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Face Detection and Tracking
Face Clustering

Face Detection and Tracking

Achieving a good localization of faces on video frames is of high importance for an application such as video indexing. Face localization on movies is an ambiguous task due to various scale, pose and lighting conditions.

Our Method

A novel deterministic approach has been developed that it applies face detection, forward tracking and backward tracking, using some predefined rules. From all the possible extracted candidates, a Dynamic Programming algorithm selects those that minimize a cost function.

Face detection:

Use of the Haar-like features to provide first face candidates. For tracking purposes, a post-processing step is added to reduce the number of false alarms and remove some of the background. Candidates are rejected if the number of pixels fulfilling the criteria below are under a certain threshold.

0 < h < 1 and 0.23 < s < 0.68 and 0.27 < v

The remaining candidates are replaced by the smallest bounding box containing the skin-like pixels. The detection is performed every 5 frames.

Forward tracking process:

  • The Morphological Elastic Graph Matching (MEGM) tracking algorithm is used.
  • The tracking is initialized by the detection output.
  • While the detection is efficient for frontal faces, the tracking provides candidates for other possible poses.

Backward tracking process:

  • If a face is detected in the middle of a shot, no information is available about its localization in the previous video frames.
  • The same tracker (MEGM) is applied backwards.
  • The backward tracker is initialized with the labeled detections.
  • Detected, forward and backward tracked faces are now grouped in actor appearances (labels).


  • The node cost C is the distance between the center of the bounding box and the centroid of the skin-like pixels.
  • The transition cost combines:
    • the overlap between two bounding boxes
    • the ratio of the bounding boxes areas to penalize big changes in the area during tracking

Structure of the trellis

  • The trellis selects the path with the minimum cost. The selection is performed using dynamic programming.

Experimental results



Relevant Publications

I. Cherif, V. Solachidis and I. Pitas, "A Tracking Framework for Accurate Face Localization", in Proc. of Int. Federation for Information Processing Conf. on Artificial Intelligence (IFIP AI 2006), Santiago, Chile, 21-24 August, 2006.

Research Projects

NM2 - “New media for a new millennium” (IST-004124), FP6


© 2006

Face Clustering

Clustering could be considered as a form of unsupervised classification imposed over a finite set of objects. Its goal is to group sets of objects into classes, such that similar objects are placed in the same cluster, while dissimilar objects are placed in different clusters.

Human faces are some of the most important and frequently encountered entities in videos and can be considered as high-level semantic features. Face clustering in videos can be used in many applications such as video indexing and content analysis, as a pre processing step for face recognition, or even as a basic step for extracting the principal cast of a feature length movie and much more.


Our Methods

A) Clustering based on Mutual Information: The capabilities of joint entropy and mutual information are exploited in order to classify face images exported from a Haar detector. We use the intensity images and we define for every image the probability density function as the histogram of the intensities of that image summed to one. In order to calculate the joint entropy between the two images we construct a 2D histogram of 256 bins which take into account the relative positions of intensities so that similarity occurs between two images, when same intensities are located in same spatial locations.

Problem Definition

  • Face clustering is the task where from a set of face images A we create n subsets .
  • We exploit the mutual information between the face images to create a similarity matrix which afterwards will be clustered.
  • The mutual information is shown to be a good measure for similarity between face images where light conditions and poses are variant.
  • The movies' context for such an algorithm gives a new dimension to the problem where no calibrated images are used as input.
  • Purpose of such an algorithm: Define primordial actors, automatic (not manually annotated) database search, registration, content analysis.

Clustering Process:

  • The clustering process is based on the Fuzzy-C Means (FCM) algorithm.
  • We provide the number of classes and the similarity matrix to the algorithm.
  • In order to use this algorithm we define every row of the aforementioned similarity matrix as a different vector in an M-dimensional L2-normed vector space over R.
Darker regions belong to the first actor and clearer ones to the second actor. The video sequence has four consecutive shots in the order FA-FA-SA-SA where FA and SA first and second actor respectively


B) Hierarchical Clustering: An algorithm to cluster face images found in feature length movies and generally in video sequences is proposed. A novel method for creating a dissimilarity matrix using SIFT image features is introduced. This dissimilarity matrix is used as an input in a hierarchical average linkage clustering algorithm, which finally yields the clustering result.

The final result is found to be quite robust to significant scale, pose and illumination variations, encountered in facial images.

Clusters 1, 2 and 4 contained only facial images from the same person. The third cluster contained the false face detections (non-facial images) as we expected, but it also included certain instances of the actor in cluster 1, due to a significant change in the person's pose.



Relevant Publications

Ν. Vretos, V. Solachidis and I. Pitas "A Mutual Information Based Algorithm for Face Clustering", in Proc. of Int. Conf. on Multimedia and Expo (ICME 2006) , Toronto Ontario, Canada, 9-12 July, 2006.

P. Antonopoulos, N. Nikolaidis and I. Pitas, “Hierarchical Face Clustering Using SIFT Image Features”, submitted in Proc. of IEEE Symposium on Computational Intelligence in Image and Signal Processing (CIISP 2007), Honolulu, HI , USA.

Research Projects

NM2 - “New media for a new millennium” (IST-004124), FP6

Pythagoras II - Funded by the Hellenic Ministry of Education in the framework of the program


© 2006