In hierarchical coding, image data are encoded in such a way that it is possible to access a given image at different quality levels or resolutions. For example, in searching an image database, hierarchical coding allows the user to initially access a low-quality version of an image (at a correspondingly low bit rate) in order to determine if the image is the desired one. Additional data can then be transmitted in stages to further refine the image. This type of scheme is termed progressive transmission. As another example, an image database might be used to support a number of output devices, each having a different resolution. A hierarchical coding scheme allows for each device to efficiently access a version of the image with the appropriate resolution. This is an example of a multiuse environment. We now discuss in more detail the concepts and desired characteristics of schemes for progressive transmission and multiuse environments. This is followed by descriptions of various image hierarchies that are useful for such applications.
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