Performance Evaluation in Laparoscopic Surgery

"SIMD Packet Techniques for Photon Mapping", Shawn Singh, Petros Faloutsos, IEEE/EG Symposium on Interactive Ray Tracing 2007, to appear.
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We present a novel photon mapping framework that uses Single Instruction, Multiple Data (SIMD) parallelism to accelerate the final gathering phase of photon mapping. By using SIMD instructions, four coherent tasks can be computed in parallel using almost the same memory traffic as it would cost to process one task alone. This approach has been very successful for real-time ray tracing, but until now it has been unclear how to effectively apply the same approach to final gathering. Our solution is to use sample-point density estimation instead of k-nearest neighbor density estimation, a technique drawn from reverse photon mapping. Sample-point estimation removes the overheads that make SIMD instructions impractical, while retaining the same benefits and image quality as traditional photon mapping.

"Fool Me Twice: Exploring and Exploiting Error Tolerance in Physics-Based Animation", Tom Yeh, Sanjay Patel, Petros Faloutsos, Glenn Reinman, ACM Transactions on Graphics, accepted with revisions, 2007.

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The error tolerance of human perception offers a range of opportunities to trade numerical accuracy for performance in physics-based simulation. However, most previous approaches either focus exclusively on understanding the tolerance of the human visual system or burden the application developer with case-specific implementations. In this paper, based on a detailed set of perceptual metrics, we propose a methodology to identify the maximum error tolerance of physics simulation. Then, we apply this methodology in the evaluation of two techniques. The first is the hardware optimization technique of precision reduction which reduces the size of floating point units (FPUs), allowing more of them to occupy the same silicon area. The increased number of FPUs can significantly improve the performance of future physics accelerators. A key benefit of our approach is that it is transparent to the application developer. The second is the software optimization of choosing the largest timestep for simulation.

"Sketching Facial Expressions", Gabriele Nataneli, Petros Faloutsos, ACM SIGGRAPH Sketch, 2007.
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We present an innovative sketch-based interface for driving facial expressions. Unlike existing solutions our approach relies on recognition and constructs a semantically relevant representation of a sketched face. This representation is parameterized and used to drive a facial model. The main appeal of our method is that the interface is completely decoupled from the underlying facial model that is used. Therefore one single interface is capable of driving a variety of different models both 2D and 3D. The connection between our tool and the face model is defined by a library of template strokes that can be generated with ease.

"Flipping with Physics: Motion Editing for Acrobatics", Anna Majkowska, Petros Faloutsos, ACM SIGGRAPH / Eurographics Symposium on Computer Animation, 2007
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Complex acrobatic stunts, such as double or triple flips, can be performed only by highly skilled athletes. On the other hand, simpler tricks, such as single-flip jumps, are relatively easy to master.We present a method for creating complex, multi-flip ballistic motions from simple, single-flip jumps. Our approach also allows an animator to interact with the system by introducing modifications to a ballistic phase of a motion. Our method automatically adjusts motion trajectories, to assure physical validity of the motion after the modifications. The presented technique is efficient and produces physically valid results without resorting to computationally expensive optimization. To validate our approach we present the results of a study of user sensitivity to errors in angular momentum and take-off angle. The study shows that small changes of these parameters introduced by our method are not perceptible to a viewer.

"On the Beat! Timing and Tension for Dynamic Characters", Brian Allen, Derek Chu, Ari Shapiro, Petros Faloutsos, ACM SIGGRAPH / Eurographics Symposium on Computer Animation, 2007.
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Dynamic simulation is a promising complement to kinematic motion synthesis, particularly in cases where simulated characters need to respond to unpredictable interactions. Moving beyond simple rag-doll effects, though, requires dynamic control. The main issue with dynamic control is that there are no standardized techniques that allow an animator to precisely specify the timing of the motion while still providing natural response to external disturbances. The few proposed techniques that address this problem are based on heuristically or manually tuning proportional-derivative (PD) control parameters and do not generalize easily. We propose an approach to dynamic character control that is able to honor timing constraints, to provide naturallooking motion and to allow for realistic response to perturbations. Our approach uses traditional PD control to interpolate between key-frames. The key innovation is that the parameters of the PD controllers are computed for each joint analytically. By continuously updating these parameters over time, the controller is able to respond naturally to both external perturbations and changes in the state of the character.

"A Dynamic Controller Toolkit", Derek Chu, Ari Shapiro, Brian Allen, Petros Faloutsos, in ACM SIGGRAPH Video Game Symposium (Sandbox), pp. 15-20, 2007.
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We introduce a toolkit for creating dynamic controllers for articulated characters under physical simulation. Our toolkit allows users to create dynamic controllers for interactive or offline use through a combination of both visual and scripting tools. Users can design controllers by specifying keyframe poses, using a high-level scripting language, or by manipulating the rules of physics through a group of helper functions that can temporarily modify the environment in order to make the desired animation more feasible under physical simulation. The goal of the toolkit is to integrate dynamic control methods into a usable interactive system for noncomputer scientists and non-roboticists, and provide the means to quickly generate physically based motion.

"Automatic Splicing for Hand and Body Animations",Anna Majkowska, Victor Zordan, Petros Faloutsos, ACM SIGGRAPH / Eurographics Symposium on Computer Animation, 2006.
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We propose a solution to a new problem in animation research: how to use human motion capture data to create character motion with detailed hand gesticulation without the need for the simultaneous capture of hands and the full-body. Occlusion and a difference in scale make it difficult to capture both the detail of the hand movement and unrestricted full-body motion at the same time. With our method, the two can be captured separately and spliced together seamlessly with little or no user input required. The algorithm relies on a novel distance metric derived from research on gestures and uses a two-pass dynamic time warping algorithm to find correspondence between the hand and full-body motions. In addition, we provide a method for supplying user input, useful to animators who want more control over the integrated animation. We show the power of our technique with a variety of common and highly specialized gesticulation examples.

"Style Components", Ari Shapiro, Yong Cao, Petros Faloutsos, In Proceedings of Graphics Interface 2006.
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We propose a novel method for interactive editing of motion data based on motion decomposition. Our method employs Independent Component Analysis (ICA) to separate motion data into visually meaningful components called style components. The user then interactively identies suitable style components and manipulates them based on a proposed set of operations. In particular, the user can transfer style components from one motion to another in order to create new motions that retain desirable aspects of the style and expressiveness of the original motion. For example, a clumsy walking motion can be decomposed so as to separate the clumsy nature of the motion from the underlying walking pattern. The clumsy style component can then be applied to a running motion, which will then yield a clumsy-looking running motion. Our approach is simple, efcient and intuitive since the components are themselves motion data. We demonstrate that the proposed method can serve as an effective tool for interactive motion analysis and editing.

"Expressive Speech-Driven Facial Animation",Yong Cao,Wen C. Tien, Petros Faloutsos, Fred Pighin, ACM Transactions on Graphics, Volume 24, Issue 4, 1283-1302, October 2005.
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Speech-driven facial motion synthesis is a well explored research topic. However, little has been done to model expressive visual behavior during speech. We address this issue using a machine learning approach that relies on a database of speech related high fidelity facial motions. From this training set, we derive a generative model of expressive facial motion that incorporates emotion control while maintaining accurate lip-synching. The emotional content of the input speech can be manually specified by the user or automatically extracted from the audio signal using a Support Vector Machine classifier.

"Real-time Speech Motion Synthesis from Recorded Motions", Yong Cao, Petros Faloutsos, Eddie Kohler, Fred Pighin, In Proceedings of ACM SIGGRAPH / Eurographics Symposium on Computer Animation 2004.
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Data-driven approaches have been successfully used for realistic visual speech synthesis. However, little effort has been devoted to real-time lip-synching for interactive applications. In particular, algorithms that are based on a graph of motions are notorious for their exponential complexity. In this paper, we present a greedy graph search algorithm that yields vastly superior performance and allows real-time motion synthesis from a large database of motions. The time complexity of the algorithm is linear with respect to the size of an input utterance. In our experiments, the synthesis time for an input sentence of average length is under a second.

"Hybrid Control For Interactive Character Animation", Ari Shapiro, Frederic Pighin, and Petros Faloutsos,Pacific Graphics, 2003.
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We implement a framework for animating interactive characters by combining kinematic animation with physical simulation. The combination of animation techniques allows the characters to exploit the advantages of each technique. For example, characters can perform naturallooking kinematic gaits and react dynamically to unexpected situations.

"Composable Controllers for Character Animation", Petros Faloutsos, Michiel van de Panne and Demetri Terzopoulos, Los Angeles, August, SIGGRAPH 2001.
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Computer and Graphics 2001 Best Paper Award winning article on the Virtual Stuntman
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First use of Support Vector machines in CG.

An ambitious goal in the area of physics-based computer animation is the creation of virtual actors that autonomously synthesize realistic human motions and possess a broad repertoire of lifelike motor skills. To this end, the control of dynamic, anthropomorphic figures subject to gravity and contact forces remains a difficult open problem. We propose a framework for composing controllers in order to enhance the motor abilities of such figures. A key contribution of our composition framework is an explicit model of the "pre-conditions" under which motor controllers are expected to function properly. We demonstrate controller composition with pre-conditions determined not only manually, but also automatically based on Support Vector Machine (SVM) learning theory. We evaluate our composition framework using a family of controllers capable of synthesizing basic actions such as balance, protective stepping when balance is disturbed, protective arm reactions when falling, and multiple ways of standing up after a fall. We furthermore demonstrate these basic controllers working in conjunction with more dynamic motor skills within a prototype virtual stuntperson. Our composition framework promises to enable the community of physics-based animation practitioners to easily exchange motor controllers and integrate them into dynamic characters

"On Power-Law Relationships of the Internet Topology", Michalis Faloutsos, Petros Faloutsos, Christos Faloutsos, ACM SIGCOMM'99, Cambridge, Massachussets,pp 251-262, 1999
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Most cited document of 1999 according to citeseer 2006.

Despite the apparent randomness of the Internet, we discover some surprisingly simple power-laws of the Internet topology. These power-laws hold for three snapshots of the Internet, between November 1997 and December 1998, despite a 45% growth of its size during that period. We show that our power-laws fit the real data very well resulting in correlation coefficients of 96% or higher. Our observations provide a novel perspective of the structure of the Internet. The power-laws describe concisely skewed distributions of graph properties such as the node outdegree. In addition, these power-laws can be used to estimate important parameters such as the average neighborhood size, and facilitate the design and the performance analysis of protocols. Furthermore, we can use them to generate and select realistic topologies for simulation purposes.

"Dynamic Free-Form Deformations for Animation Synthesis", Petros Faloutsos, Michiel van de Panne, Demetri Terzopoulos, IEEE Transactions on Visualization and Computer Graphics, Vol. 3, No 3. July-September 1997.
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Free-form deformations (FFDs) are a popular tool for modeling and keyframe animation. This paper extends the use of FFDs to a dynamic setting. Our goal is to enable normally inanimate graphics objects, such as teapots and tables, to become animated, and learn to move about in a charming, cartoon-like manner. To achieve this goal, we implement a system that can transform a wide class of objects into dynamic characters. Our formulation is based on parameterized hierarchical FFDs augmented with Lagrangian dynamics, and provides an efficient way to animate and control the simulated characters. Objects are assigned mass distributions and elastic deformation properties, which allow them to translate, rotate, and deform according to internal and external forces. In addition, we implement an automated optimization process that searches for suitable control strategies. The primary contributions of the work are threefold. First, we formulate a dynamic generalization of conventional, geometric FFDs. The formulation employs deformation modes which are tailored by the user and are expressed in terms of FFDs. Second, the formulation accommodates a hierarchy of dynamic FFDs that can be used to model local as well as global deformations. Third, the deformation modes can be active, thereby producing locomotion.