Each scene consists of a single background drawing,
Each scene consists of a single background drawing, with a number of characters and foreground objects. Each is drawn separately on a clear sheet of acetate called a cel. Then each frame of a scene is photographed, with the cels stacked in the appropriate order. The skier cel can be moved for each exposure, or more likely a sequence of cels will represent the movement of the skier. So a great deal of drawing effort has been saved. Only one background has been drawn, and only one flag for the foreground. The savings by drawing each frame in its entirety is immense. This deconstruction of a video sequence into different components or objects is the basis of MPEG-4 video coding, which is the big difference between MPEG-4 and earlier standards. Rather than the restrictions of coding a single two-dimensional rectangular video image, MPEG-4 allows both twodimensional and three-dimensional objects to be mixed in a synchronized presentation. MPEG-4 breaks away from the cinematic representation and moves toward the virtual reality world of video games. This paradigm shift leads MPEG- 4 to be a natural vehicle for rich media presentations. Immersive and interactive presentations can combine synthetic three-dimensional objects with two-dimensional stills and conventional video images. An object has shape, texture, and motion. The object texture is equivalent to the information that was intraframe encoded in blocks by the earlier MPEG standards by the discrete cosine transform. Consider a downhill racer again. The skier is moving, but the background (called a sprite) is essentially a still image. If the skier could be separated from the background, the background could be sent once, and the skier alone could be transmitted as a moving image. This potentially would save a large amount of data. A sprite is defined as a large video object, possibly panoramic, and it is persistent over time. The media player can crop and spatially transform (warp) the sprite; for example, as a camera pans around the foreground objects. Rendering a number of video objects together in the player is relatively easy. Separating video objects from a two-dimensional scene within the encoder is nontrivial. This process is called video segmentation and is a subject of ongoing research. The concept is a natural process for the human brain; we look at a scene and immediately decompose it into a number of objects. So, although the idea of object coding may seem foreign to a video engineer, to the neural physiologist it must seem like the obvious way to encode visual images. The scanned raster with which we are familiar is an engineering convenience. In practice, object coding is not used for rectangular video scenes. Objects are used to combine video audio and graphics objects in interactive content, much like the DVD. A rectangular video image is encoded in much the Video compression 93
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