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A Combined Optical Flow and Graph Cut Approach for Foreground Extraction in Videoconference Applications

Mihai Făgădar-Cosma, Marwen Nouri, Vladimir-Ioan Creţu, Mihai Victor Micea
Alcatel-Lucent Bell N.V.
Copernicuslaan 50, 2018 Antwerp, Belgium
“Politehnica” University of Timişoara, Department of Computer Science
Bd. Vasile Pârvan 2, 300223 Timişoara, Romania

Alcatel-Lucent Bell Labs, Centre de Villarceaux
Route de Villejust, 91620 Nozay, France

“Politehnica” University of Timişoara, Department of Computer Science
Bd. Vasile Pârvan 2, 300223 Timişoara, Romania

“Politehnica” University of Timişoara, Department of Computer Science
Bd. Vasile Pârvan 2, 300223 Timişoara, Romania

Abstract: Immersive videoconferences have added a new dimension to remote collaboration by bringing participants together in a common virtual space. To achieve this, the conferencing system must extract in real-time the foreground from each incoming video stream and translate it into the shared virtual space. The method presented in this paper differentiates itself in the sense that no prior training or assumptions on the video content are used during foreground extraction. A temporally coherent mask is created based on motion cues obtained from the video stream and is used to provide a set of hard constraints. Based on these constraints, a graph cut algorithm is employed to produce the pixel-accurate foreground segmentation. The obtained results are evaluated using a state-of-the-art perceptual metric to provide an objective assessment of the method accuracy and reliability. Furthermore, the presented approach makes use of parallel execution in order to achieve real-time processing capabilities.

Keywords: Foreground extraction, videoconference, optical flow, graph cut, motion segmentation, real-time video processing.

>>Full text
Mihai FĂGĂDAR-COSMA, Marwen NOURI, Vladimir-Ioan CREŢU, Mihai Victor MICEA, A Combined Optical Flow and Graph Cut Approach for Foreground Extraction in Videoconference Applications, Studies in Informatics and Control, ISSN 1220-1766, vol. 21 (4), pp. 413-422, 2012.

1. Introduction and Related Work

In the last decade videoconferencing has gained a lot of momentum, supported by the introduction of fixed and mobile broadband Internet and the availability of affordable and easy to use video capture hardware. Having achieved the desiderate of real-time audio, video and document sharing, the next step in videoconferencing is to deliver an immersive experience by gathering participants into a common virtual space that further enhances collaboration options [1]. At the root of this concept rests the ability of the conferencing system to accurately extract foreground information from each incoming video stream and use it to populate the virtual space which is shared with all participants. The perceived quality of foreground segmentation represents a key aspect for achieving a true immersive experience, further accentuated by the fact that foreground segmentation is in itself an ill-posed problem [2]. The implementation of an immersive videoconferencing system must therefore rely on a real-time, automatic foreground extraction algorithm, capable of handling monocular video sequences that may exhibit illumination changes, multimodal and cluttered backgrounds, camera noise and video compression artifacts.

Foreground / background segmentation has been an active research area of video sequence processing, with many algorithms and methods being developed [3-6]. The most accurate results are obtained by methods that rely on dedicated setups involving stereoscopic [7] or multiple cameras [8, 9]. While highly robust, these methods are not feasible for common videoconferencing scenarios which use off-the-shelf or integrated monocular webcams.

In the field of monocular object segmentation, the majority of encountered algorithms rely on background subtraction [4] based on an empty image of the scene provided during the initialization stage. In [10] the foreground layer is extracted by combining background subtraction with color and contrast cues. The key concept revolves around background contrast attenuation, which reduces contrast in the background layer while preserving it around object boundaries. The method proposed in [11] uses a known and stationary background image and a frontal human body detector in order to perform an initial segmentation of the person in the scene. The result is subject to a coarse to fine segmentation process [12] that relies on a GMM model of foreground and background pixels to provide the input for an unsupervised graph cut segmentation [13, 14]. In addition, a self-adaptive initialization level sets scheme is applied in order to find the most salient edges along the person’s contour. The major drawback of these otherwise accurate methods is the requirement for an initial clean background image. This cannot be satisfied in videoconference scenarios, since people are usually in the scene starting from the first frame and the number of potential backgrounds is virtually infinite.

Another approach is to replace the need for an initial background image with a learning model trained using manually labeled video sequences. Criminisi et al. [15] have adapted stereoscopic approaches to monocular video by using a probabilistic framework to fuse motion, color and contrast cues with spatio-temporal (S-T) priors generated during training phase. The accuracy of this method is similar to the one in [7], except for cases when foreground color distribution resembles the one in the background or when there is insufficient motion. Further improvements described in [16] have replaced the Hidden Markov Model with tree-based classifiers trained on ground-truth segmentations that imitate depth masks used in stereoscopic vision. The classifiers operate on motion information encoded in the form of motons (motion descriptors similar to textons, which encode texture information). This method allows a better segmentation of the foreground which is closest to the camera, being able to discard background motion. Despite their relatively high accuracy, both methods can be prohibitive due to the need to calibrate the learned priors for different types of scenes using manually labeled sequences.

A third way of addressing the foreground segmentation problem takes the form of constraints placed on the nature and position of foreground objects. Kim et al. [17, 18] propose an algorithm which targets the part of the MPEG-4 standard related to object-based video compression and handling. The algorithm performs S-T motion segmentation by combining a low-complexity spatial technique with a marker extraction and update process followed by a region growing phase. The low complexity and relative accuracy of the approach makes it suitable for use in mobile devices, but the a priori assumption that the foreground object is placed in the center of the frame limits the number of applicable scenarios. For videoconferencing, segmentation needs to take into account extra movements, other than only those related to head and torso. For example, the system must handle cases in which a person uses hand gestures and body language in order to support the presentation of a topic or to show an exhibit.

The foreground extraction method proposed in the present paper eliminates the need for initial training as well as any a priori assumptions or knowledge related to the nature of the observed scene. Starting from accurate motion cues obtained through aggregation of dense and sparse optical flow information [19], the system builds a temporally coherent mask (TCM) of foreground detected through motion. The temporal coherence of the mask in absence of motion is achieved through the use of image statistics, similar to other methods encountered in the state-of-the-art [3, 18].

To obtain the final pixel-accurate segmentation, a heuristic approach combines the TCM and sparse optic flow information in order to generate the hard foreground and background constraints for a graph-cut algorithm. The accuracy and reliability of the obtained results are evaluated using the state-of-the-art perceptual objective metric described in [20]. The proposed approach supports parallelization, enabling it to achieve real-time execution capabilities.


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