9.1 Video Object Plane (VOP)

In object-based coding the video frames are defined in terms of layers of video object planes (VOP). Each video object plane is then a video frame of a specific object of interest to be coded, or to be interacted with. Figure 9.1(a) shows a video frame that is made of three VOPs. In this figure, the two objects of interest are the balloon and the aeroplane. They are represented with their video object planes of VOP ₁ and VOP ₂. The remaining part of the video frame is regarded as a background, represented with VOP ₀. For coding applications, the background is coded only once, and the other object planes are encoded through the time. At the receiver the reconstructed background is repeatedly added to the other decoded object planes. Since in each frame the encoder only codes the objects of interest (e.g. VOP ₁ and/or VOP ₂), and usually these objects represent a small portion of the video frame, then the bit rate of the encoded video stream can be extremely low. Note that, had the video frame of Figure 9.1(a) been coded with a conventional codec such as H.263, since clouds in the background move, then the H.263 encoder will inevitably encode most parts of the picture, with a much higher bit rate than that generated from the two objects.

Figure 9.1: (a) A Video Frame Composed of (b) Balloon VOP ₁, (c) Aeroplane VOP ₂