In most image transmission applications, the requirement of a short transmission time (and correspondingly low bit rate) often precludes the use of a numerically lossless compression algorithm at the outset. However, in certain applications, it may be necessary to send a lossless version of the image at some later stage of the transmission. Such an application might exist in the medical field, where two physicians are discussing a possible patient referral from remote locations. One of the physicians may wish to transmit a digital radiograph over the phone line, and in the interest of a short transmission time, a lossy (but high quality) version of the image is sent. If the referral is accepted, the remaining difference (residual) image required to perfectly reconstruct the original image could be sent, perhaps at the conclusion of the telephone conversation. In general, a lossy plus lossless residual encoding scheme consists of the following steps:
- Generate a low bit rate image through the use of an efficient lossy compression scheme.
- Form a residual by computing the difference between the lossy reconstruction and the original image.
- Encode the residual using an appropriate lossless technique.
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