During the past two decades, various lossless and lossy image coding techniques have been developed (for a list of references see Ref. ). Typical lossless coders can attain compression ratios of only 2:1 or 3:1 for most images; thus, users often prefer to deal with lossy algorithms that can achieve high compression rates, e.g., 50:1 or more. The problem is that high compression ratios are only possible at the cost of imperfect source representation. Compression is lossy in that the decoded images are not exact copies of the originals, but if the properties of the human visual system are correctly exploited, original and decoded images will be almost indistinguishable. The trade-off between image distortion and coding rate may be stated as follows (Ref. ): How much fidelity in the representation is one willing to give up in order to reduce the storage or the number of bits required to transmit the data?
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