Abstract:
A system and method for object detection are provided where the system includes a component detection unit for detecting components in an image, a component fusion unit in signal communication with the component detection unit for fusing the components into an object, and a CPU in signal communication with the detection and fusion units for comparing the fused components with a statistical model; and the method includes receiving observation data for a plurality of training images, forming at least one statistical model from the plurality of training images, receiving an input image having a plurality of pixels, detecting a plurality of components in the input image, determining a fusion of the detected components, comparing the fusion with the statistical model, and detecting an object in accordance with the comparison.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application Ser. No. 60/470,578, filed May 14, 2003, and entitled “Component Fusion for Face Detection in the Presence of Heteroscedastic Noise”, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     In typical methods for face detection and/or recognition, it is known that component-based face detection can yield better performance than global approaches, particularly when pose and illumination variations or occlusions are considered. While pose and illumination can significantly change the global face appearance, components are less prone to these changes since the components are smaller than the whole face. The component detectors may accurately locate the face components as well. 
     The component information may be used to register and normalize the face to a “standard” one, which is appropriate for face recognition. Also, component based methods can be used to build a detector that may handle partial occlusions. Component-based methods have also been used in other areas, such as people detection, for example. 
     In one prior example, a component-based face detector with a two-level hierarchy of Support Vector Machine (“SVM”) classifiers is used. The face components are detected independently with the trained SVMs at the first level. At the second level, a single SVM checks if the geometric locations of the components comply with a face. However, only the largest responses from the component detectors are used when checking the validity of the geometry. Unfortunately, SVMs are relatively slow and it would be quite challenging to employ them in real-time systems. 
     Another prior example employs four types of rectangular features, and uses AdaBoosting to automatically build a strong classifier from feature-based weak classifiers. This example then computes the integral image to accelerate the computation of features. This gives a high detection rate and a low false detection rate, while the boosted face detector may work in real-time. 
     Unfortunately, prior fusion methods typically neglect the uncertainties that characterize the component locations, and are generally unsuitable for use in the presence of noise. Accordingly, what is needed is an approach to Component Fusion for Face Detection that is suitable for use in the presence of heteroscedastic noise. 
     SUMMARY 
     These and other drawbacks and disadvantages of the prior art are addressed by a system and method of Component Fusion for Face Detection. 
     The system includes a component detection unit for detecting components in an image, a component fusion unit in signal communication with the component detection unit for fusing the components into an object, and a CPU in signal communication with the detection and fusion units for comparing the fused object with a statistical model. 
     The corresponding method includes steps for receiving observation data for a plurality of training images, forming at least one statistical model from the plurality of training images, receiving an input image having a plurality of pixels, detecting a plurality of components in the input image, determining a fusion of the detected components, comparing the fusion with the statistical model, and detecting an object in accordance with the comparison. 
     These and other aspects, features and advantages of the present disclosure will become apparent from the following description of exemplary embodiments, which is to be read in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure teaches a system and method of Component Fusion for Face Detection in accordance with the following exemplary tables and figures, in which: 
       Table A shows equations 1 through 10; 
       Table B shows equations 11 through 21; 
         FIG. 1  shows a block diagram of a system for Component Fusion for Face Detection according to an illustrative embodiment of the present disclosure; 
         FIG. 2  shows image diagrams for face examples and components for use in accordance with the system of  FIG. 1 ; 
         FIG. 3  shows graphical and image diagrams for observation distributions and corresponding face examples for use in accordance with the system of  FIG. 1 ; and 
         FIG. 4  shows graphical diagrams for evaluation data versus sample number index in accordance with the system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Face detection using components provides results superior to global methods due to its robustness to occlusions, pose and illumination changes. In embodiments of the present disclosure, a first level of processing is devoted to the detection of individual components, while a second level deals with the fusion of the component detectors. Prior fusion methods neglect the uncertainties that characterize the component locations. These uncertainties carry important information that, when exploited, lead to increased face localization accuracy. Preferred embodiments of the present disclosure provide solutions that take geometrical constraints into account. The efficiency and usefulness of these techniques are tested with both synthetic and real world examples. 
     Thus, the present disclosure provides a new framework for component fusion in the context of the face detection task. The fusion relies on modeling the noise as heteroscedastic, and is constrained by a geometric face model. To achieve real-time performance, exemplary embodiments employ AdaBoosting when training component detectors. However, the presently disclosed framework is not limited to such detectors, and alternate embodiments are open to various types of component detectors, such as Support Vector Machines (“SVMs”), for example. 
     As shown in  FIG. 1 , a system for Component Fusion for Face Detection according to an illustrative embodiment of the present disclosure is indicated generally by the reference numeral  100 . The system  100  includes at least one processor or central processing unit (“CPU”)  102  in signal communication with a system bus  104 . A read only memory (“ROM”)  106 , a random access memory (“RAM”)  108 , a display adapter  110 , an I/O adapter  112 , a user interface adapter  114 , a communications adapter  128 , and a video adapter  130  are also in signal communication with the system bus  104 . 
     A display unit  116  is in signal communication with the system bus  104  via the display adapter  110 . A disk storage unit  118 , such as, for example, a magnetic or optical disk storage unit, is in signal communication with the system bus  104  via the I/O adapter  112 . A mouse  120 , a keyboard  122 , and an eye tracking device  124  are in signal communication with the system bus  104  via the user interface adapter  114 . A video imaging device or camera  132  is in signal communication with the system bus  104  via the video adapter  130 . 
     A component detection unit  170  and a component fusion unit  180  are also included in the system  100  and in signal communication with the CPU  102  and the system bus  104 . While the detection unit  170  and the fusion unit  180  are illustrated as coupled to the at least one processor or CPU  102 , these components are preferably embodied in computer program code stored in at least one of the memories  106 ,  108  and  118 , wherein the computer program code is executed by the CPU  102 . 
     As will be recognized by those of ordinary skill in the pertinent art based on the teachings herein, alternate embodiments are possible, such as, for example, embodying some or all of the computer program code in registers located on the processor chip  102 . Given the teachings of the disclosure provided herein, those of ordinary skill in the pertinent art will contemplate various alternate configurations and implementations of the detection unit  170  and the fusion unit  180 , as well as the other elements of the system  100 , while practicing within the scope and spirit of the present disclosure. 
     Turning to  FIG. 2 , face examples and components for use with the system  100  are indicated generally by the reference numeral  200 . The examples include faces  210 ,  212 ,  214 ,  216 ,  218 ,  220 ,  222 ,  224 ,  226 ,  228 ,  230  and  232 , respectively. A face  234  includes potentially overlapping components  236 ,  238  and  240 , representing a right eye component, a left eye component, and a lower face component, respectively. In this exemplary example, the left eye component  236  and the right eye component  238  are each 36 by 28 pixels. The lower face component  240  is 52 by 40 pixels. The face examples in the first row  210 - 216  and the second row  218 - 224  are frontal and turning left faces, respectively, with 4 different illumination settings. The face examples in the third row  226 - 232  show faces with different expressions. 
     It shall be recognized by those of ordinary skill in the pertinent art that the shapes and sizes of the component areas are merely exemplary. Embodiments of the present disclosure will perform well with components of many contiguous shapes and sizes, without undue experimentation. 
     Exemplary Component Detectors use three components for a face. All the faces are aligned to a 64 by 64 pixel image. The detectors then use three rectangles to cut three components, left eye  238 , right eye  236  and lower face  240 , as shown in  FIG. 2 . 
     The exemplary face database has 1862 different faces. The images were taken with 5 poses (frontal, turning left, turning right, tilting up, and tilting down) and 4 illumination conditions (dark overall, lighting from left, lighting from right, and bright overall). There are also some faces with different expressions. The face examples  210 - 232  are from the database. More than 6000 pictures were collected as negative examples for detector training. 
     The AdaBoosting theory states that by adding weak classifiers one can obtain better strong classifiers. However, in practice, this might not be true since the weak classifiers are often correlated. To deal with this issue, preferred embodiments use a modified AdaBoosting method that trains the component detectors such that the trained strong classifier is verified to be empirically better at each boosting step. 
     The Component-Based Face Model has many advantages. Suppose we have a probabilistic face model, where each component position has some uncertainty. With the uncertainties, the face model is flexible to describe a variety of possible faces. Assuming Gaussian distributions, in the face model we have a set of 2D points with means m i , and covariance matrices C i , i=1, 2, . . . , N, where N is the number of components. The face model provides a constraint such that the components should comply with the geometrical configurations. That is, the components should not be too far away, as represented by observation distributions such as those of  FIG. 3 . 
     The face model is trained from known face examples. We know the exact locations of the components in each training face example, so we can estimate the mean and covariance matrix of each component from these locations. 
     After the component detectors are trained, we scan the input image to get the component confidence maps, A i (x), i=1, 2, . . . , N, where x is the location in an image, and N is the number of components. We assume that the confidence map A i (x) is normalized across all of the components. 
     With the face model {m i , C i } i=1,2, . . . ,N , the overall face likelihood is described by Equation (1) of Table A, where {x′ i } are rigidly transformed from {x i } into the face model space, subject to rotation, translation and scaling. Equations (1) through (10) are provided in Table A. 
     Note the simple maxima of individual component detector responses are not necessarily best choices for component locations under face model constraints. Our goal is to find the best component localization {x i } with maximal L. We could do an exhaustive search with all A i (x), but that is generally too computationally expensive. 
     Since the shape of A i (x) is often smooth and Gaussian-like, we use a Gaussian shape to approximate it. In other words, the underlying noise model is assumed to be heteroscedastic, i.e., the noise is both anisotropic and inhomogeneous. We can identify the local maximum as s i =A i (μ i ), where μ i  is the location of maximum and considered the center of the Gaussian shape. A non-parametric method to estimate the “covariance” matrix Q i  in an area B around μ i  is given by Equation (2) of Table A. 
     Then the confidence map can be rewritten as Equation (3). Therefore, Equation (4) follows, where Equations (5) and (6) apply. In order to maximize L one should minimize d 2 . When d 2  is computed for an observation, L or In L can be thresholded to make a detection or rejection decision. 
     Least square fitting is now addressed. For the beginning, let us simplify the problem so that we only have a fixed-point face model {m i } and fixed-point observations {x i }, for example, taking the means of the face model and maxima of the confidence maps. Suppose we find the scaling factor s, the rotation R and translation x 0 , so that an observation point x can be mapped to a point x′ in model space. This is shown by Equation (7), where the rotation matrix R is a function of theta, as shown in Equation (8). 
     Our goal is to minimize the sum of squared error d 2  by choosing the right s, R and x 0  as shown in Equation (9). By taking the partial derivatives of Equation (9) with respect to theta, s and x 0 , and setting them to zeros (denoting m i =(m i , n i ) T  and x i =(x i , y i ) T ), we get the solution defined by Equations (10), (11) and (12), where Equations (11) and (12) are shown in Table B. Equations (11) through (21) are provided in Table B. 
     Using the above solution, we can evaluate Equation (9) to get the least square error. A smaller d 2  suggests a larger similarity between the observation and model geometrical configurations. This simple method does not take the individual component confidences into consideration, or the heteroscedastic model of the noise. 
     Fitting points to a probabilistic model is now addressed. Within this section, assume that we have a probabilistic model of 2D points {m i , C i } i=1,2, . . . ,N . We want to match the observed points x i  to the model. This case has been analyzed, and here is the summary. 
     An observation point x can be mapped to a point x′ in model space as shown in Equation (13), where t=(t x , t y ) T  and the scaling and rotation matrix R is given by Equation (14). 
     Let us denote a=(a, b) T , and the goal is to find the best a and t to minimize the Mahalanobis distance, as shown in Equation (15). 
     Taking the partial derivatives of Equation (15) with respect to a and t, and setting them to zeros, we get the solution shown in Equation (16), where Y i =(x i , Jx i ) and J is defined by Equation (17). 
     In order to match probabilistic observations to a probabilistic model with the model {m i , C i } and observation {μ i , Q i }, i=1, 2, . . . , N, we want to find the best choices of component locations x i , and the associated transformation a and t to minimize the combined Mahalanobis distance d 2  in Equation (6), where x′ i  is a function of x i , a and t according to Equation (13). Unfortunately, it is hard to find the close form solution to this problem, because the partial derivatives are not linear with respect to x i , a and t. 
     We can use two strategies to solve this optimization problem. One employs numerical optimization methods, such as Levenberg-Marquardt or Newton iterative optimization, which require iterations before convergence. 
     The other approximates the solution. Notice in Equation (6) there are two terms. The first term is the Mahalanobis distance in the model space, and the second term is the Mahalanobis distance in the observation space. If we pick μ i  as the solution for x i  (this is the first approximation of the solution, though very rough), and match μ i  to the probabilistic model {m i , C i } i=1,2, . . . ,N , we end up a biased minimization d 2   obs  of Equation (6) where the second term is zero. On the other hand, if we pick m i  as the matched points x′ i  in the model space, and match x′ i  back to the observation {μ i , Q i } i=1,2, . . . , N  (denote that the choices in the observation space are x″ i ), we end up another biased minimization d 2   mod  of Equation (6) where the first term is zero. The real minimization must be a tradeoff between these two biased ones. The second approximation of the solution we choose is then the equal average as defined by Equation (18). 
     Further more, we can refine the equal average to get the third approximation, the weighted average approximation, by using the Mahalanobis distances in weighting the average according to Equation (19). 
     The advantage of the approximations is that they are fast. If the solutions are close to the real minimum, the approximations are more favorable for real-time face detection systems. 
     Turning now to  FIG. 3 , observation distributions and corresponding face examples are indicated generally by the reference numeral  300 . Here, a real-world face  310  includes a right eye component  312 , a left eye component  314 , and a lower face component  316 . The next real-world face  320  includes a right eye component  322 , a left eye component  324 , and a lower face component  326 . The next real-world face  330  includes a right eye component  332 , a left eye component  334 , and a lower face component  336 . The next real-world face  340  includes a right eye component  342 , a left eye component  344 , and a lower face component  346 . The next real-world face  350  includes a right eye component  352 , a left eye component  354 , and a lower face component  356 . Likewise, the next real-world face  360  includes a right eye component  362 , a left eye component  364 , and a lower face component  366 . 
     Location distributions  370  include traces (thicker ellipses) representing the distributions of the model for the right eye component  372 , the left eye component  374 , and the lower face component  376 . The location distributions  370  also include traces (thinker ellipses) for 50 synthesized sets of components including right eye components  382 , left eye components  384 , and lower face components  386 , which were randomly generated. In this experiment, we assume a face model where the centers of the left eye, right eye and lower face components are indicated by Equation (20), and the associated covariance matrices are indicated by Equation (21). 
     We randomly generate observation data by adding noise to both the means and covariance matrices of the components in the face model. A 0-mean Gaussian noise with a standard deviation of 4 pixels is added to both x and y directions of the means, and the covariance matrices are also added with a 0-mean Gaussian noise having a standard deviation of 3. Thus, the face model and observation examples are shown in by the distribution  370 . 
     As shown in  FIG. 4 , evaluation data versus sample number index plots are indicated generally by the reference numeral  400 . The plot  410  shows the d 2  computed with various approximations. Results for observation mean  412 , equal average  414 , weighted average  416 , and Levenberg-Marquardt  418  are included. The observation mean approximation has large errors. The equal average and weighted average approximations are very close to the true d 2  obtained by Levenberg-Marquardt optimization. The plot  450  shows the distance error of the best match for each component in average in the observation space. Results for observation mean  452 , equal average  454 , and weighted average  456  are included. We can see small but noticeable displacement errors for the equal and weighted average methods, compared to the plot  410 . This suggests that the when d 2  is close to the minimum, the d 2  surface is quite flat, which is because of the fact that we have-relatively large covariances in the face model and observation examples. 
     The real world face detection examples  310  through  360  of  FIG. 3  are from a video with different poses. In real world examples with AdaBoosting component detectors, an exemplary embodiment face detection system runs comfortably at a frame rate on a standard laptop with 640 by 480 image resolution. The techniques, as tested with these real world examples, successfully handled pose changes as shown with respect to  FIG. 3 . System embodiments may be applied to other real world data, including standard face databases. 
     As will be understood by those skilled in the pertinent art, the present disclosure has provided a statistical fusion framework for component-based face detection. The framework has been successfully tested with component face detectors trained using AdaBoosting, and running in real-time. The provided systems and methods are effective with both synthetic and real world data. 
     The disclosed technique can be applied to many appearance-based image acquisition problems in addition to surveillance images. Alternate examples may include automatic object detection on assembly lines by machine vision, human face detection in security control, and the like. As shall be recognized by those of ordinary skill in the pertinent art, the term “image” as used herein may also represent three-dimensional, four-dimensional, and higher dimensional datasets in alternate embodiments. 
     These and other features and advantages of the present disclosure may be readily ascertained by one of ordinary skill in the pertinent art based on the teachings herein. It is to be understood that the teachings of the present disclosure may be implemented in various forms of hardware, software, firmware, special purpose processors, or combinations thereof. 
     Most preferably, the teachings of the present disclosure are implemented as a combination of hardware and software. Moreover, the software is preferably implemented as an application program tangibly embodied on a program storage unit. The application program may be uploaded to, and executed by, a machine comprising any suitable architecture. Preferably, the machine is implemented on a computer platform having hardware such as one or more central-processing units (“CPU”), a random access memory (“RAM”), and input/output (“I/O”) interfaces. The computer platform may also include an operating system and microinstruction code. The various processes and functions described herein may be either part of the microinstruction code or part of the application program, or any combination thereof, which may be executed by a CPU. In addition, various other peripheral units may be connected to the computer platform such as an additional data storage unit and a printing unit. 
     It is to be further understood that, because some of the constituent system components and methods depicted in the accompanying drawings are preferably implemented in software, the actual connections between the system components or the process function blocks may differ depending upon the manner in which the present disclosure is programmed. Given the teachings herein, one of ordinary skill in the pertinent art will be able to contemplate these and similar implementations or configurations of the present disclosure. 
     Although the illustrative embodiments have been described herein with reference to the accompanying drawings, it is to be understood that the present disclosure is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one of ordinary skill in the pertinent art without departing from the scope or spirit of the present disclosure. All such changes and modifications are intended to be included within the scope of the present disclosure as set forth in the appended claims.