虚拟的“真实”——交互动画中仿真蒙皮的探索

柔性蒙皮的皮肤不像刚性蒙皮那么僵硬,本文的目的就在于将长期应用于传统影视动画制作中的柔性蒙皮技术移植到交互领域中,即改变目前应用于交互领域中的角色几何体及其动画信息的数据结构,重新塑造一种信息文件的结构方式,使得在实时骨骼蒙皮的过程中,皮肤顶点受骨骼的影响值不会受到限制,达到真正意义上的柔性节点蒙皮,从而为交互状态下的角色动画提供一种新的动画解决方案。     

支持多模即时控制的角色动画库

该文主要介绍了一个支持多模即时控制的角色动画库,包括其整体架构、脚本格式和动画引擎。提出了一种通过以XML作为主要格式的角色定义,将角色、骨架和动作定义相分离以提高动画库的复用性的方法。采用动作有限状态机使得系统能够通过一系列的动画指令来控制动画角色的行为,简化了对动画角色的控制。同时系统通过支持多模输入并在虚拟化身上做出即时反应,增强了虚拟现实应用中的沉浸感、临场参与感和交互性。     

Markerless Multiview Motion Capture with 3D Shape Model Adaptation

In this paper, we address simultaneous markerless motion and shape capture from 3D input meshes of partial views onto a moving subject. We exploit a computer graphics model based on kinematic skinning as template tracking model. This template model consists of vertices, joints and skinning weights learned a priori from registered full‐body scans, representing true human shape and kinematics‐based shape deformations. Two data‐driven priors are used together with a set of constraints and cues for setting up sufficient correspondences. A Gaussian mixture model‐based pose prior of successive joint configurations is learned to soft‐constrain the attainable pose space to plausible human poses. To make the shape adaptation robust to outliers and non‐visible surface regions and to guide the shape adaptation towards realistically appearing human shapes, we use a mesh‐Laplacian‐based shape prior. Both priors are learned/extracted from the training set of the template model learning phase. The output is a model adapted to the captured subject with respect to shape and kinematic skeleton as well as the animation parameters to resemble the observed movements. With example applications, we demonstrate the benefit of such footage. Experimental evaluations on publicly available datasets show the achieved natural appearance and accuracy.     

Realistic modeling and animation of human body based on scanned data

In this paper we propose a novel method for building animation model of real human body from surface scanned data. The human model is represented by a triangular mesh and described as a layered geometric model. The model consists of two layers: the control skeleton generating body animation from motion capture data, and the simplified surface model providing an efficient representation of the skin surface shape. The skeleton is generated automatically from surface scanned data using the feature extraction, and then a point-to-line mapping is used to map the surface model onto the underlying skeleton. The resulting model enables real-time and smooth animation by manipulation of the skeleton while maintaining the surface detail. Compared with earlier approach, the principal advantages of our approach are the automated generation of body control skeletons from the scanned data for real-time animation, and the automatic mapping and animation of the captured human surface shape. The human model constructed in this work can be used for applications of ergonomic design,garment CAD, real-time simulating humans in virtual reality environment and so on.     

草绘3维人体建模的模板形变方法

目的 采用草绘交互方式直接构造3维人体模型是当前人体建模研究的重要课题之一.提出一种草绘3维人体建模的模板形变方法.方法 针对输入的草图,首先,采用关节点定位方法获取草图中的人体关节点,根据人体结构学约束识别人体骨架结构,通过解析人体轮廓草图获取人体草图特征;其次,通过骨架模板和外观轮廓模板形变,将草图特征映射到3维人体模型,实现3维人体建模.结果 草图解析方法能有效提取草图特征,通过模板形变方法生成3维人体模型,并在模型上保持草图特征;能适应不同用户的绘制习惯,且生成的3维人体模型可用于人体动画设计.结论 提出一种草绘3维人体建模的模板形变方法,支持用户采用草绘方式进行3维人体模型设计,方法具有良好的用户适应性,对3维动画创作具有重要意义.     

Multi‐layer Lattice Model for Real‐Time Dynamic Character Deformation

Due to the recent advancement of computer graphics hardware and software algorithms, deformable characters have become more and more popular in real‐time applications such as computer games. While there are mature techniques to generate primary deformation from skeletal movement, simulating realistic and stable secondary deformation such as jiggling of fats remains challenging. On one hand, traditional volumetric approaches such as the finite element method require higher computational cost and are infeasible for limited hardware such as game consoles. On the other hand, while shape matching based simulations can produce plausible deformation in real‐time, they suffer from a stiffness problem in which particles either show unrealistic deformation due to high gains, or cannot catch up with the body movement. In this paper, we propose a unified multi‐layer lattice model to simulate the primary and secondary deformation of skeleton‐driven characters. The core idea is to voxelize the input character mesh into multiple anatomical layers including the bone, muscle, fat and skin. Primary deformation is applied on the bone voxels with lattice‐based skinning. The movement of these voxels is propagated to other voxel layers using lattice shape matching simulation, creating a natural secondary deformation. Our multi‐layer lattice framework can produce simulation quality comparable to those from other volumetric approaches with a significantly smaller computational cost. It is best to be applied in real‐time applications such as console games or interactive animation creation.     

Local Volume Preservation for Skinned Characters

Generating plausible deformations of a character skin within the standard production pipeline is a challenge. This paper presents a volume preservation method dedicated to skinned characters. As usual, the character is defined by a skin mesh at some rest pose and an animation skeleton. At each animation step, skin deformations are first computed using standard SSD. Our method corrects the result using a set of local deformations which model the fold‐over‐free, constant volume behavior of soft tissues. This is done geometrically, without the need of any physically‐based simulation. To make the method easily applicable, we also provide automatic ways to extract the local regions where volume is to be preserved and to compute adequate skinning weights, both based on the character's morphology.     

A Data‐driven Segmentation for the Shoulder Complex

The human shoulder complex is perhaps the most complicated joint in the human body being comprised of a set of three bones, muscles, tendons, and ligaments. Despite this anatomical complexity, computer graphics models for motion capture most often represent this joint as a simple ball and socket. In this paper, we present a method to determine a shoulder skeletal model that, when combined with standard skinning algorithms, generates a more visually pleasing animation that is a closer approximation to the actual skin deformations of the human body. We use a data‐driven approach and collect ground truth skin deformation data with an optical motion capture system with a large number of markers (200 markers on the shoulder complex alone). We cluster these markers during movement sequences and discover that adding one extra joint around the shoulder improves the resulting animation qualitatively and quantitatively yielding a marker set of approximately 70 markers for the complete skeleton. We demonstrate the effectiveness of our skeletal model by comparing it with ground truth data as well as with recorded video. We show its practicality by integrating it with the conventional rendering/animation pipeline.     

A mobile environment for sketching-based skeleton generation

Articulated character animation is typically performed by manually creating and rigging a skeleton into an unfolded 3D object. However, such tasks are not trivial, as they require a substantial amount of training and practices. Although automatic skeleton extraction methods have been proposed, they generally may not guarantee that the resulting skeleton can help produce desired animations according to user intention. In this paper, we present a sketching-based skeleton generation method suitable for use in the mobile environment. This method takes user sketching as an input, and based on the mesh segmentation result of a 3D object, it estimates a skeleton for articulated character animation. In addition, we are currently developing a Web-based mobile platform to support mesh editing by a group of collaborative users and we depict the system architecture of such a platform. Results show that our method can produce better skeletons in terms of joint positions and topological structure.     

Interactive motion mapping for real‐time character control

It is now possible to capture the 3D motion of the human body on consumer hardware and to puppet in real time skeleton‐based virtual characters. However, many characters do not have humanoid skeletons. Characters such as spiders and caterpillars do not have boned skeletons at all, and these characters have very different shapes and motions. In general, character control under arbitrary shape and motion transformations is unsolved ‐ how might these motions be mapped? We control characters with a method which avoids the rigging‐skinning pipeline — source and target characters do not have skeletons or rigs. We use interactively‐defined sparse pose correspondences to learn a mapping between arbitrary 3D point source sequences and mesh target sequences. Then, we puppet the target character in real time. We demonstrate the versatility of our method through results on diverse virtual characters with different input motion controllers. Our method provides a fast, flexible, and intuitive interface for arbitrary motion mapping which provides new ways to control characters for real‐time animation.     

Skeleton driven animation based on implicit skinning

Skeleton-driven animation methods have been commonly used in animating 3D characters. In these methods, a skinning process that binds the character surface onto the skeleton is required. This process is usually accomplished manually and is a time-consuming task. In this paper, we propose a novel method for automatically skinning skeletal character models. Given the motion of a skeleton, our method can animate the character model automatically. In our method, each joint coordinate is parameterized by its surrounding local surface. In such a way, the character's surface is implicitly bound onto the skeleton. Character animation is achieved by minimizing an energy function that is carefully designed to prevent unnatural volume changes and to guarantee smooth deformations. Experiments demonstrate the efficiency and excellent performance of our method.     

Animation of Biological Organ Growth Based on L-systems

In contrast with the growth of plants and trees, human organs can undergo significant changes in shape through a variety of global transformations during the growth period, such as bending or twisting. In our approach, the topology of a human organ is represented by a skeleton in the form of a tree or cycled graph. The length of skeleton growth can be simulated by an algebraic L-system that also produces discrete events. The paper shows how to include global transformations into the formalism of L-systems to obtain a continuous process. The shape of the organ is approximated by a number of ellipsoidal clusters centred at points on the skeleton. The proposed growth model of the organ continually responds to the positional changes of surrounding organs, thereby changing the organ shape locally. In our study, the stomach of a human embryo is used for the demonstration of organ development, and the methodology employed is also applicable to the animation of animal organs and their development.     

Animating Volumetric Models

This paper describes a technique to animate three-dimensional sampled volumes. The technique gives the animator the ability to treat volumes as if they were standard polygonal models and to use all of the standard animation/motion capture tools on volumetric data. A volumetric skeleton is computed from a volumetric model using a multi-resolution thinning procedure. The volumetric skeleton is centered in the object and accurately represents the shape of the object. The thinning process is reversible in that the volumetric model can be reconstructed from the volumetric skeleton. The volumetric skeleton is then connected and imported into a standard graphics animation package for animation. The animated skeleton is used for reconstruction, which essentially recreates a deformed volume around the deformed skeleton. Polygons are never computed and the entire process remains in the volumetric domain. This technique is demonstrated on one of the most complex 3D datasets, the Visible Male, resulting in actual “human animation”.     

DEMONSTRATION‐BASED BEHAVIOR PROGRAMMING FOR EMBODIED VIRTUAL AGENTS

We present a novel technique for behavioral animation through data‐driven behavior synthesis. This technique has two key features: it provides natural character behavior and has a programming‐by‐demonstration interface. Thus we can quickly create compelling autonomous virtual agents that exhibit stylized behavior. First, the human user demonstrates behavior for the character by specifying its high‐level actions (e.g., with a joystick) during an interactive session. Each demonstration is recorded as a sequence of discrete actions. Later, the character synthesizes novel behavior by concatenating segments of action sequences. The choice of segments is guided by simulations that predict fitness. Thus our technique operates such as a cognitive model, providing a character with deliberative decision making. The actions are abstract and can be mapped to any pertinent motions, even procedurally synthesized motions. Thus our technique complements character animation algorithms. We empirically show that our O(log n) technique is scalable, robust when provided with sufficient data, produces effective behavior for a number of problem domains, and is faster than traditional planning. Also, the interface is intuitive enough that character behavior can be created by nontechnical users.     

Order-controlled free-form animation

This paper introduces an animation system directed by order. High-level operations such as moving, turning, rolling and bouncing on a flat surface provide an easy-to-use interface to build animations. The underlying animation system relies on a constraint-based deformation model. Previously, to build an animation the user had to break up the desired animation into a list of deformations composed of a set of constraints. In addition to each constraint, he had to control the size and the shape of the deformed area as well as the shape of the deformation. The goal of the order-controlled animation is to encapsulate all the parameters of the deformations. Indeed, using these high-level operations the underlying deformations are completely transparent to the user. Before introducing these operations, we will present some extensions of the deformation model such as the generalized shape of the deformed area and a rotating movement combined with the deformation. We also explain how to control velocity and acceleration.     

Skeleton-aware size variations in digital mannequins

The general trend in character modeling is toward the personalization of models with higher levels of visual realism. This becomes possible with the development of commodity computation resources that are capable of processing massive data in parallel across multiple processors. On the other hand, there is always a trade-off between the quantity of the model features that are simulated and the plausibility of the visual realism because of the limited computation resources. Also, to keep the resources’ to be used efficiently within the other modeling approaches such as skin reflectance, aging, animation, etc., one must consider the efficiency of the method being used in the simulation. In this paper, we present an efficient method to customize the size of a human body model to personalize it with industry standard parameters. One of the major contributions of this method is that it is possible to generate a range of different size body models by using anthropometry surveys. This process is not limited by data-driven mesh deformation but also adapts the skeleton and motion to keep the consistency between different body layers.     

Animation‐Aware Quadrangulation

Geometric meshes that model animated characters must be designed while taking into account the deformations that the shape will undergo during animation. We analyze an input sequence of meshes with point‐to‐point correspondence, and we automatically produce a quadrangular mesh that fits well the input animation. We first analyze the local deformation that the surface undergoes at each point, and we initialize a cross field that remains as aligned as possible to the principal directions of deformation throughout the sequence. We then smooth this cross field based on an energy that uses a weighted combination of the initial field and the local amount of stretch. Finally, we compute a field‐aligned quadrangulation with an off‐the‐shelf method. Our technique is fast and very simple to implement, and it significantly improves the quality of the output quad mesh and its suitability for character animation, compared to creating the quad mesh based on a single pose. We present experimental results and comparisons with a state‐of‐the‐art quadrangulation method, on both sequences from 3D scanning and synthetic sequences obtained by a rough animation of a triangulated model.     

一种具有逼真效果的虚拟人动画生成方法

针对传统的虚拟人动画逼真度低、动画生成复杂、且动画存在飘移现象的问题,提出了一种基于视频关键帧获取运动数据的虚拟人动画生成方法。首先利用线性混合蒙皮算法建立了新的人体几何模型;其次利用正向运动学方法驱动虚拟人的骨骼模型,并利用平移补偿原则消除虚拟人运动中的飘移现象;最终实现了基于双目正交视觉获取关节运动参数的虚拟人动画。实验结果表明,新方法能逼真地模拟真实人体动作,且方法易于实现、实用性强。     

Automatic cage construction for retargeted muscle fitting

The animation of realistic characters necessitates the construction of complicated anatomical structures such as muscles, which allow subtle shape variation of the character’s outer surface to be displayed believably. Unfortunately, despite numerous efforts, the modelling of muscle structures is still left for an animator who has to painstakingly build up piece by piece, making it a very tedious process. What is even more frustrating is the animator has to build the same muscle structure for every new character. We propose a muscle retargeting technique to help an animator to automatically construct a muscle structure by reusing an already built and tested model (the template model). Our method defines a spatial transfer between the template model and a new model based on the skin surface and the rigging structure. To ensure that the retargeted muscle is tightly packed inside a new character, we define a novel spatial optimization based on spherical parameterization. Our method requires no manual input, meaning that an animator does not require anatomical knowledge to create realistic accurate musculature models.