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96 The Technology of Video and Audio Streaming same way as the earlier H.263 and MPEG-1 and -2 encoders. There are simpler ways of extracting objects that are suited to rich media presentations. The speaker could be shot on a blue-scene set, then extracted as an object. Figure 5.8 shows a typical application of objects for interactive content. The navigation elements and live video are combined over a background layer. Each object can be used as a hot-spot or link to other objects, giving the interactivity we are familiar with from the web page or DVD. The arrangement of the objects within a scene is described by the scene descriptor, the Binary Format for Scenes (BIFS). Both natural video (pixelbased) and synthetic two- and three-dimensional objects can be encoded. The player stacks the object layers together to render a two-dimensional representation of the scene. Shape coding A video object can be coded as an arbitrary shape. In the example the foreground flag could be coded as a separate video object. The shape of the object is defined by an alpha channel. To reproduce a complex edge structure the shape can be coded as a grayscale, giving an 8-bit resolution. A simple shape can be represented as a binary object (present or absent). Texture coding The texture of a video object is conventionally coded by the DCT. The codec is similar to H.263, MPEG-1, and MPEG-2. The texture of static objects can be coded using the wavelet transform. Motion coding The motion coding adds a global motion prediction. If an object is moving as a whole (like a camera pan), rather than code the motion vectors for each macroblock, the motion parameters of the entire object are transmitted. This can be very compact data. Video encoder Just like MPEG-1 and MPEG-2, the standard for MPEG-4 defines a standard player or terminal. The details of the encoder are left to the vendor, as long as the encoded bit stream can be played back on an MPEG-4 terminal. A typical MPEG-4 encoder is shown in Figure 5.9. The texture and shape coding are new components. The basic encoder architecture is similar to MPEG-1 and MPEG-2 encoders. The enhancements to the coding do offer a small increase in efficiency over MPEG-2, typically 10 15 percent for typical material with the advanced simple profile. The AVC gives much better efficiencies, up to twice MPEG-2, albeit with a much higher complexity for the encoder and decoder. MPEG-4 has a number of different profiles for natural video content. The most popular are:

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