Pedestrian Detection: A Survey of Methodologies, Techniques and Current Advancements

Transcription

1 31 Pedestrian Detection: A Survey of Methodologies, Techniques and Current Advancements Tanmay Bhadra 1, Joydeep Sonar 2, Arup Sarmah 3,Chandan Jyoti Kumar 4 Dept. of CSE & IT, School of Technology Assam Don Bosco University Abstract Detecting pedestrian in an image is a challenging task in the field of object detection. With the increase in the number of pedestrian fatalities in roads the significance of pedestrian detection is also increasing. In this paper we do a brief study on some of the existing pedestrian detection systems and also discuss in detail some of the benchmark data sets which are currently used in the field of pedestrian detection. Most data sets are also made publicly available so that it can also be used by other researchers in their survey/research. We also take a look at some of the state of the art techniques used by many pedestrian detection system and give a brief description on them. Furthermore we also demonstrate a typical working structure of pedestrian detection system. Hence we concluded that to be able to detect and track people plays a key role in the area of research, and machine vision plays a crucial role in this task. I. INTRODUCTION Pedestrian detection is a canonical instance of object detection. It has various applications such as car safety, surveillance, robotics etc. which enabled it to acquire some much needed attention in the previous years. On the contrary pedestrian detection remains to be a challenging task in the field of object detection. The detection of pedestrian is becoming more significant as the number of pedestrians fatalities are increasing day after day (more than pedestrians are killed and injured in traffic around world every year)[1]. One of the main concerns of car manufacturers is to have an automated system that is able to detect pedestrians in the surroundings of a vehicle. To be able to effectively detect pedestrians based on vision is challenging for number of reasons. Few such challenges are pedestrians appear in different backgrounds with a wide variety of appearances and also different body sizes, poses, clothing and outdoor lighting conditions. Distance of the pedestrian from the camera also plays a vital role as standing relatively far away from the camera may make them appear small in the image [2]. Most pedestrian detectors can achieve satisfactory performance on high resolution datasets, however they encounter difficulties in low resolution images [3] [4] II. DATASETS Despite having various benchmark datasets, Bastian Leibe et al. has used 44 recorded sequence of 33 different people walking parallel to the camera image plane for their training dataset [5]. Figure 1 shows a sample data from the training dataset used by Bastian Leibe et al. Figure 1:Training Dataset Over the years, there have been many public pedestrian datasets. INRIA [6], ETH [7], TUD-Brussels [8], Daimler [9] (Daimler stereo [10]), Caltech-USA [11], and KITTI [12] are the most commonly used ones. INRIA is one of the oldest dataset and hence has comparatively lesser number of images. However the dataset has high quality of annotation of pedestrians in various settings and as such it is widely used for training. ETH and TUD-Brussels fall under the category of midsized video datasets. Daimler lacks color channels and as such is not considered by all methods. Whereas Daimler stereo, ETH and KITTI provide stereo information. Except INRIA all other datasets are obtained from video, and hence enable the use of optical flow. Caltech-USA and KITTI are currently one of the most predominant benchmarking for a large number of methods they have been evaluated, whereas KITTI is often used because of the diversity of its test set. KITTI is not yet frequently used [12,13]. INRIA, ETH (monocular), TUD-Brussel, Daimler (monocular) and Caltech-USA are available under a unified evaluation toolbar whereas KITTI uses its own separate one with unpublished test data. Both toolboxes maintain an online ranking where published methods can be compared.

2 32 III. BLOCK DIAGRAM Figure 2: Flow Chart of a Typical Pedestrian Detection Training: In machine learning one of the most important mechanisms is to train our algorithm on our training set. The training set must be different and distinct from the test set. If we use the same data set for both training and testing, the resulting model may not be able to detect unseen data. Hence it is important to separate data into training and test set. Once a model has been created using the training set, we can test the model with the help of the test set. In the training part the main motive is to extract features of the object so that we can feed the extracted features to the classifier. After normalization the data in the training set we can extract features like Haar-like features or Histogram of oriented Gradient (HOG) features. Certain algorithms like AdaBoost uses a number of training samples to help select appropriate features from the dataset. AdaBoost is able to combine classifiers with poor performance into a bigger classifier with much higher performance [14]. Once training the system is done and we have a classifier, we can feed the test set to the classifier to check the efficiency of our algorithm. Once the image to be processed is loaded the system can abstract the subimages or the Region of Interest (ROI) from the image and load it onto the classifier. Based on the features extracted in the training phase the classifier will be able to classify whether in a particular image a pedestrian is present or not. IV. SURVEY ON EXISTING SYSTEM Bastian Leibe et. al. [15]in their paper have evaluated descriptors, shape context descriptors and local chamfer descriptor and also evaluated four different interest point detectors for pedestrian detection. These results were then compared to the standard global chamfer matching approach. In this paper they try to show that shape context trained on real edge images outperforms those trained on clear pedestrian silhouettes. This paper first makes a comparison between global chamfer matching and local chamfer matching. The outcome of the above comparison clearly indicated that local chamfer matching outperformed the former technique. A second comparison between local chamfer matching and local shape context was carried out. In this comparison local chamfer matching is again found to be a better technique when silhouette information is used alone. However when real edge images were used for training local shape context outperforms all other tested approaches. Global chamfer matching is a very popular detection approach based on global features. It matches object shape silhouettes of the images to image structures. In order to achieve this a silhouette is first shifted over the image. Then a distance between a silhouette T and the edge image at each location l is calculated (D chamfer (T,l)). This distance is based on a distance transform DT. The distance transform DT computes the distance of one pixel to the nearest feature pixel: D chamfer (T,l) = 1 T DT(t + l). This feature stands out mainly because the resulting similarity measure is smoother[21] and hence speeds up the matching process of pedestrian detector by employing a hierarchical search. Another Local Approach they described was the Implicit Shape Model(ISM), which is trained by extracting local features from training images and then making a model from their spatial occurrence on the object. From each training image an interest point detector D is applied, and local features F are calculated around the extracted points. These local features are clustered and form a visual vocabulary of typical local features. Then the spatial occurrence distributions on the training data are recorded for each typical features. This step is known as Model Training. After this step comes Hypothesis Generation in which codebook entries are matched to one of the extracted features and votes are cast for possible object locations according to the occurrence distribution learned in the training phase. Then a segmentation mask can be inferred for each hypothesis. This is done by projecting supporting features of a hypothesis back to the image and using the stored segmentation masks in order to get the local features. Finally, they applied a Minimum Description Length (MDL) based verification step for disambiguating overlapping hypothesis. In order to extract feature points, they used interest point detector within the ISM approach. In this paper they evaluated the use of four different interest point detectors: Harris[16,17], Difference-of-Gaussian[18], Harris- Laplace[19], and Hessian-Laplace[20]. Also, in this tεt

3 33 paper, two shape-based local feature descriptors were compared and were applied within the ISM framework. (a) (b) (c) (c) Figure 3: Interest points (in yellow) on an example image: (b) Harris (c) DoG (d) Harris-Laplace (e)hessian- Laplace Local chamfer descriptor and shape context descriptors are generally trained on silhouette images. They can, however, be applied to real edge images of which the silhouette images are an idealized approximation. Both the descriptors were trained not only on silhouette images but also on real edge images to make the descriptors robust and realistic. Foreground as well as background structures influence the result of features extracted when learning shape-based features on real edge images rather than on silhouette images. Finally from their experimental results they concluded that shape context descriptor trained on real edge images performed best. Compared to raw image patches and Local Chamfer, it was able to achieve a 20% gain in EER performance. The different interest point detectors also had a big impact on the performance of detection. It was concluded that the Hessian-Laplace detector outperformed the other detectors. Hence, using shape Context descriptors trained on real edge images and the Hessian-Laplace detector represents a good combination for pedestrian detection. The shape context descriptor also speeds up the computation process because of its relatively low dimensionality. E. Naranjo et al. in his study provided us a brief idea about combination of feature extraction methods for vision based pedestrian detection. In such cases there are two basic components-first location of pedestrian in the image, second combining it with a SVM based classifier[22]. A candidate selection mechanism is applied in order to ease the pedestrian recognition task in Intelligent transport System. This selection of candidates can be implemented in 3D scene or 2D image plane by performing an object segmentation.for 4D scene object segmentation stereo vision is used[24][25]. 2D image (d) object segmentation tackles the problem of candidate selection using a single camera. Candidate selection-candidate selection method plays a crucial role in the global performance of the pedestrian detection system. This method must assure that no miss detection occurs. All real pedestrians must be detected effectively. Moreover the candidates that are described by a bounding box in the image must be detected precisely as possible. As the detection accuracy plays an important role on the performance of the recognition stage. Information from 3-D images are extracted using disparity map techniques [27], also segmentation based on v-disparity[25][26].here they have proposed a candidate selection method which is based on the direct combination of the 3D co-ordinates of the relevant points. Accordingly for candidate segmentation purposes a nondense 3D geometrical representation is created and used. Such kind of representation allows robust object segmentation whenever the number of relevant points in the image is high. Major advantage is outliers can be easily filtered out in the 3D space which makes the method less sensitive to noise. SVM classifier- Support vector machine first proposed in [28][29] is a common approach for pedestrian detection. SVM provides a method to calculate the hyper plane that optimally separates two high-dimensional classes of objects. Other important aspects while constructing a classifier are global classification structures, use of single/multiple cascaded classifiers and the training strategy. The by-components approach mentioned here divides the candidate body into several parts. Each body part is then independently learnt by a specialized classifier in the first learning stage. The body are then integrated by another classifier in a second learning stage which helps in cases like partially occluded pedestrians. Here independent classifiers are used for each body part to make the learning process easy and simple. After a huge number of trials they have proposed 6 different sub regions for each candidate region of interest. 1 st sub region located in the head zone. Arm and leg are covered by 2 nd, 3 rd, 4 th, 5 th sub region. Each classifier produces a theoretical output by - 1(non-pedestrian) and +1(pedestrian). A set of features must be extracted from each sub region and fed to the classifier. Here 7 different feature extraction methods are tested namely-canny filter, Haar wavelets, gradient magnitude and orientation. Coocurrence matrix, Normalized Histograms (HON), Number of Texture Unit (NTU).After applying these features to various images it is found that NTU and Histograms are fit for Head and Arms, HON is fit for Legs and NTU is fit for area between the legs. Finally we can conclude that a comparative study of feature extraction methods for vision-based pedestrian detection was carried out. To reduce variability of

4 34 pedestrians the learning process was simplified by decomposing dividing candidates into 6 local sub-regions which are fed to individual SVM classifiers. Edgar Seemann et al. in his paper considers the problem of pedestrian in crowded real-world scenarios. The core method comprises of local and global cues via probabilistic top-down segmentation. The method consists of a series of iterative evidence aggregation steps. The main objective is to detect a pedestrian s presence (it may be occluded) in an image, localize them in the image. Here still gray images are considered. The first step is to sample local features from the image and combine them to generate hypothesis about possible object locations. Then for every hypothesis a probabilistic top-down segmentation is computed to determine its area of support in the image. The evaluation criteria not only consists of yes/no detection decision but also precise locations and extents of the pedestrians.[30] Training approach-first a codebook is learned of local appearances which are characteristics for the object category. In order to achieve it a scale-invariant DoG interest point operator [31] is applied to all training images and by extracting image patches with a radius of 3σ of the detected scale. Grouping of the extracted patches is done using an agglomerative algorithm, prior to which all these patches are rescaled to a fixed size. While recognition the patch extraction process is used and the local information is collected in a probabilistic Hough voting procedure from sample patches. Each patch is matched to the codebook and those entries matches that are found cast votes for possible objects position. The possibility to generate top-down segmentations using learned knowledge about an object category is a recent approach [32,33,34]. Here this approach is used to improve recognition. For each hypothesis, they trace back the image to determine a per-pixel level where its support came from, thus segmenting the object from the background. We have observed that a central topic of the paper consists of aggregation of evidences from the image in multiple iterations. The first step is this direction is to use the top-down segmentation to refine object hypothesis. The main idea behind this step is the integration of information only about the object and discards misleading information from the background. Navneet Dalal et al. experimentally show that grids of Histogram of Oriented Gradient (HOG) descriptors significantly outperform other feature sets that are used to detect pedestrians. They further study the influence on performance in each stage. Finally they concluded that fine-scale gradients, fine orientation binning relatively coarse spatial binning, and high-quality local contrast normalization are all important for better results. This approach gives a near-perfect result. And so they have used a more challenging dataset containing over 1800 annotated human images with a range of pose variations and backgrounds. [35] Locally normalized Histogram of Oriented Gradient (HOG) descriptors have better performance than those feature sets including wavelets [36, 37]. These descriptors are oblivious of edge orientation histograms [38, 39], and shape context[40], but they are able to improve their performance by using overlapping local contrast normalizations. They have used linear SVM classifier throughout for simplicity and speed. These detectors gave perfect results on the MIT pedestrian test set [41,36]. As for the feature extraction methods it is based on evaluating well-normalized local histograms of image gradient orientations in a dense grid. Local object appearance can be characterized well by the distribution of local intensity gradients. The combination of histogram entries forms the representation. It is also necessary to contrast-normalize the local responses before using them. Oriented histograms have had many predecessors [42,38,39], but it reached its peak when it was combined with local spatial histogram and normalization. The shape context work [40] made representation effective using only edge pixel counts without the orientation histogram. These sparse feature based representation minimized the power and simplicity of HOG's as dense image descriptors. The HOG/SIFT representation has various advantages. It is able to capture edge or gradient structure which is important for local shape, and it is done in representation with an easily controllable degree of invariance to local geometric transformation. But for human detection coarse spatial sampling, fine orientation sampling and strong local photometric normalization turns out to be the best strategy. May be because it permits limbs and body segments to change appearance and move from side to side a lot provided they are upright. They tested their detector on two different datasets. The first one is an MIT pedestrian database[29], which contains 509 training and 20 test images of pedestrians in a city. It has mostly front or back views with a few limited in different poses. The second dataset was produced by them which was a significantly more challenging dataset, 'INRIA'. This set contained x128 images of humans acquired after cropping from personal photos. Most people are usually standing, but appear in any orientation with huge variety of background image. Dariu M. Gavrila and M. Enzweiler in their paper provides us withan overview of the current state of the art systems in pedestrian detection, which is a rapidly evolving field.[9] They covered the main aspects of a typical pedestrian detection system and also a brief experimental study on certain systems such as the Haar wavelet-cased AdaBoost cascade [23], HOG/linSVM

5 35 [35] and a few others. They have used a large data set of approximately 8.5 GB size containing more than 20,000 images. The data set was also made public for benchmarking. After performing the experiments on the data set they could come to a conclusion that HOG/linSVM had had a clear advantage when used with images with higher resolution and also had lower processing speeds whereas Haar wavelet-based AdaBoost cascade approach had an advantage with images at lower resolution and showed (near) real-time processing speeds. They decomposed the task of detecting pedestrians into three types: ROI selection, classification and tracking or temporal integration. V. CONCLUSION Detection of pedestrian based on vision is still an open challenge. To be able to detect pedestrians in different backgrounds, some being in motion and some stationary while some change directions unpredictably. Different approaches have been developed to try to address the above mentioned and other such complexities. Although pedestrian traffic fatalities remain to be a concerning area, car manufacturers are working hard to protect the safety of the car and pedestrian by the use of pedestrian detection systems which alerts drivers when any pedestrian is detected in front of the car. Various cars like Volvo S60, Mercedes S65AMG, Audi A8L have already started the use of pedestrian detection and also in other high-end cars. Although to purchase these systems may sting a little upfront, but they are worth every single penny, when it comes to saving a pedestrian's life. REFERENCES [1] D. Gavrila, Sensor-based pedestrian protection, in IEEE Intelligent Systems, vol. 16, no. 6, pp ,November [2] D. Gavrila, J. Giebel and S. Munder, Vision-based pedestrian detection: the PROTECTOR system, in Proc. IEEE Intelligent Vehicles Symposium, pp , June [3] P. Doll ar, C. Wojek, B. Schiele, and P. Perona. Pedestrian detection: An evaluation of the state of the art. TPAMI, [4] D. Hoiem, Y. Chodpathumwan, and Q. Dai. Diagnosing error in object detectors. ECCV, [5] Pedestrian Detection in Crowded Scenes,Bastian Leibe, Edgar Seemann, and Bernt Schiele,Multimodal Interactive Systems, TU Darmstadt, Germany [6] Dalal, N., Triggs, B.: Histograms of oriented gradients for human detection. In: CVPR. (2005) [7] Ess, A., Leibe, B., Schindler, K., Van Gool, L.: A mobile vision system for robust multi-person tracking. In: CVPR, IEEE Press (June 2008) [8] Wojek, C., Walk, S., Schiele, B.: Multi-cue onboard pedestrian detection. In: CVPR. (2009) [9] Enzweiler, M., Gavrila, D.M.: Monocular pedestrian detection: Survey and experiments.pami (2009) [10] Keller, C., Fernandez, D., Gavrila, D.: Dense stereobased roi generation for pedestrian detection. In: DAGM. (2009) [11] Dollar, P., Wojek, C., Schiele, B., Perona, P.: Pedestrian detection: A benchmark. In: CVPR. (2009) [12] Geiger, A., Lenz, P., Urtasun, R.: Are we ready for autonomous driving? the kitti vision benchmark suite. In: Conference on Computer Vision and PatternRecognition (CVPR). (2012) [13] Dollár, P., Wojek, C., Schiele, B., Perona, P.: Pedestrian detection: An evaluation of the state of the art. TPAMI (2011) [14] IN/library/bbbb aspx [15] Edgar Seemann, Bastian Leibe, Krystian Mikolajczyk, Bernt Schiele,"An Evaluation of Local Shape-Based Features for Pedestrian Detection" [16] C. Harris and M. Stephens. A combined corner and edge detector. In Alvey Vision Conference, pages , [17] C. Schmid and R. Mohr. Local grayvalue invariants for image retrieval. PAMI, pages , [18] D. G. Lowe. Distinctive image features from scaleinvariant keypoints. In IJCV, [19] K. Mikolajczyk and C. Schmid. Indexing based on scale invariant interest points. In ICCV, 2001 [20] K. Mikolajczyk and C. Schmid. A performance evaluation of local descriptors. Submitted to PAMI, 2004 [21] D. Gavrila. Pedestrian detection from a moving vehicle. In ECCV, pages Springer, 2000.

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