DigitalCompressionCh9

DigitalCompressionCh9 - D IG I T AL C OMPRESSION for M...

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DIG I TAL COMPRESSION for MULTIMEDIA Principles and Standards Jerry D. Gibson Toby Berger Tom Lookabaugh Dave Lindbergh Richard L. Baker Morgan Kaufmann Publishers, Inc. San Francisco, California Southern Methodist University Cornell University DiviCom PictureTel Corporation PictureTel Corporation I W!§
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9 JPEG Still-Image Compression CHAPTER Standard 9.1 Introduction Although several standards for voice and facsimile compression have existed for a number of years, the JPEG still-image compression standard has to be one of the most widely recognized standards in existence today. The baseline JPEG compression method has truly become ubiquitous, with a wide variety of hardware and software implementations available for a host of applications. The broad goal of the Joint Photographic Experts Group (JPEG), meeting as a working group under the ISO but closely coordinated with CCITT SGVIII, was to develop a general-purpose still-image compression standard that would be applicable to virtually all continuous-tone still-image transmission and storage problems. To achieve this, JPEG draws upon years of prior research in image compression and complements this with numerous innovations and refinements that bring both performance and complexity to levels that allow it to have the wide utility that it has today. The goals of JPEG are the following (Wallace 1991; Pennebaker and Mitchell 1993): 1. Achieve rate and reconstructed image quality "at or near the state of the art" with image fidelity classifiable as "very good" to "excellent" 2. Be useful for compressing almost any continuous-tone still image, including both gray-scale and color, any color space, and most image sizes 3. Have complexity that would allow software implementations on many com- mon computing platforms and affordable hardware implementations
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292 JPEG Still-Image Compression Standard The JPEG standard has several lossy encoding modes, starting with the base- line sequential mode, and a lossless encoding mode. In the following sections, we provide an overview of the JPEG compression methods, a discussion of rei a- tiveperformance, and insights on what led to various parameter selections and approaches. 9.2 Baseline JPEG The basic JPEG encoder and decoder structures are illustrated in Figure 9.1; FDCT stands for forward DCT and IDCT stands for inverse DCT. The baseline JPEG encoding method is called sequential encoding. First the image is parti- tioned into 8 x 8 blocks of pixels that are ordered according to a "rasterlike" left-to-right, top-to-bottom scan, as depicted in Figure 9.2. The FDCT is com- puted on each of the 8 x 8 blocks of pixels, and the resulting 64 DCT coefficients are scalar quantized using uniform quantization tables based upon psychovisual experiments (Lohscheller 1984). These scalar quantization tables are provided as part of the standard but are not a requirement. After the DCT coefficients are quantized, the coefficients within the block are ordered according to the zigzag
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DigitalCompressionCh9 - D IG I T AL C OMPRESSION for M...

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