Ebook Fundamentals of multimedia: Part 2
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Ebook Fundamentals of multimedia: Part 2
CHAPTER 9Image Compression StandardsRecent years have seen an explosion in the availability of digital images, because of the incivase in numbers of d Ebook Fundamentals of multimedia: Part 2 digital imaging devices, such as scanners and digital cameras. The need to efficiently process and store images in digital form has motivated the development of many image compression standards for various applications and needs. In general, standards have greater longevity than particular programs Ebook Fundamentals of multimedia: Part 2 or devices and therefore warrant careful study. In this chapter, wc examine some current standards and demonstrate how topics presented in Chapters 7Ebook Fundamentals of multimedia: Part 2
and 8 are applied in practice.We first explore the standard JPEG definition, used in most images on the web, then go on to look at the wavelet-baser! CHAPTER 9Image Compression StandardsRecent years have seen an explosion in the availability of digital images, because of the incivase in numbers of d Ebook Fundamentals of multimedia: Part 2 ge compression, arc inchided for completeness.9.1THE JPEG STANDARDJPEG is an image compression standard developed by the Joint Photographic Experts Group. it was formally accepted as an international standard in 1992 [1],JPEG consists of a number of steps, each of which contributes to compression. W Ebook Fundamentals of multimedia: Part 2 e’ll look at (he motivation behind these steps, then take apart the algorithm piece by piece.9.1.1Main Steps in JPEG Image CompressionAs we know, unliEbook Fundamentals of multimedia: Part 2
ke one-dimensional audio signals, a digital image /(i, j) is not defined over the time domain. Instead, it is defined over a spatial domain — that is,CHAPTER 9Image Compression StandardsRecent years have seen an explosion in the availability of digital images, because of the incivase in numbers of d Ebook Fundamentals of multimedia: Part 2 onse that is a function ?*(w, v) in the spatial frequency domain, indexed by two integers w and V.JPEG is a lossy image compression method. The effectiveness of the DCT transform coding method in JPEG relies on three major observations:Observation 1. Useful image contents change relatively slowly ac Ebook Fundamentals of multimedia: Part 2 ross the image — that is, it is unusual for intensity values to vary widely several times in a small area — for example, in an 8 X 8 image block. SpatEbook Fundamentals of multimedia: Part 2
ial frequency indicates how many times pixel values change across an image block. The DCT formalizes this notion with a measure of how much the image CHAPTER 9Image Compression StandardsRecent years have seen an explosion in the availability of digital images, because of the incivase in numbers of d Ebook Fundamentals of multimedia: Part 2 less likely to notice the loss of very high-spatial-frequency components than lower-frequency components.253254 Chapter 9 Image Compression StandardsJPEG’s approach to the use of DCTis basjcaUyt0 reduce high-frequency contents and then efficiently code the result into Ệ hit-string, ilic term spaiia! Ebook Fundamentals of multimedia: Part 2 redundancy indicates that much of die jnfurination ill an image is repeated: if a pixel is red, then its neighbor is likely red also. Because of ObseEbook Fundamentals of multimedia: Part 2
rvation 2 ahovc, the DCT coefficients for the lowest frequencies are most important. Therefore, as frequency gets Iiigher, ilbecojiis less.important tCHAPTER 9Image Compression StandardsRecent years have seen an explosion in the availability of digital images, because of the incivase in numbers of d Ebook Fundamentals of multimedia: Part 2 eros can be represented efficiently as the length of such a run of zeros, and compression of bits required is possible. Since we end up using fewer numbers to represent the pixels in blocks, by removing some location-dependentjnfonna.tion, we have effectively removed spatial redundancy.JPEG works fo Ebook Fundamentals of multimedia: Part 2 r both color and grayscale images. In the Case of color images, such as Y1Ọ or YU V, the encoder works on each component separately, using the same roEbook Fundamentals of multimedia: Part 2
utines. If the source image is in a different color formal, lite encoder performs a color-space conversion to YIQ or YL’V. As discussed in Chapter 5, CHAPTER 9Image Compression StandardsRecent years have seen an explosion in the availability of digital images, because of the incivase in numbers of d Ebook Fundamentals of multimedia: Part 2 uity (accuracy in distinguishing closely .spaced lines') is much greater for gray ("black and white”) than for color. We simply cannot see much change in color if it occurs in close proximity — think of the blobby ink used 111 comic books. This works .simply because our eye secs the black lines best Ebook Fundamentals of multimedia: Part 2 , and our brain just pushes the color into place. Ill fact, ordinary broadcast TV makes use of this phenomenon to transmit much less color informationEbook Fundamentals of multimedia: Part 2
than giay information.Section 9.1 The JPEG Standard 2S5When the JPEG image is needed for viewing, tile three compressed component images can be decodCHAPTER 9Image Compression StandardsRecent years have seen an explosion in the availability of digital images, because of the incivase in numbers of d Ebook Fundamentals of multimedia: Part 2 , we will simply use one of them — for example, the y image, in the description of the compression algorithm below1.Figure 9.1 shows a block diagram for a JPEG encoder. If we reverse Che arrows in the figure, we basically obtain a JPEG decoder, The JPEG encoder consists of die following main steps:• Ebook Fundamentals of multimedia: Part 2 Transform RGB to Y1Ọ or YU V anti subsample color•Perform DCT on image blocks•Apply Quantization•Perform 'Zigzag ordering and run-length encoding•PerfEbook Fundamentals of multimedia: Part 2
orm Entropy codingDOT on Image Blocks. Each image is divided into 8x8blocks. The 2D OCT (Equation 8.17) is applied to each block image with output beiCHAPTER 9Image Compression StandardsRecent years have seen an explosion in the availability of digital images, because of the incivase in numbers of d Ebook Fundamentals of multimedia: Part 2 8 would have made accuracy at low frequencies better, but using 8 makes the DCT-CandJpCT) computation very fast.Using blocks at all. however, has the effect of isolating each block from its neighboring context. This is why JPEG images look choppy (•‘block)'”) when the user specifies a high compress Ebook Fundamentals of multimedia: Part 2 ion ratio — we can see these blocks. (And in fact removing such “blocking artifacts” is an important concern of researchers.)To calculate a particularEbook Fundamentals of multimedia: Part 2
/•■(», u), we select the basis image in Figure 8.9 that corresponds . to the appropriate n and K and use ir in Equation 8.17 to derive one of the freCHAPTER 9Image Compression StandardsRecent years have seen an explosion in the availability of digital images, because of the incivase in numbers of d Ebook Fundamentals of multimedia: Part 2 consists of simply dividing each entry in lite frequency space block by an integer, then rounding:F (w. V) - round ()(9.1}\Ổ(«. u)/Here, F(u, v) represents a ĐCT coefficient, Q(ii, v) is a quantization matrix entry, and F(w7 ụl represents the qtfimrizfltiDCfcoejftcientsJREGwin nssũnIhc.spccixding e Ebook Fundamentals of multimedia: Part 2 ntropy cojmg.T he default values in the 8 X 8 quantization matrix Ỡ(«, u) are listed in Tables 9.1 and 9.2 lor luminance and chrominance images, respeEbook Fundamentals of multimedia: Part 2
ctively. Tfiesc numbers resnltcd Bom psychophysical studies, with the goal of maximizing the Compression ratio while minimizing perceptual losses in JCHAPTER 9Image Compression StandardsRecent years have seen an explosion in the availability of digital images, because of the incivase in numbers of dCHAPTER 9Image Compression StandardsRecent years have seen an explosion in the availability of digital images, because of the incivase in numbers of dGọi ngay
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