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The recent audio-visual standard MPEG4 emphasizes content- based information representation and coding. Rather than operating at the level of pixels, MPEG4 operates at a higher level of abstraction, capturing the information based on the content of a video sequence. Video object plane (VOP) extraction is an important step in defining the content of any video sequence, except in the case of authored applications which involve creation of video sequences using synthetic objects and graphics. The generation of VOPs from a video sequence involves segmenting the objects from every frame of the video sequence. The problem of object segmentation is also being addressed by the Computer Vision community. The major problem faced by the researchers is to define object boundaries such that they are semantically meaningful. Finding a single robust solution for this problem that can work for all kinds of video sequences still remains to be a challenging task. The object segmentation problem can be simplified by imposing constraints on the video sequences. These constraints largely depend on the type of application where the segmentation technique will be used. The purpose of this paper is twofold. In the first section, we summarize the state-of- the-art research in this topic and analyze the various VOP generation and object segmentation methods that have been presented in the recent literature. In the next section, we focus on the different types of video sequences, the important cues that can be employed for efficient object segmentation, the different object segmentation techniques and the types of techniques that are well suited for each type of application. A detailed analysis of these approaches from the perspective of accuracy of the object boundaries, robustness towards different kinds of video sequences, ability to track the objects through the video sequences, and complexity involved in implementing these approaches along with other limitations will be discussed. In the final section, we concentrate on the specific problems that require special attention and discuss the scope and direction for further research.
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