Visual Perception of Dynamic Properties and Events: Collisions and Throws

The central topic of this dissertation is visual perception of dynamic events. The topic is worth of interest, as witnessed by its long tradition in the history of Experimental Psychology, starting with the seminal work of Albert Michotte (1881 - 1965) on phenomenal causality. Thus, the topic I chose is not original in itself. However, a distinctive element of novelty in my dissertation is the use of Computer Graphics techniques as a means for creating realistic experimental stimuli in psychological experiments. Besides the advantage of reducing the gap between laboratory experiments and everyday experience, this may reveal the importance of experimental variables which traditionally have been ignored in research on visual perception of dynamic events. The reader should be informed that this dissertation is characterized by various lines of research, which are intrinsically connected with the central topic of visual perception of dynamic events. In some of the experiments, I investigate visual perception of dynamic events, whereas in others I investigate cognition of the same events. Two distinct dynamic events will be especially studied: horizontal collisions and throws. Moreover, the results of the experiments will be discussed not only in relation to their theoretical implications for psychological models, but also in relation to their potential applications to Physics education and Computer Graphics. As a result, the content of the dissertation is quite heterogeneous, but I hope to provide the reader with a broad and multidisciplinary perspective on the subject at hand. The dissertation is composed of five chapters, which may be divided into three groups. (i) In Chapters 1-3, after a presentation of the theoretical background of visual perception of dynamic events, I investigate the influence of dynamic properties of virtual objects on visual perception of horizontal collisions. The results of this research are important for the old and still active debate on phenomenal causality. (ii) In Chapter 4 I present a research on Naïve Physics of horizontal collisions between virtual spheres differing in simulated mass and velocity. In this chapter I take a more cognitive (rather than perceptual) perspective on dynamic events, investigating how people reason about the proposed physical event. (iii) In Chapter 5, I present a research on visual perception of virtual throwing animations, which are complex and rarely studied dynamic events. This chapter stands out for its multidisciplinary nature, as in it I discuss how the results can be applied to Computer Graphics. The research presented in this last chapter has been conducted as a part of my doctorate studies when I was a visiting PhD student at the Graphics, Vision, and Visualisation Group at Trinity College Dublin, where I collaborated with Professor Carol O’Sullivan and Doctor Ludovic Hoyet, who are computer scientists working on applications of visual perception to Computer Graphics. In more detail, in Chapter 1 I discuss the theoretical background of visual perception of dynamic events and phenomenal causality. Firstly, I focus on Michotte’s classical work. Secondly, I discuss some prominent issues which have been debated for a long time in this field of research. Lastly, I present White’s schema-matching model of visual perception of dynamic events, discussing its differences and similarities as compared with Michotte’s model. This chapter is intended to serve as a theoretical point of reference for the entire dissertation. In Chapter 2 I discuss the hypothesis that visually perceived dynamic properties of objects involved in dynamic events do influence visual perception of the dynamic events themselves. Firstly, I try to confute two popular arguments against this hypothesis. Then, I highlight the evolutionary advantage of visual perception of dynamic properties, discussing their possible influence on visual perception of dynamic events. Lastly, I discuss Runeson’s KSD model in relation to the presented hypothesis. In Chapter 3 I present three experiments which confirm the hypothesis discussed in Chapter 2. In particular, I show that simulated material (Experiment 1) and size (Experiments 2 and 3) of virtual objects involved in horizontal collisions strongly influence how observers perceive the event. I also discuss the theoretical implications of these findings by referring to Michotte’s and White’s models. In Chapter 4 I present a research on Naïve Physics of horizontal collisions. Firstly, I discuss the general importance of studying Naïve Physics for improving basic education in Physics. Secondly, I present Information Integration Theory and Functional Measurement methodology as suitable tools for the assessment of students’ intuitive knowledge of physical events, evidencing their advantages over multiple-choice surveys. Lastly, I present two experiments (conducted using Information Integration Theory and Functional Measurement) on Naïve Physics of horizontal collisions between simulated spheres differing in size, velocity, and material. The importance of the results for Physics instruction will also be discussed. Finally, in Chapter 5 I present a research on visual perception of edited virtual throwing animations. First I discuss the relations between visual perception of dynamic events (human motion in particular) and Computer Graphics. Then, I present two experiments on observers’ sensitivity to anomalies in realistic virtual throwing animations, discussing the importance of the results for videogames and movies industry

Post/documentary: referential multimodality in "Animated Documentaries" and "Documentary Games"

From John Grierson’s influential early definition of documentary as “the creative treatment of actuality” through documentary studies’ reconstruction of the multitude of existing forms to philosophers’ attempts to develop comprehensive accounts of documentary as a specific kind of nonfiction film, the concept of documentary has been both fiercely contested and, through these debates, continuously expanded to refer to an ever more extensive corpus of works. By now, there is a broad consensus that documentary film as a genre cannot be reduced to supposedly “objective” recordings of the “actual world,” as both various kinds of reenactments and sometimes radical forms of subjectivity have (yet again) become well-established elements of many documentary films. However, it would seem that summarily treating “hybrid” documentary films, “animated documentaries,” and “documentary games” as nothing but the most recent chapter of the history of documentary occludes more than it illuminates. Instead, this article proposes to examine “animated documentaries” such as Chicago 10: Say Your Peace (Brett Morgen, 2007) or Waltz with Bashir (Ari Folman, 2008) and “documentary games” such as JFK Reloaded (Traffic Games, 2004) or The Cat and the Coup (Peter Brinson and Kurosh ValaNejad, 2011) through the lens of post/documentary, thus emphasizing not just their semiotic but also their occasionally rather complex referential multimodality

Perceptual Effects in Physically Based Animation with Rigid and Deformable Objects

We perform four psychophysical studies to investigate the perceptual effect of factors in the rendering and simulation stages of physically based animation production. Our study provides helpful insights in how to improve visual plausibility or reduce computational cost, which may allow artists to adjust their designs to enhance or minimize the perceived deformation in a model, or to choose a more efficient dynamics model and simpler mesh used in simulation without harming the visual plausibility. In our first study, we find that appearance can potentially influence people’s sensitivity to differences of deformation as well as subjective rating of softness. Further analysis shows that, in simple scenarios, the effect of low-level visual details in appearance can be dominant, even if high-level information delivered by appearance has the opposite implication. Another experiment shows that as the number of objects in a scenario increases, objects are perceived to be stiffer. In the second study, we quantitatively measure how different low-level visual details can influence people’s perceived stiffness of a deformable sphere under physically based simulation. We find that checkerboard pattern with certain combinations of spatial frequency and contrast can reduce the perceived stiffness. Our study further shows that adding a high-contrast checkerboard background can reduce such effect. In our third study, we discover that the resolution of a mesh used in the simulation of deformable objects can be reduced to a certain level without being noticed. For complex deformation, it is easier for people to recognize such reduction. Lastly, we verify two hypotheses which are assumed to be true only intuitively in many rigid body simulations in our third study. I: In large scale rigid body simulation, viewers may not be able to perceive distortion incurred by an approximated simulation method. II: Fixing objects under a pile of objects does not affect the visual plausibility. Our analysis of results supports the truthfulness of the hypotheses under certain simulation environments, but discovers four factors which may affect the authenticity of these hypotheses: number of collisions simulated simultaneously, homogeneity of colliding object pairs, distance from scene under simulation to camera position, and simulation method used

Referential multimodality in “animated documentaries” and “documentary games”

From John Grierson’s influential early definition of documentary as “the creative treatment of actuality” through documentary studies’ reconstruction of the multitude of existing forms to philosophers’ attempts to develop comprehensive accounts of documentary as a specific kind of nonfiction film, the concept of documentary has been both fiercely contested and, through these debates, continuously expanded to refer to an ever more extensive corpus of works. By now, there is a broad consensus that documentary film as a genre cannot be reduced to supposedly “objective” recordings of the “actual world,” as both various kinds of reenactments and sometimes radical forms of subjectivity have (yet again) become well-established elements of many documentary films. However, it would seem that summarily treating “hybrid” documentary films, “animated documentaries,” and “documentary games” as nothing but the most recent chapter of the history of documentary occludes more than it illuminates. Instead, this article proposes to examine “animated documentaries” such as Chicago 10: Say Your Peace (Brett Morgen, 2007) or Waltz with Bashir (Ari Folman, 2008) and “documentary games” such as JFK Reloaded (Traffic Games, 2004) or The Cat and the Coup (Peter Brinson and Kurosh ValaNejad, 2011) through the lens of post/documentary, thus emphasizing not just their semiotic but also their occasionally rather complex referential multimodality

Physics-based character locomotion control with large simulation time steps.

Physical simulated locomotion allows rich and varied interactions with environments and other characters. However, control is di cult due to factors such as a typical character's numerous degrees of freedom and small stability region, discontinuous ground contacts, and indirect control over the centre of mass. Previous academic work has made signi cant progress in addressing these problems, but typically uses simulation time steps much smaller than those suitable for games. This project deals with developing control strategies using larger time steps. After describing some introductory work showing the di culties of implementing a handcrafted controller with large physics time steps, three major areas of work are discussed. The rst area uses trajectory optimization to minimally alter reference motions to ensure physical validity, in order to improve simulated tracking. The approach builds on previous work which allows ground contacts to be modi ed as part of the optimization process, extending it to 3D problems. Incorporating contacts introduces di cult complementarity constraints, and an exact penalty method is shown here to improve solver robustness and performance compared to previous relaxation methods. Trajectory optimization is also used to modify reference motions to alter characteristics such as timing, stride length and heading direction, whilst maintaining physical validity, and to generate short transitions between existing motions. The second area uses a sampling-based approach, previously demonstrated with small time steps, to formulate open loop control policies which reproduce reference motions. As a prerequisite, the reproducibility of simulation output from a common game physics engine, PhysX, is examined and conditions leading to highly reproducible behaviour are determined. For large time steps, sampling is shown to be susceptible to physical inva- lidities in the reference motion but, using physically optimized motions, is successfully applied at 60 time steps per second. Finally, adaptations to an existing method using evolutionary algorithms to learn feedback policies are described. With large time steps, it is found to be necessary to use a dense feedback formulation and to introduce phase-dependence in order to obtain a successful controller, which is able to recover from impulses of several hundred Newtons applied for 0.1s. Additionally, it is shown that a recent machine learning approach based on support vector machines can identify whether disturbed character states will lead to failure, with high accuracy (99%) and with prediction times in the order of microseconds. Together, the trajectory optimization, open loop control, and feedback developments allow successful control for a walking motion at 60 time steps per second, with control and simulation time of 0.62ms per time step. This means that it could plausibly be used within the demanding performance constraints of games. Furthermore, the availability of rapid failure prediction for the controller will allow more high level control strategies to be explored in future

Implicit muscle models for interactive character skinning

En animation de personnages 3D, la déformation de surface, ou skinning, est une étape cruciale. Son rôle est de déformer la représentation surfacique d'un personnage pour permettre son rendu dans une succession de poses spécifiées par un animateur. La plausibilité et la qualité visuelle du résultat dépendent directement de la méthode de skinning choisie. Sa rapidité d'exécution et sa simplicité d'utilisation sont également à prendre en compte pour rendre possible son usage interactif lors des sessions de production des artistes 3D. Les différentes méthodes de skinning actuelles se divisent en trois catégories. Les méthodes géométriques sont rapides et simples d'utilisation, mais leur résultats manquent de plausibilité. Les approches s'appuyant sur des exemples produisent des résultats réalistes, elles nécessitent en revanche une base de données d'exemples volumineuse, et le contrôle de leur résultat est fastidieux. Enfin, les algorithmes de simulation physique sont capables de modéliser les phénomènes dynamiques les plus complexes au prix d'un temps de calcul souvent prohibitif pour une utilisation interactive. Les travaux décrits dans cette thèse s'appuient sur Implicit Skinning, une méthode géométrique corrective utilisant une représentation implicite des surfaces, qui permet de résoudre de nombreux problèmes rencontrés avec les méthodes géométriques classiques, tout en gardant des performances permettant son usage interactif. La contribution principale de ces travaux est un modèle d'animation qui prend en compte les effets des muscles des personnages et de leur interactions avec d'autres éléments anatomiques, tout en bénéficiant des avantages apportés par Implicit Skinning. Les muscles sont représentés par une surface d'extrusion le long d'axes centraux. Les axes des muscles sont contrôlés par une méthode de simulation physique simplifiée. Cette représentation permet de modéliser les collisions des muscles entre eux et avec les os, d'introduire des effets dynamiques tels que rebonds et secousses, tout en garantissant la conservation du volume, afin de représenter le comportement réel des muscles. Ce modèle produit des déformations plus plausibles et dynamiques que les méthodes géométriques de l'état de l'art, tout en conservant des performances suffisantes pour permettre son usage dans une session d'édition interactive. Elle offre de plus aux infographistes un contrôle intuitif sur la forme des muscles pour que les déformations obtenues se conforment à leur vision artistique.Surface deformation, or skinning is a crucial step in 3D character animation. Its role is to deform the surface representation of a character to be rendered in the succession of poses specified by an animator. The quality and plausiblity of the displayed results directly depends on the properties of the skinning method. However, speed and simplicity are also important criteria to enable their use in interactive editing sessions. Current skinning methods can be divided in three categories. Geometric methods are fast and simple to use, but their results lack plausibility. Example-based approaches produce realistic results, yet they require a large database of examples while remaining tedious to edit. Finally, physical simulations can model the most complex dynamical phenomena, but at a very high computational cost, making their interactive use impractical. The work presented in this thesis are based on, Implicit Skinning, is a corrective geometric approach using implicit surfaces to solve many issues of standard geometric skinning methods, while remaining fast enough for interactive use. The main contribution of this work is an animation model that adds anatomical plausibility to a character by representing muscle deformations and their interactions with other anatomical features, while benefiting from the advantages of Implicit Skinning. Muscles are represented by an extrusion surface along a central axis. These axes are driven by a simplified physics simulation method, introducing dynamic effects, such as jiggling. The muscle model guarantees volume conservation, a property of real-life muscles. This model adds plausibility and dynamics lacking in state-of-the-art geometric methods at a moderate computational cost, which enables its interactive use. In addition, it offers intuitive shape control to animators, enabling them to match the results with their artistic vision