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Stereoscopic and Multi-View Video Coding Standards

layer depth disparity defined

Definition: The Multiviezv Profile (MVP) extends the wellknown hybrid coding towards exploitation of interview channel redundancies by implicitly defining disparity compensated prediction.

Stereo video coding is already supported by the MPEG-2 technology, where a corresponding multi-view profile, defined in 1996, is available to transmit two video signals. The main application area of MPEG-2 multiview profile (MVP) is stereoscopic TV. The MVP extends the well-known hybrid coding towards exploitation of inter-view channel redundancies by implicitly defining disparity-compensated prediction. The main new elements are the definition of usage of the temporal scalability (TS) mode for multi-camera sequences, and the definition of acquisition parameters in the MPEG-2 syntax. The TS mode was originally developed to allow the joint encoding of a low frame rate base layer stream and an enhancement layer stream comprised of additional video frames. In the TS mode, temporal prediction of enhancement layer macro-blocks could be performed either from a base layer frame, or from the recently reconstructed enhancement layer frame. To encode stereo or multi-channel signals, frames from one camera view are defined as the base layer, and frames from the other one(s) as enhancement layer(s). Thus both disparity-compensated and motion-compensated prediction can simultaneously be used, while compatibility with monoscopic decoders is achieved, since the base layer represents a monoscopic sequence. However, there are important disadvantages: disparity vectors fields are sparse and thus the disparity-compensation is not efficient so motion-compensation is usually preferred. Furthermore, the technology is outdated and interactive applications that involve view interpolation cannot be supported.

To provide support for interactive applications, enhanced depth and/or disparity information about the scene has to be included in the bit stream, which can also be used for synthesizing virtual views from intermediate viewpoints. Experiments for encoding depth data using different video codecs by putting the depth data into the luminance channel, and simply changing the semantics of its description have been conducted by MPEG and the ATTEST IST project. Results show that this approach makes it possible to achieve extreme compression of depth data while still maintaining a good quality level for both decoded depth and any generated novel views. Furthermore, hidden (occlusion) information can be included in the form of additional MPEG-4 Video Object Planes (VOPs) or, preferably, Layered Depth Images, which are defined in a new part of MPEG-4 called Animation Framework extension (AFX).

Another useful tool for storing such information are the Multiple Auxiliary Components (MAC), defined by MPEG-4, where grayscale shape is generalized to include further information, besides describing the transparency of the video object. MACs are defined for a video object plane (VOP) on a pixel-by-pixel basis, and contain additional data related to the video object, such as disparity, depth, and additional texture. Up to three auxiliary components (including the grayscale or alpha shape) are possible. Only a limited number of types and combinations are defined and identified by a 4-bit integer so far, but more applications are possible by selection of a USER DEFINED type or by definition of new types. All the auxiliary components can be encoded by the shape coding tools and usually have the same shape and resolution as the texture of the video object.

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