Mahmoud Mejdoub

Classification improvement of local feature vectors over the KNN algorithm

The KNN classification algorithm is particularly suited to be used when classifying images described by local features. In this paper, we propose a novel image classification approach, based on local descriptors and the KNN algorithm. The proposed scheme is based on a hierarchical categorization tree that uses both supervised and unsupervised classification techniques. The unsupervised one is based on a hierarchical lattice vector quantization algorithm, while the supervised one is based on both feature vectors labelling and supervised feature selection method. The proposed tree improves the effectiveness of local feature vector classification and outperforms the exact KNN algorithm in terms of categorization accuracy.

Graph-based approach for human action recognition using spatio-temporal features

Due to the exponential growth of the video data stored and uploaded in the Internet websites especially YouTube, an effective analysis of video actions has become very necessary. In this paper, we tackle the challenging problem of human action recognition in realistic video sequences. The proposed system combines the efficiency of the Bag-of-visual-Words strategy and the power of graphs for structural representation of features. It is built upon the commonly used Space-Time Interest Points (STIP) local features followed by a graph-based video representation which models the spatio-temporal relations among these features. The experiments are realized on two challenging datasets: Hollywood2 and UCF YouTube Action. The experimental results show the effectiveness of the proposed method.

Human Action Recognition Using Temporal Segmentation and Accordion Representation

In this paper, we propose a novel motion descriptor Seg-SIFT-ACC for human action recognition. The proposed descriptor is based both on the accordion representation of the video and its temporal segmentation into elementary motion segments. The accordion representation aims to put in space adjacency the columns of the video frames having a high temporal correlation. For complex videos containing many different elementary actions, the accordion representation may put in spatial adjacency temporally correlated pixels that belong to different elementary actions. To surmount this problem, we divide the video into elementary motions segments and we apply the accordion representation on each one separately.

Spatio-temporal pyramidal accordion representation for human action recognition

We propose in this paper a spatio-temporal pyramid representation (STPR) of the video based Accordion image. The Accordion image allows the pixels having a high temporal correlation to be put in space adjacency. The STPR introduces spatial and temporal layout information to the local SIFT features computed on the Accordion image. It consists in applying firstly, a temporal pyramid decomposition on the video to divide it into a sequence of increasingly finer temporal blocks and secondly in performing a spatial pyramid representation on the Accordion images relative to the temporal blocks. The Multiple Kernel Learning approach is used to combine the multi-histograms coming from different Spatio-Temporal Pyramid levels. Experiments using the human action recognition datasets (Hollywood2 and Olympic sports) show the effectiveness of the proposed approach.

Histogram of dense subgraphs for image representation

Modelling spatial information of local features is known to improve performance in image categorisation. Compared with simple pairwise features and visual phrases, graphs can capture the structural organisation of local features more adequately. Besides, a dense regular grid can guarantee a more reliable representation than the interest points and give better results for image classification. In this study, the authors introduced a bag of dense local graphs approach that combines the performance of bag of visual words expressing the image classification process with the representational power of graphs. The images were represented with dense local graphs built upon dense scale-invariant feature transform descriptors. The graph-based substructure pattern mining algorithm was applied on the local graphs to discover the frequent local subgraphs, producing a bag of subgraphs representation. The results were reported from experiments conducted on four challenging benchmarks. The findings show that the proposed subgraph histogram improves the categorisation accuracy.

Laplacian Tensor sparse coding for image categorization

To generate the visual codebook, a step of quantization process is obligatory. Several works have proved the efficiency of sparse coding in feature quantization process of BoW based image representation. Furthermore, it is an important method which encodes the original signal in a sparse signal space. Yet, this method neglects the relationships among features. To reduce the impact of this issue, we suggest in this paper, a Laplacian Tensor sparse coding method, which will aim to profit from the relationship among the local features. Precisely, we propose to apply the similarity of tensor descriptors to create a Laplacian Tensor similarity matrix, which can better present in the same time the closeness of local features in the data space and the topological relationship among the spatially near local descriptors. Moreover, we integrate statistical analysis applied to the local features assigned to each visual word in the pooling step. Our experimental results prove that our method prevails or exceeds existing background results.

Flickr-based semantic context to refine automatic photo annotation

Automatic photo annotation task aims to describe the semantic content by detecting high level concepts in order to further facilitate concept based video retrieval. Most of existing approaches are based on independent semantic concept detectors without considering the contextual correlation between concepts. This drawback has its impact over the efficiency of such systems. Recently, harnessing contextual information to improve the effectiveness of concepts detection becomes a promising direction in such field. In this paper, we propose a new contextbased annotation refinement process. For this purpose, we define a new semantic measure called “Second Order Co-occurence Flickr context similarity” (SOCFCS) which aims to extract the semantic context correlation between two concepts by exploring Flickr resources (Flickr related-tags). Our measure is an extension of FCS measure by taking into consideration the FCS values of common Flickr related-tags of the two target concepts. Our proposed measure is applied to build a concept network which models the semantic context inter-relationships among concepts. A Random Walk with Restart process is performed over this network to refine the annotation results by exploring the contextual correlation among concepts. Experimental studies are conducted on ImageCLEF 2011 Collection containing 10000 images and 99 concepts. The results demonstrate the effectiveness of our proposed approach.

Extending Laplacian sparse coding by the incorporation of the image spatial context

Diverse studies have shown the efficiency of sparse coding in feature quantization. However, its major drawback is that it neglects the relationships among features. To reach the spatial context, we proposed in this paper, a novel sparse coding method called Extended Laplacian Sparse Coding. Two successive stages are required in this method. In the first stage, the sparse visual phrases based on Laplacian sparse coding are generated from the local regions in order to represent the geometric information in the image space. The second stage aims to incorporate the spatial relationships among local features in the image space into the objective function of the Laplacian sparse coding. It takes into account the similarity among local regions in the Laplacian sparse coding process. The matching between the local regions is based on the Hungarian method as well as the histogram intersection measure between sparse visual phrases already assigned to the local regions in the first stage. Furthermore, we suggested to improve the pooling step that succeeds the encoding step by introducing the discretized max pooling method that estimates the distribution of the responses of each local feature to the dictionary of basis vectors. Our experimental results prove that our method outperforms the existing background results.

Extended Laplacian Sparse Coding for Image Categorization

In image classification task, several recent works show that sparse representation plays a basic role in dictionary learning. However, this approach neglects the spatial relationships in the image space during dictionary learning. However, this approach neglects the neighboring relationship in dictionary learning. To alleviate the impact of this problem, we propose a novel dictionary learning based on Laplacian sparse coding method that profits from the neighboring relationship among the local features. For that purpose, we incorporate the matching between local regions in the Laplacian sparse coding formula. Moreover, we integrate statistical analysis of the distribution of the responses of each local feature to the dictionary basis in the final image representation. Our experimental results prove that our method performs existing background results based on sparse representation.

Bag of frequent subgraphs approach for image classification

The bag of words approach describes an image as a histogram of visual words. Therefore, the structural relation between words is lost. Since graphs are well adapted to represent these structural relations, we propose, in this paper, an image classification framework which draws benefit from the efficiency of the graph in modeling structural information and the good classification performances given by the bag of words method. For each image in the dataset, a graph is created by modeling the spatial relations between dense local patches. Thus, we obtain a graph dataset. From the graph dataset, we select the most frequent subgraphs to construct the bag of subgraphs (BoSG) and we associate to each image a subgraph histogram that describes its visual content. For experiments, we have used the two challenging datasets: 15 Scenes and Pascal VOC 2007. Experimental results show that the proposed method outperforms the bag of words and the spatial pyramid models in terms of recognition rate.