B-VOP - Bidirectional Video Object Plane

By: Christine Martz

Meaning of B-VOP – “Bidirectional Video Object Plane”, is used for motion-compensated prediction processing in the creation of MPEG video streams.

MPEG-4 video streams can be divided into hierarchical layers. The lowest layer is the Video Object Plane (VOP) layer. It corresponds to a single frame of the video stream. VOP is used to represent rectangular-plane frames or arbitrary-shaped object plane.

Video scenes can be composed from several objects which may change in position, appearance, or size, all independent of each other. A past and a future reference Video Object Plane (VOP) is encoded in a way that for each object or shape motion vector, a predictor is built from neighboring motion vectors that were already encoded allowing for bidirectional prediction. B-VOP are bidirectionaly predicted from a past and a future reference Video Object Plane (VOP).

Other Related Definitions:

“…Efficient coding of digital video is achieved in accordance with this invention, by integrating the bidirectional prediction modes of the MPEG-1 and the H.263 standards into a single adaptive scheme, while eliminating the restrictions and limitations imposed in these standards. This results in an efficient yet flexible method for performing the bidirectionally predictive coding of pictures (improved B-pictures) that is capable of efficiently operating with good performance over a wider range of bitrates than that possible by equivalent techniques in the individual MPEG-1 and H.263 standards. The present invention is thus suitable for B-picture coding of the H.263+ standard. Furthermore, the inventive method can be applied to the bidirectionally predictive coding of either rectangular regions or arbitrary shaped objects/regions in video pictures (so-called B-VOPs) for MPEG-4. ” [United States Patent ]

“…A B-VOP is a VOP which is coded bidirectionally. For example, macroblocks in a B-VOP can be predicted using the forward, the backward or both using the forward and backward motion vectors; this has similarities to MPEG-1/2 in which B-pictures can use such motion vectors. However, MPEG-4 video also supports an H.263 based mode for motion compensation, referred to as the direct mode. In direct mode, the motion vector for a macroblock in a B-VOP is obtained by scaling of the P-VOP motion vector, and further correcting it by a small (delta) motion vector. ” [AT&T Laboratories]

“…A VOP may be interpolated from data contained in previous VOPs and VOPs that have yet to be displayed. These would be called Bidirectional Interpolated VOPs or B-VOPs. A B-VOP would only be based on data contained in I-VOPs or P-VOPs and not on other B-VOPs. Motion estimation is necessary for encoding P-VOPs and B-VOPs and works by matching 16 x 16 or 8 x 8 pixel blocks with special attention being given to blocks that lie on the boundary of the VOP.” [David Mellor - Audio Media Magazine]

Related Links:

An Introduction to MPEG Video Compression - MPEG Video Basics
MPEG-4 video coding - Audio & Multimedia MPEG-4 Video Coding.
MPEG and multimedia communications - Overview of the MPEG Standard.

Technical Resources:

Object-Based Rate Control for the MPEG-4 Visual Simple Profile
The MPEG4 Standard: Evolution or Revolution?
Overview of the MPEG-4 Standard
MPEG-4 Requirements
MPEG-4, The Multimedia Standard

Products and Solutions:

Video enabled iPod

Blogs, News, Feeds, Discussion Lists:

MPEG Industry Forum News
Mp4-tech mailing list

Books:

The MPEG-4 Book - by Touradj Ebrahimi, Fernando Pereira
H.264 and MPEG-4 Video Compression: Video Coding for Next Generation Multimedia - by Iain E. G. Richardson
Digital Video Compression - by Peter Symes

See Also:

B-VOP Resources

 

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