Abstract Humans need to shift their center of mass during standing for several purposes such as preparing for the upcoming motion or increasing their stability. In this paper, we present a control strategy for robust center of mass shifting motions during standing. In our strategy, the desired posture can be defined with only a few high level features, such as the desired character center of mass position and the foot configurations. An online optimization process is designed for generating a kinematic lower body and pelvis posture that satisfies these high level features together with some criteria that guide a natural standing pose. Natural knee bending behaviours automatically arise as a result of this optimization process. Internal joint torques for tracking this optimized posture together with the given desired upper body pose are calculated by the physics‐based control framework. Moreover, a physics‐based arm control strategy that regulates the angular momentum of the character is devised in order to increase the robustness of the character under external disturbances. Several experiments are conducted to demonstrate the effectiveness of the proposed strategy. Because the strategy does not include any off‐line parameter optimization, equations of motion, or inverse dynamics, it is highly suitable for online applications.

Abstract With the growth in available computing power, we see increasingly crowded virtual environments. In densely crowded situations, collisions are likely to occur, and the choice in collision detection technique can impact the perceived realism of a real‐time crowd. This paper presents an investigation into the accuracy of human observers with regard to the recognition of collisions between virtual characters. We show the result of two user studies, where participants classify scenarios as “colliding” or “not colliding”; a pilot study investigates the perception of static images, whereas the main study expands on this by employing animated videos. In the pilot experiment, we investigated the effect of two variables on the ability to recognize collisions: distance between the character meshes and visibility of the inter‐character gap. In the main experiment, we investigate the angle between the character paths and the severity of the (near) collision. On average, respondents correctly classified 72% (static) and 68% (animated) of the scenarios. A notable result is that the maximum uncertainty in determining existence of collisions occurs when the characters are overlapping and that there is a significant bias towards answering “not colliding.” We also discuss differences in bias in the recognition of upper‐ and lower‐body collisions.

Abstract This paper presents a gaze behavior model for an interactive virtual character situated in the real world. We are interested in estimating which user has an intention to interact, in other words which user is engaged with the virtual character. The model takes into account behavioral cues such as proximity, velocity, posture, and sound; estimates an engagement score; and drives the gaze behavior of the virtual character. Initially, we assign equal weights to these features. Using data collected in a real setting, we analyze which features have higher importance. We found that the model with weighted features correlates better with the ground‐truth data.

We present a novel dense crowd simulation method. In real crowds of high density, people manoeuvring the crowd need to twist their torso to pass between others. Our proposed method does not use the traditional disc-shaped agent, but instead employs capsule-shaped agents, which enables us to plan such torso orientations. Contrary to other crowd simulation systems, which often focus on the movement of the entire crowd, our method distinguishes between active agents that try to manoeuvre through the crowd, and passive agents that have no incentive to move. We introduce the concept of a focus point to influence crowd agent orientation. Recorded data from real human crowds are used for validation, which shows that our proposed model produces equivalent paths for 85 percent of the validation set. Furthermore, we present a character animation technique that uses the results from our crowd model to generate torso-twisting and side-stepping characters.

In this paper we describe a novel technique for solving the inverse kinematics problem for human fingers. We derive a closed-form solution that places the fingertips precisely in the desired location by allowing minor deviations in the rotations of the Proximal Interphalangeal and Distal Interphalangeal joints compared to the fixed ratio that is known to exist between them. The obvious advantage is that with our approach there is no need to iterate until the distance between the fingertips and the desired locations is small enough. We show that this method is reliable and exact while showing minimal differences to the finger poses generated by the original closed-form solution. In our experiments we found the positions of the intermediate joints of the finger to deviate only around 1.5 mm in the worst case from those resulting from a numerical approximation of the original closed-form solution. On average the deviation is less than 0.5 millimeter.

In this paper we present a control framework which creates robust and natural balance shifting behaviours during standing. Given high-level features such as the position of the center of mass projection and the foot configurations, a kinematic posture satisfying these features is synthesized using online optimization. The physics-based control framework of the system calculates internal joint torques that enable tracking the optimized posture together with balance and pelvis control. Our system results in a very stable pose regardless of the position of the COM projection within the foot support polygon. This is achieved using an online knee bending and hip joint position optimization scheme. Moreover, we improve the robustness of the character under external perturbations by an arm control strategy that regulates the body's angular momentum. The capabilities of the system are demonstrated under different scenarios. The proposed framework doesn't include equations of motions or inverse dynamics. The simulations run in real-time on a standard modern PC without needing any preprocessing like offline parameter optimization. As a result, our system is suitable for commercial real-time graphics applications such as games.

In high-density crowds, one can observe torso twists; people rotate their upper body to decrease their width perpendicular to the motion path, in order to squeeze through narrow spaces between other crowd members. In this paper we investigate such behaviour, by recording and analysing dense crowds. Apart from the common approach, where only the position of each person in the crowd is recorded, we also record and analyse the torso orientations. To the best of our knowledge, this has not been done before in the context of dense crowds. We show that the paths chosen by the participants can be predicted by Generalized Voronoi Diagrams based on line segment representations of the participants' torsos, and attest that the medial axis of a capsule-shaped representation of the torso is a good choice for such line segments.

Skinning using point handles has experimentally shown its effectiveness for stretching, twisting, and supple deformations [Jacobson et al. 2011] which are difficult to achieve using rigid bones. However, point handles are much less effective for limbs bending since their influence weights vary over the skin mesh. This poster presents an efficient scheme, which expands the space of deformations possible to the point handles by supporting rigid bending using a rigidity energy minimization framework. Thus, 3D points that are much easier to design while provide larger space of deformations than a skeleton can be used as alternative handles in character skinning.

ABSTRACT Simulating a crowded scene like a busy shopping street requires tight packing of virtual characters. In such cases, collisions are likely to occur, and the choice in collision detection shape will influence how characters are allowed to intermingle. Full collision detection is too expensive for crowds, so simplifications are needed. The most common simplification, the fixed‐width, pose‐independent cylinder, does not allow intermingling of characters, as it will either cause too much empty space between characters or undetected penetrations. As a possible solution to this problem, we introduce the bounding cylinder hierarchy (BCH), a bounding volume hierarchy that uses vertical cylinders as bounding shapes. Because the BCH is a generalization of the single cylinder, we expect that this representation can be easily integrated with existing crowd simulation systems. We compare our BCH with commonly used collision shapes, namely the single cylinder and oriented bounding box tree, in terms of query time, construction time, and represented volume. To get an indication of possible crowd densities, we investigate how close characters can be before collision is detected and finally propose a critical maximum depth for the BCH. Copyright © 2014 John Wiley & Sons, Ltd.

This abstract discusses a perceptual study investigating the perception of emotion in conversational body language. Specifically, we wished to determine the parts of the body most important for the identification of emotional expressions. We conducted a perceptual study using motion captured clips of an actor conducting a number of emotional conversations, exhibiting a set of emotions with negative connotations. These motions were represented on a virtual character, showing either the full body, or in the absence of the motion of arms, legs or head. Participants were then asked to indicate the level of presence of both the correct emotion and a corresponding positive pair (e.g., relaxed/stressed). We found participants were able to identify the emotions correctly, but depended strongly on the motions of the arms when doing so.

Often in games, a virtual character is required to interact with objects in the surrounding environment. These interactions can occur in different locations, with different items, often in combination with environment navigation tasks. This results in switching and blending between different motions in order to fit to restrictions due to the position of the character and the interaction circumstances. In this paper, we conduct perceptual experiments to gain knowledge about such interactions and deduce important factors about their design for game animators. Our results identify at what point interaction information is obvious, and which body parts are most important to consider. We find that general information about target position is evident from early on in a combined navigation and manipulation task, and can be deduced from very few visual cues. We also learn that participants are highly sensitive to target positions during the interaction phase, relying mostly on indicators in the motion of the character's arm in the final steps.

It has been shown that humans are sensitive to the portrayal of emotions for virtual characters. However, previous work in this area has often examined this sensitivity using extreme examples of facial or body animation. Less is known about how attuned people are at recognizing emotions as they are expressed during conversational communication. In order to determine whether body or facial motion is a better indicator for emotional expression for game characters, we conduct a perceptual experiment using synchronized full-body and facial motion-capture data. We find that people can recognize emotions from either modality alone, but combining facial and body motion is preferable in order to create more expressive characters.

Abstract This paper presents an efficient technique for synthesizing motions by stitching, or splicing, an upper‐body motion retrieved from a motion space on top of an existing lower‐body locomotion of another motion. Compared to the standard motion splicing problem, motion space splicing imposes new challenges as both the upper and lower body motions might not be known in advance. Our technique is the first motion (space) splicing technique that propagates temporal and spatial properties of the lower‐body locomotion to the newly generated upper‐body motion and vice versa. Whereas existing techniques only adapt the upper‐body motion to fit the lower‐body motion, our technique also adapts the lower‐body locomotion based on the upper body task for a more coherent full‐body motion. In this paper, we will show that our decoupled approach is able to generate high‐fidelity full‐body motion for interactive applications such as games.

Abstract Natural locomotion of virtual characters is very important in games and simulations. The naturalness of the total motion strongly depends on both the path the character chooses and the animation of the walking character. Therefore, much work has been done on path planning and generating walking animations. However, the combination of both fields has received less attention. Combining path planning and motion synthesis introduces several problems. In this paper, we will identify two problems and propose possible solutions. The first problem is selecting an appropriate distance metric for locomotion synthesis. When concatenating clips of locomotion, a distance metric is required to detect good transition points. We have evaluated three common distance metrics both quantitatively (in terms of footskating, path deviation and online running time) and qualitatively (user study). Based on our observations, we propose a set of guidelines when using these metrics in a motion synthesizer. The second problem is the fact that there is no single point on the body that can follow the path generated by the path planner without causing unnatural animations. This raises the question how the character should follow the path. We will show that enforcing the pelvis to follow the path will lead to unnatural animations and that our proposed solution, which uses path abstractions, generates significantly better animations. Copyright © 2011 John Wiley & Sons, Ltd.

Abstract Virtual humans are employed in many interactive applications using 3D virtual environments, including (serious) games. The motion of such virtual humans should look realistic (or ‘natural’) and allow interaction with the surroundings and other (virtual) humans. Current animation techniques differ in the trade‐off they offer between motion naturalness and the control that can be exerted over the motion. We show mechanisms to parametrize, combine (on different body parts) and concatenate motions generated by different animation techniques. We discuss several aspects of motion naturalness and show how it can be evaluated. We conclude by showing the promise of combinations of different animation paradigms to enhance both naturalness and control.

Abstract Especially in a constrained virtual environment, precise control of foot placement during character locomotion is crucial to avoid collisions and to ensure a natural locomotion. In this paper, we present the step space: a novel technique for generating animations of a character walking over a set of desired foot steps in real‐time. We use an efficient greedy nearest‐neighbor approach and warp the resulting animation such that it adheres to both spatial and temporal constraints. We use smart aligning and inverse kinematics to ensure spatial constraints and time warping for temporal constraints. We will show that our technique can generate realistic locomotion animations very efficiently even though we impose many constraints on the animation. Copyright © 2010 John Wiley & Sons, Ltd.

Reusing motion capture data is of great importance in games and simulations. Therefore, many techniques from motion synthesis aim at concatenating or blending existing pieces of motion. For concatenation, a distance metric is required to detect good transition points. In this paper, three common distance metrics are evaluated both quantitatively and qualitatively. In the quantitive analysis we evaluate foot-skate, path deviation and on-line running time. The qualitative analysis is done by a user study. We observed that the point cloud metric results in a very slow graph search and a high path deviation compared to the other metrics. However, this metric results in less foot skating. We also show that increasing the graph size does not alway introduce more visual artifacts. Based on our observations, we propose a set of guidelines when using these metrics in a motion synthesizer.

This issue contains a selection of the best papers of the workshop Motion in Games that was held in Utrecht last June.It contains also a regular paper on Combined Compression and Simplification of Dynamic 3D Meshes by Libor Vasa and Vaklav Scala from the University of West Bohemia in Pilsen in Czech Republic.The paper deals with a new approach to dynamic mesh compression, which combines compression with simplification to achieve improved compression results, a natural support for incremental transmission and level of detail.The algorithm allows fast progressive transmission of dynamic 3D content.

This volume of Computer Animation and Virtual Worlds (CAVW) contains a selection of papers submitted to CASA 2009, the 22nd International Conference on Computer Animation and Social Agents. CASA 2009 was organized in Amsterdam, the Netherlands from the 17th to the 19th of June 2009. CASA is organized under the auspices of the Computer Graphics Society (CGS) and is the premier academic conference in the field of computer animation and behavior simulation of social agents. CASA was founded in 1988. Over the last years, the conference has been organized in Philadelphia (1998, 2000), Seoul (2001, 2008), Geneva (2002, 2004, 2006), New Jersey (2003), Hong Kong (2005), and Hasselt (2007). In 2009 the conference was held in Amsterdam, the Netherlands. The organization was done by the Human Media Interaction (HMI) research group of the University of Twente, the Netherlands. The CASA 2009 edition received 112 submissions. Each submission received at least three reviews and 35 of these submissions have been chosen to appear in this special issue of Computer Animation and Virtual Worlds. Other papers have been selected to appear in a proceedings published by the University of Twente. We thank all the authors for having submitted their work to this conference and the members of the International Program Committee and the additional external reviewers for their time and efforts invested in the reviewing process. We would like to thank the editors of Wiley for their support to publishing these papers in a very short time. A particular word of thanks goes to the HMI support team, that is, to Hendri Hondorp, Lynn Packwood, Charlotte Byron, and Alice Vissers for their support in organizing the conference, taking care of financial and organizational matters, and taking care of editing and publishing matters. CASA 2009 has been sponsored by the innovation agency IOP-MMI of SenterNovem (Dutch Ministry of Economic Affairs) and the GATE (Game research for Training and Entertainment) project.

In this paper, we present a simple and robust mixed reality (MR) framework that allows for real-time interaction with virtual humans in mixed reality environments under consistent illumination. We will look at three crucial parts of this system: interaction, animation and global illumination of virtual humans for an integrated and enhanced presence. The interaction system comprises of a dialogue module, which is interfaced with a speech recognition and synthesis system. Next to speech output, the dialogue system generates face and body motions, which are in turn managed by the virtual human animation layer. Our fast animation engine can handle various types of motions, such as normal key-frame animations, or motions that are generated on-the-fly by adapting previously recorded clips. Real-time idle motions are an example of the latter category. All these different motions are generated and blended on-line, resulting in a flexible and realistic animation. Our robust rendering method operates in accordance with the previous animation layer, based on an extended for virtual humans precomputed radiance transfer (PRT) illumination model, resulting in a realistic rendition of such interactive virtual characters in mixed reality environments. Finally, we present a scenario that illustrates the interplay and application of our methods, glued under a unique framework for presence and interaction in MR.

In this paper, we present a simple and robust mixed reality (MR) framework that allows for real-time interaction with virtual humans in real and virtual environments under consistent illumination. We look at three crucial parts of this system: interaction, animation and global illumination of virtual humans for an integrated and enhanced presence. The interaction system comprises of a dialogue module, which is interfaced with a speech recognition and synthesis system. Next to speech output, the dialogue system generates face and body motions, which are in turn managed by the virtual human animation layer. Our fast animation engine can handle various types of motions, such as normal key-frame animations, or motions that are generated on-the-fly by adapting previously recorded clips. All these different motions are generated and blended on-line, resulting in a flexible and realistic animation. Our robust rendering method operates in accordance with the previous animation layer, based on an extended for virtual humans precomputed radiance transfer (PRT) illumination model, resulting in a realistic display of such interactive virtual characters in mixed reality environments. Finally, we present a scenario that illustrates the interplay and application of our methods, glued under a unique framework for presence and interaction in MR

In this paper we present a discussion about believability for Virtual Environments, emotional simulation and also Embodied Conversational Agents (ECAs). We will discuss about the definition of believability and the three elements of believability environments (immersion, presentation and interaction). We also present a discussion about believability and interfaces. Finally, ECA, emotional and personnality simulation are explained and presented.

In this paper, we propose an animation approach based on principal component analysis that allows generating two layers of subtle motions: small posture variations and personalised change of balance. Such a motion generator is needed in many cases when one attempts to create an animation sequence out of a set of existing clips. In nature there exists no motionless character, while in computer animation we often encounter cases where no planned actions, such as waiting for another actor finishing his/her part, is implemented as a stop/frozen animation. We identify many situations where a flexible idle motion generator can help: from synchronisation of speech/body animation duration, to dynamic creation of stand still variations in between two active plays. Our approach overcomes the limitations of using a small set of existing clips as a basis for synthesizing idle motions, such as unnatural repetition of movements and difficulties to insert idle motions into an animation without breaking its continuity. A realistic animation is obtained by blending small posture variations with personalised balance shifting animations.

Abstract This paper describes a generic model for personality, mood and emotion simulation for conversational virtual humans. We present a generic model for updating the parameters related to emotional behaviour, as well as a linear implementation of the generic update mechanisms. We explore how existing theories for appraisal can be integrated into the framework. Then we describe a prototype system that uses the described models in combination with a dialogue system and a talking head with synchronized speech and facial expressions. Copyright © 2004 John Wiley & Sons, Ltd.