Gabor texture in active appearance models

In computer vision applications, Active Appearance Models (AAMs) is usually used to model the shape and the gray-level appearance of an object of interest using statistical methods, such as PCA. However, intensity values used in standard AAMs cannot provide enough information for image alignment. In this paper, we firstly propose to utilize Gabor filters to represent the image texture. The benefit of Gabor-based representation is that it can express local structures of an image. As a result, this representation can lead to more accurate matching when condition changes. Given the problem of the excessive storage and computational complexity of the Gabor, three different Gabor-based image representations are used in AAMs: (1) GaborD is the sum of Gabor filter responses over directions, (2) GaborS is the sum of Gabor filter responses over scales, and (3) GaborSD is the sum of Gabor filter responses over scales and directions. Through a large number of experiments, we show that the proposed Gabor representations lead to more accurate and robust matching between model and images.     

Generalized sparse MRF appearance models

Image segmentation or registration approaches that rely on a local search paradigm (e.g, Active Appearance Models, Active Contours) require an initialization that provides for considerable overlap or a coarse localization of the object to be segmented or localized. In this paper we propose an approach that does not need such an initialization, but localizes anatomical structures in a global manner by formulating the localization task as the solution of a Markov Random Field (MRF).     

A comparative study of facial appearance modeling methods for active appearance models

Active appearance models (AAMs) have been widely used in many face modeling and facial feature extraction methods. One of the problems of AAMs is that it is difficult to model a sufficiently wide range of human facial appearances, the pattern of intensities across a face image patch. Previous researches have used principal component analysis (PCA) for facial appearance modeling, but there has been little analysis and comparison between PCA and many other facial appearance modeling methods such as non-negative matrix factorization (NMF), local NMF (LNMF), and non-smooth NMF (ns-NMF). The main contribution of this paper is to find a suitable facial appearance modeling method for AAMs by a comparative study. In the experiments, PCA, NMF, LNMF, and ns-NMF were used to produce the appearance model of the AAMs and the root mean square (RMS) errors of the detected feature points were analyzed using the AR and BERC face databases. Experimental results showed that (1) if the appearance variations of testing face images were relatively non-sparser than those of training face images, the non-sparse methods (PCA, NMF) based AAMs outperformed the sparse methods (nsNMF, LNMF) based AAMs. (2) If the appearance variations of testing face images are relatively sparser than those of training face images, the sparse methods (nsNMF) based AAMs outperformed the non-sparse methods (PCA, NMF) based AAMs.     

Towards a low bandwidth talking face using appearance models

This paper is motivated by the need to develop low bandwidth virtual humans capable of delivering audio-visual speech and sign language at a quality comparable to high bandwidth video. Using an appearance model combined with parameter compression significantly reduces the number of bits required for animating the face of a virtual human. A perceptual method is used to evaluate the quality of the synthesised sequences and it appears that 3.6 kb s⁻¹ can yield acceptable quality.     

一种改进主动形状模型的研究

主动形状模型（Active Shape Model,ASM）是一种较为流行的用于特征定位的统计学形状模型,主要应用于灰度图像中物体的跟踪与定位。在传统主动形状模型基础上,研究了一种构建局部纹理模型的新方法。该模型充分利用特征点之间的联系,构建加权模型。在ORL人脸数据库、JAFFE人脸数据库中进行特征定位实验,并进行了ASM性能分析。结果表明,改进ASM方法提高了搜索速度与特征点定位的精度。     

基于多表观模型竞争的视觉跟踪算法

在视觉跟踪中,如何适时地更新目标模型是影响跟踪算法跟踪精度和鲁棒性的关键性因素,也是当前研究中面临的重点和难点问题。对此,提出了一种基于多表观模型竞争的模型更新策略,通过多表观模型中各子模型的贡献度大小确定竞争优势排序,当最优子模型的贡献度满足多表观模型更新阈值时,对各子模型及其对应的系数进行更新,否则,仅对部分子模型进行更新。在此基础上,以粒子滤波算法为跟踪框架,提出了基于多表观模型竞争的视觉跟踪算法。实验结果表明,所提算法能够较好地处理视觉跟踪中的模型更新问题,跟踪性能较无模型更新策略的粒子滤波算法有明显提高。     

Modified AIC rule for model selection in combination with prior estimated noise models

In this paper we adapt the AIC model selection rule to be used in combination with prior estimated noise models in the presence of model errors. It is shown that under these conditions a slightly modified cost function extension is needed, resulting in a multiplicative complexity term instead of an additive one as would be found for fixed noise models and no model errors. The equivalency of this result with the classical prediction error framework is discussed.     

两种超声颈动脉血管斑块图像分割方法比较与改进

针对颈动脉超声图像,实现了两种颈动脉血管斑块的分割方法—活动形状模型(Active Shape Models,ASM)和活动表观模型(Active Appearance Models,AAM),对38组颈动脉超声图像进行了内外轮廓分割,并比较了两类算法对颈动脉内外轮廓分割的有效性。在综合分析实验结果的基础上,结合颈动脉超声图像的特点,通过引入比例不变性改进了ASM算法。统计结果表明,在运行时间上,ASM和改进ASM的运行时间相近,AAM大约为ASM和改进ASM的16倍。同时,采用FOM和RAY两种方法对分割效果进行评价,结果表明,改进ASM算法的分割效果较ASM有了很大的提高,是最适合颈动脉血管斑块超声图像分割的算法。     

Online learning and fusion of orientation appearance models for robust rigid object tracking

We introduce a robust framework for learning and fusing of orientation appearance models based on both texture and depth information for rigid object tracking. Our framework fuses data obtained from a standard visual camera and dense depth maps obtained by low-cost consumer depth cameras such as the Kinect. To combine these two completely different modalities, we propose to use features that do not depend on the data representation: angles. More specifically, our framework combines image gradient orientations as extracted from intensity images with the directions of surface normals computed from dense depth fields. We propose to capture the correlations between the obtained orientation appearance models using a fusion approach motivated by the original Active Appearance Models (AAMs). To incorporate these features in a learning framework, we use a robust kernel based on the Euler representation of angles which does not require off-line training, and can be efficiently implemented online. The robustness of learning from orientation appearance models is presented both theoretically and experimentally in this work. This kernel enables us to cope with gross measurement errors, missing data as well as other typical problems such as illumination changes and occlusions. By combining the proposed models with a particle filter, the proposed framework was used for performing 2D plus 3D rigid object tracking, achieving robust performance in very difficult tracking scenarios including extreme pose variations.     

A background robust active appearance model using active contour technique

This paper proposes an active contour-based active appearance model (AAM) that is robust to a cluttered background and a large motion. The proposed AAM fitting algorithm consists of two alternating procedures: active contour fitting to find the contour sample that best fits the face image and then the active appearance model fitting over the best selected contour. We also suggest an effective fitness function for fitting the contour samples to the face boundary in the active contour technique; this function defines the quality of fitness in terms of the strength and/or the length of edge features. Experimental results show that the proposed active contour-based AAM provides better accuracy and convergence characteristics in terms of RMS error and convergence rate than the existing robust AAM. The combination of the existing robust AAM and the proposed active contour-based AAM (AC-R-AAM) had the best accuracy and convergence performances.     

Efficient constrained local model fitting for non-rigid face alignment

Active appearance models (AAMs) have demonstrated great utility when being employed for non-rigid face alignment/tracking. The “simultaneous” algorithm for fitting an AAM achieves good non-rigid face registration performance, but has poor real time performance (2–3 fps). The “project-out” algorithm for fitting an AAM achieves faster than real time performance (>200 fps) but suffers from poor generic alignment performance. In this paper we introduce an extension to a discriminative method for non-rigid face registration/tracking referred to as a constrained local model (CLM). Our proposed method is able to achieve superior performance to the “simultaneous” AAM algorithm along with real time fitting speeds (35 fps). We improve upon the canonical CLM formulation, to gain this performance, in a number of ways by employing: (i) linear SVMs as patch-experts, (ii) a simplified optimization criteria, and (iii) a composite rather than additive warp update step. Most notably, our simplified optimization criteria for fitting the CLM divides the problem of finding a single complex registration/warp displacement into that of finding N simple warp displacements. From these N simple warp displacements, a single complex warp displacement is estimated using a weighted least-squares constraint. Another major advantage of this simplified optimization lends from its ability to be parallelized, a step which we also theoretically explore in this paper. We refer to our approach for fitting the CLM as the “exhaustive local search” (ELS) algorithm. Experiments were conducted on the CMU MultiPIE database.     

Application of the active shape model in a commercial medical device for bone densitometry

Osteoporosis is a common disorder characterised mainly by low bone mineral density (BMD), and leading to an increased risk of fracture. We have developed a new device that estimates BMD from ordinary hand radiographs. A crucial element of this method is the reconstruction of the metacarpals. This paper describes how this was solved using the active shape model (ASM). Standard ASM is unable to locate the metacarpal shafts in the direction along the bones. Therefore ASM was extended with a translation operator, which solved the problem. A hierarchical filtering method was used to construct a sufficient list of initial guesses for the ASM. The performance of ASM and the experience with the integration of ASM in a commercial medical device is reported. The ASM achieves 99.5% reconstruction success and is able to validate its own reconstruction in 97% of the cases. The system (Pronosco X-posure system, version 2.0) has been approved by the FDA, and more than 100 units have been sold.The concept of the translation operator is generalised to the more active shape model (MASM), which also allows a natural integration with the active appearance model.     

基于外观模型的目标跟踪算法研究进展

基于视觉的目标跟踪是模式识别、计算机视觉、机器学习等多个学科的交叉研究课题,在视频监控、视频压缩编码、视频检索、智能交通等领域有着十分广泛的应用。为了使国内外同行对基于外观模型的目标跟踪方法有一个较为全面的了解,对其进行了系统总结。在介绍跟踪算法原理的基础上,重点阐述了两大类基于外观模型的目标跟踪方法:产生式方法和判别式方法,深入讨论了其中的典型算法和研究成果,并对这些算法在公开数据集上的测试结果进行了分析比较,最后展望了该领域未来的发展方向。     

Building optimal 2D statistical shape models

Statistical shape models are used widely as a basis for segmenting and interpreting images. A major drawback of the approach is the need, during training, to establish a dense correspondence across a training set of segmented shapes. We show that model construction can be treated as an optimisation problem, automating the process and guaranteeing the effectiveness of the resulting models. This is achieved by optimising an objective function with respect to the correspondence. We use an information theoretic objective function that directly promotes desirable features of the model. This is coupled with an effective method of manipulating correspondence, based on re-parameterising each training shape, to build optimal statistical shape models. The method is evaluated on several training sets of shapes, showing that it constructs better models than alternative approaches.     

A low complexity approximation of probabilistic appearance models

Appearance models yield a compact representation of shape, pose and illumination variations. The probabilistic appearance model, introduced by Moghaddam et al. (Proceedings of the International Conference on Computer Vision, Cambridge, MA, June 1995, p. 687; IEEE Trans. Pattern Anal. Mach. Intell. 19 (7) (1997) 696) has recently shown excellent performances in pattern detection and recognition, outperforming most other linear and non-linear approaches. Unfortunately, the complexity of this model remains high. In this paper, we introduce an efficient approximation of this model, which enables fast implementations in statistical estimation-based schemes. Gains in complexity and cpu time of more than 10 have been obtained, without any loss in the quality of the results.     

Measurement error in statistical models of shape

Active shape models (ASMs) are popular and sophisticated methods of extracting features in (especially medical) images. Here we analyse the error in placing ASM points on the boundary of the feature. By using replications, a corrected covariance matrix is presented that should reduce the effects of placement error. We show analytically and via simulations that the cumulative variability for a given number of eigenvalues retained in principal components analysis (PCA) ought to be reduced by increasing levels of point-placement error. Results for predicted errors are in excellent agreement with the set-up parameters of two simulated shapes and with anecdotal evidence from the trained experts for real data taken from the OSTEODENT project. We derive an equation for the reliability of placing the points and we find values of 0.79 and 0.85 (where 0 = bad and 1 = good) for the two clinical experts for the OSTEODENT data. These analyses help us to understand the sources and effects of measurement error in shape models.     

Active blobs: region-based, deformable appearance models

A region-based approach to nonrigid motion tracking is described. Shape is defined in terms of a deformable triangular mesh that captures object shape plus a color texture map that captures object appearance. Photometric variations are also modeled. Nonrigid shape registration and motion tracking are achieved by posing the problem as an energy-based, robust minimization procedure. The approach provides robustness to occlusions, wrinkles, shadows, and specular highlights. The formulation is tailored to take advantage of texture mapping hardware available in many workstations, PCs, and game consoles. This enables nonrigid tracking at speeds approaching video rate.     

Techniques for evaluating fault prediction models

Many statistical techniques have been proposed to predict fault-proneness of program modules in software engineering. Choosing the “best” candidate among many available models involves performance assessment and detailed comparison, but these comparisons are not simple due to the applicability of varying performance measures. Classifying a software module as fault-prone implies the application of some verification activities, thus adding to the development cost. Misclassifying a module as fault free carries the risk of system failure, also associated with cost implications. Methodologies for precise evaluation of fault prediction models should be at the core of empirical software engineering research, but have attracted sporadic attention. In this paper, we overview model evaluation techniques. In addition to many techniques that have been used in software engineering studies before, we introduce and discuss the merits of cost curves. Using the data from a public repository, our study demonstrates the strengths and weaknesses of performance evaluation techniques and points to a conclusion that the selection of the “best” model cannot be made without considering project cost characteristics, which are specific in each development environment.

基于改进主动表观模型算法的人脸特征定位

人脸特征点的精确定位一直是人脸图像处理的重要研究内容,特征点定位精确与否直接影响后续工作结果的好坏。在基于反向组合AAM（Active Appearance Models）人脸特征点定位算法的基础上,提出结合特征点局部纹理模型来对AAM初始形状参数做最优化以及对AAM匹配模板升级的改进。改进的算法采用特征点局部纹理模型和AAM全局纹理模型结合的方法来最优化AAM初始形状参数,并在此前提下对AAM匹配模板进行升级,使其更接近待匹配图像的信息。在精确的匹配模板和优化的初始形状参数下,匹配的最终精度会得到提升。实验和理论证明,改进后的算法比传统反向组合AAM算法以及现有改进的PAAM（Progressive AAM）算法以及简单的结合ASM和AAM的改进算法都有更好的特征点定位精度。