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Appendix E
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The goal of compression, then, is to discard all information in the image that is not absolutely necessary from the standpoint of what the HVS is capable of resolving. Such a system can be described as psychovisually lossless.
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DCT is one of the building blocks of the JPEG standard. All JPEG DCTbased coders start by portioning the input image into nonoverlapping blocks of 8 8 picture elements. The 8-bit samples are then level-shifted so that the values range from 128 to 127. A fast Fourier transform is then applied to shift the elements into the frequency domain. Huffman coding is mandatory in a baseline system; other arithmetic techniques can be used for entropy coding in other JPEG modes. The JPEG specification is independent of color space or gray scale. A color image typically is encoded and decoded in the YUV color space with four pixels of Y for each U, V pair. In the sequential DCT-based mode, processing components are transmitted or stored as they are calculated in one single pass. Figure E.5 provides a simplified block diagram of the coding system.
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Figure E.5 Block diagram of a DCT-based imagecompression system. Note how the 8 8 source image is processed through a forward-DCT (FDCT) encoder and related systems to the inverse-DCT (IDCT) decoder and reconstructed into an 8 8 image. (From [1]. Used with permission.)
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The progressive DCT-based mode can be convenient when it takes a perceptibly long time to send and decode the image. With progressive DCT-based coding, the picture first will appear blocky and the details
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will subsequently appear. A viewer may linger on an interesting picture and watch the details come into view or move onto something else, making this scheme well suited, for example, to the Internet. In the lossless mode, the decoder reproduces an exact copy of the digitized input image. The compression ratio, naturally, varies with picture content. The varying compression ratio is not a problem for sending still photos, but presents significant challenges for sequential images that must be viewed in real time. The efficiency of JPEG coding for still images led to the development of motion JPEG (M-JPEG) for video applications, primarily studio use (early nonlinear editing systems, such as Avid and Lightworks, were based on M-JPEG). Motion JPEG uses intraframe compression, where each frame is treated as an individual signal; a series of frames is basically a continous stream of JPEG-compressed images. The benefit of this construction is easy editing, since video sequences can be cut at frame boundaries, without any further computation or compression of the images. Also, any individual frame is self-supporting and can be accessed as a stand-alone image. The intraframe system is based, again, on DCT. Because a picture with high-frequency detail will generate more data than a picture with low detail, the data stream will vary, in terms of bits per second. This is problematic for most real-time systems, which would prefer to see a constant data rate at the expense of varying levels of quality. In practice, many systems that played back motion JPEG video sequences, such as nonlinear editing machines, employed sophisticated multilayer buffering schemes and limited the amount of data allocated for the encoding of each image, so that while the data rate varied according to image detail, the variation was not completely unbounded. This allowed these systems to maintain crisp response to play, stop, fast-forward, and rewind commands, but also limited the data rate (and hence quality) of highly detailed images, while compressing images with low detail with high quality. Because nonlinear editing systems need to record as well as play, the symmetry of the complexity of encoders and decoders was also an important advantage of motion JPEG. The major disadvantage of motion JPEG is bandwidth and storage requirements, which are a direct consequence of the limited range of compression ratios. Because stand-alone frames are coded, there is no opportunity to code only the differences between frames (to remove redundancies). Large compression ratios, which exploit this interframe correlation are not available to motion JPEG systems.
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