werden können, endlose, neue Sequenzen zu erzeugen. Die Übertragung von animierten 3D Daten über bandbreitenlimitierte Kanäle wie ausgedehnte Netzwerke, Mobilfunk oder zu sogenannten thin clients erfordert eine effiziente Komprimierung. Die zweite, in dieser Arbeit vorgestellte Methode, ist ein Kompressionsschema für Geometriedaten. Ähnlich wie bei der Kompression von BTF Daten wird die PCA in Verbindung mit Clustering benutzt, um die animierte Geometrie zu analysieren und in sich ähnlich bewegende Teile zu segmentieren. Diese erkannten Segmente lassen sich dann hoch komprimieren. Der Algorithmus arbeitet automatisch und erlaubt zudem eine sehr exakte Rekonstruktionsqualität nach der Dekomprimierung

Novel Degrees of Freedom, Constraints, and Stiffness Formulation for Physically Based Animation

I identify and improve upon three distinct components of physically simulated systems with the aim of increasing both robustness and efficiency for the application of computer graphics: A) the degrees of freedom of a system; B) the constraints put on that system; C) and the stiffness that derives from force differentiation and in turn enables implicit integration techniques. These three components come up in many implementations of physics-based simulation in computer animation. From a combination of these components, I explore four novel ideas implemented and experimented on over the course of my graduate degree. Eulerian-on-Lagrangian Cloth Simulation resolves a longstanding problem of simulating contact-mediated interaction of cloth and sharp geometric features by exploring a combination of all three of our components. Bilateral Staggered Projections for Joints explores the constrained degrees of freedom of articulated rigid bodies in a reduced state to extend the popular Staggered Projects technique into a novel formulation for rapid evaluation of frictional articulated dynamics. Condensation Jacobian with Adaptivity looks at using reduction methods to improve the efficiency of soft body deformations by allowing larger time step in dynamics simulations. Finally, Ldot: Boosting Deformation Performance with Cholesky Extrapolation explores the inner workings of sparse direct solvers to introduce a Cholesky factorization that is linearly extrapolated in time, which can improve the performance when encapsulated inside an iterative nonlinear solver