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(IJCSIS) International Journal of Computer Science and Information Security, Vol. 9, No. 9, September 2011 Lossless Image Compression For Transmitting Over Low Bandwidth Line G. Murugan, Research Dr. E. Kannan, Supervisor S. Arun , ECE Dept. Scholar , Singhania University ,Singhania University and Asst.Professor Veltech High & Sri Venkateswara College Dean Academic Veltech Engg college,Chennai email- of Engg , Thiruvallur University yesarun001@yahoo.com Abstract solution, as, on the one hand, it provides significantly higher The aim of this paper is to develop an effective loss less compression gains vis-à-vis lossless algorithms, and on the algorithm technique to convert original image into a compressed one. other hand it provides guaranteed bounds on the nature of loss Here we are using a lossless algorithm technique in order to convert introduced by compression. original image into compressed one. Without changing the clarity of Another way to deal with the lossy-lossless dilemma the original image. Lossless image compression is a class of image compression algorithms that allows the exact original image to be faced in applications such as medical imaging and remote reconstructed from the compressed data. sensing is to use a successively refindable compression technique that provides a bit stream that leads to a progressive We present a compression technique that provides reconstruction of the image. Using wavelets, for example, one progressive transmission as well as lossless and near-lossless can obtain an embedded bit stream from which various levels compression in a single framework. The proposed technique produces a bit stream that results in a progressive and ultimately of rate and distortion can be obtained. In fact with reversible lossless reconstruction of an image similar to what one can obtain integer wavelets, one gets a progressive reconstruction with a reversible wavelet codec. In addition, the proposed scheme capability all the way to lossless recovery of the original. Such provides near-lossless reconstruction with respect to a given bound techniques have been explored for potential use in tele- after decoding of each layer of the successively refineable bit stream. radiology where a physician typically requests portions of an We formulate the image data compression problem as one of image at increased quality (including lossless reconstruction) successively refining the probability density function (pdf) estimate of while accepting initial renderings and unimportant portions at each pixel. Experimental results for both lossless and near-lossless lower quality, and thus reducing the overall bandwidth cases indicate that the proposed compression scheme, that requirements. In fact, the new still image compression innovatively combines lossless, near-lossless and progressive coding attributes, gives competitive performance in comparison to state-of- standard, JPEG 2000, provides such features in its extended the-art compression schemes. form [2]. In this paper, we present a compression technique that incorporates the above two desirable characteristics, 1.INTRODUCTION namely, near-lossless compression and progressive refinement Lossless or reversible compression refers to from lossy to lossless reconstruction. In other words, the compression techniques in which the reconstructed data proposed technique produces a bit stream that results in a exactly matches the original. Near-lossless compression progressive reconstruction of the image similar to what one denotes compression methods, which give quantitative bounds can obtain with a reversible wavelet codec. In addition, our on the nature of the loss that is introduced. Such compression scheme provides near-lossless (and lossless) reconstruction techniques provide the guarantee that no pixel difference with respect to a given bound after each layer of the between the original and the compressed image is above a successively refinable bit stream is decoded. Note, however given value [1]. Both lossless and near-lossless compression that these bounds need to be set at compression time and find potential applications in remote sensing, medical and cannot be changed during decompression. The compression space imaging, and multispectral image archiving. In these performance provided by the proposed technique is applications the volume of the data would call for lossy comparable to the best-known lossless and near-lossless compression for practical storage or transmission. However, techniques proposed in the literature. It should be noted that to the necessity to preserve the validity and precision of data for the best knowledge of the authors, this is the first technique subsequent reconnaissance diagnosis operations, forensic reported in the literature that provides lossless and near- analysis, as well as scientific or clinical measurements, often lossless compression as well as progressive reconstruction all imposes strict constraints on the reconstruction error. In such in a single framework. situations near-lossless compression becomes a viable 2. METHODOLOGY 140 http://sites.google.com/site/ijcsis/ ISSN 1947-5500 (IJCSIS) International Journal of Computer Science and Information Security, Vol. 9, No. 9, September 2011 2.1COMPRESSION TECHNIQUES or dropping some of the chrominance information in the image. LOSSLESS COMPRESSION Transform coding. This is the most commonly used Where data is compressed and can be reconstituted method. A Fourier-related transform such as DCT or (uncompressed) without loss of detail or information. These the wavelet transform are applied, followed are referred to as bit-preserving or reversible compression by quantization and entropy coding. systems also [11]. Fractal compression. LOSSY COMPRESSION Where the aim is to obtain the best possible fidelity for a 2.3COMPRESSION given bit-rate or minimizing the bit-rate to achieve a given The process of coding that will effectively reduce the fidelity measure. Video and audio compression techniques are total number of bits needed to represent certain information. most suited to this form of compression [12]. STORAG DECODE If an image is compressed it clearly needs to be E OR R uncompressed (decoded) before it can ENCOD NETWOR viewed/listened to. Some processing of data may be ER KS (DECOM possible in encoded form however. INPUT (COMPR PRESSIO Lossless compression frequently involves some form ESSION) N) of entropy encoding and are based in information theoretic techniques Lossy compression use source encoding techniques that may involve transform encoding, differential encoding or vector quantisation Fig.1. a general data compression scheme Image compression may be lossy or lossless. Lossless compression is preferred for archival purposes and often for medical imaging, technical drawings, clip art, or comics. This is because lossy compression methods, especially when used at low bit rates, introduce compression artifacts. Lossy methods are especially suitable for natural images such as photographs in applications where minor (sometimes imperceptible) loss of fidelity is acceptable to achieve a substantial reduction in bit rate. The lossy compression that produces imperceptible differences may be called visually Fig.2 lossy image compressionresult result lossless. 2.2METHODS FOR LOSSLESS IMAGE COMPRESSION ARE Run-length encoding – used as default method in PCX and as one of possible in BMP, TGA, TIFF DPCM and Predictive Coding Entropy encoding Adaptive dictionary algorithms such as LZW – used in GIF and TIFF Deflation – used in PNG, MNG, and TIFF Chain codes 2.3METHODS FOR LOSSY COMPRESSION Reducing the color space to the most common colors in the image. The selected colors are specified in the color palette in the header of the compressed image. Each pixel just references the index of a color in the color palette. This method can be combined with dithering to avoid posterization. Fig. 3 lossless image comparison ratio Chroma sub sampling. This takes advantage of the fact that the human eye perceives spatial changes of brightness more sharply than those of color, by averaging 141 http://sites.google.com/site/ijcsis/ ISSN 1947-5500 (IJCSIS) International Journal of Computer Science and Information Security, Vol. 9, No. 9, September 2011 Divide image up into 8x8 blocks Each block is a symbol to be coded Compute Huffman codes for set of block Encode blocks accordingly 3.2HUFFMAN CODING ALGORITHM Fig.4lossy and lossless comparison ratio 3.HUFFMAN CODING Huffman coding is based on the frequency of occurrence of a data item (pixel in images). The principle is to use a lower number of bits to encode the data that occurs more frequently. Codes are stored in a Code Book which may be constructed for each image or a set of images. In all cases the code book plus encoded data must be transmitted to enable decoding. The Huffman algorithm is now briefly summarised: A bottom-up approach 1. Initialization: Put all nodes in an OPEN list, keep it No Huffman code is the prefix of any other Huffman codes so sorted at all times (e.g., ABCDE). decoding is unambiguous 2. Repeat until the OPEN list has only one node left: • The Huffman coding technique is optimal (but we (a) From OPEN pick two nodes having the lowest must know the probabilities of each symbol for this frequencies/probabilities, create a parent node of to be true) them. • Symbols that occur more frequently have shorter (b) Assign the sum of the children's frequencies/ Huffman codes probabilities to the parent node and insert it into 4.LEMPEL-ZIV-WELCH (LZW) ALGORITHM OPEN. THE LZW COMPRESSION ALGORITHM CAN (c) Assign code 0, 1 to the two branches of the tree, SUMMARISED AS FOLLOWS and delete the children from OPEN. w = NIL; The following points are worth noting about the while ( read a character k ) above algorithm: { Decoding for the above two algorithms is trivial as long if wk exists in the dictionary as the coding table (the statistics) is sent before the data. w = wk; (There is a bit overhead for sending this, negligible if the data else file is big.) add wk to the dictionary; Unique Prefix Property output the code for w; w = k; No code is a prefix to any other code (all symbols } are at the leaf nodes) great for decoder, unambiguous. If prior THE LZW DECOMPRESSION ALGORITHM IS AS statistics are available and accurate, then Huffman coding is FOLLOWS very good. read a character k; 3.1HUFFMAN CODING OF IMAGES output k; w = k; In order to encode images: while ( read a character k ) 142 http://sites.google.com/site/ijcsis/ ISSN 1947-5500 (IJCSIS) International Journal of Computer Science and Information Security, Vol. 9, No. 9, September 2011 /* k could be a character or a code. */ • Code this pair using a lossless method such as { Huffman coding entry = dictionary entry for k; The difference is usually small so entropy output entry; coding gives good results add w + entry[0] to dictionary; w = entry; Can only use a limited number of methods on the edges of the image } 5.LOSSY AND LOSSLESS ALGORITHMS 4.2ENTROPY ENCODING TREC includes both lossy and lossless compression Huffman maps fixed length symbols to variable algorithms. The lossless algorithm is used to compress data for length codes. Optimal only when symbol the Windows desktop which needs to be reproduced exactly as probabilities are powers of 2. it’s decompressed. The lossy algorithm is used to compress 3D Arithmetic maps entire message to real number range image and texture data when some loss of detail is tolerable. based on statistics. Theoretically optimal for long Let me just explain the point about the Windows messages, but optimality depends on data model. desktop since it’s perhaps not obvious why I even mentioned Also can be CPU/memory intensive. it. A Talisman video card in a PC is not only going to be Lempel-Ziv-Welch is a dictionary-based compression producing 3D scenes but also the usual desktop for a Windows method. It maps a variable number of symbols to a platform. Since there is no frame buffer, the entire desktop fixed length code. needs to be treated as a sprite which in effect forms a background scene on which 3D windows might be Adaptive algorithms do not need a priori estimation superimposed. Obviously we want to use as little memory as of probabilities, they are more useful in real possible to store the Windows desktop image so it makes applications. sense to try to compress it, but it’s also vital that we don’t 4.2.1LOSSLESS JPEG distort any of the pixel data since it is possible that an application might want to read back a pixel it just wrote to the • JPEG offers both lossy (common) and lossless display via GDI. So some form of lossless algorithm is vital (uncommon) modes. when compressing the desktop image. • Lossless mode is much different than lossy (and also 5.1LOSSLESS COMPRESSION gives much worse results) Let’s take a look at how the lossless compression • Added to JPEG standard for completeness algorithm works first as it the simpler of the two. Figure 4.1 shows a block diagram of the compression process. • Lossless JPEG employs a predictive method combined with entropy coding. • The prediction for the value of a pixel (greyscale or RGBA DATA color component) is based on the value of up to three COMPRESSED DATA neighboring pixels RGB TO PREDICTI HUFFMAN • One of 7 predictors is used (choose the one which YUV ON /RLE gives the best result for this pixel). CONVERS ENCODIN PREDICTOR PREDICTION Fig. 4.1 the lossless compression process P1 A The RGB data is first converted to a form of YUV. P2 B Using a YUV color space instead of RGB provides for better compression. The actual YUV data is peculiar to the TREC P3 C algorithm and is derived as follows: P4 A+B-C Y=G P5 A+(B-C)/2 U=R-G P6 B+(A-C)/2 V=B-G P7 (A+B)/2 The conversion step from RGB to YUV is optional. Following YUV conversion is a prediction step which takes advantage of the fact that an image such as a typical Windows Table lossless jpeg desktop has a lot of vertical and horizontal lines as well as • Now code the pixel as the pair (predictor-used, large areas of solid color. Prediction is applied to each of the difference from predicted method) R, G, B and alpha values separately. For a given pixel p(x, y) it’s predicted value d(x, y) is given by 143 http://sites.google.com/site/ijcsis/ ISSN 1947-5500 (IJCSIS) International Journal of Computer Science and Information Security, Vol. 9, No. 9, September 2011 d(0, 0) = p(0, 0) The first step is to convert the RGB data to a form of YUV d(0, y) = p(0, y) - p(0, y-1) for y > 0 called YOrtho using the following: d(x, y) = p(x, y) - p(x-1, y) for x > 0 Y = (4R + 4G + 4B) / 3 - 512 The output values from the predictor are fed into a U=R-G Huffman/RLE encoder which uses a set of fixed code tables. V = (4B -2R -2G) / 3 The encoding algorithm is the same as that used in JPEG for encoding the AC coefficients. (See ISO International Standard Note that the alpha value is not altered by this step. 10918, “ Digital Compression and Coding of Continuous-Tone The next step is to apply a two-dimensional Discrete Cosine Still Images”.) The Huffman/RLE encode outputs a series of Transform (DCT) to each color and alpha component. This variable-length code words. These code words describe the produces a two-dimensional array of coefficients for a length from 0 to 15 of a run of zeroes before the next frequency domain representation of each color and alpha component. The next step is to rearrange the order of the coefficient and the number of additional bits required to coefficients so that low DCT frequencies tend to occur at low specify the sign and mantissa of the next non-zero coefficient. positions in a linear array. This tends to place zero coefficients The sign and mantissa of the non-zero coefficient then follow in the upper end of the array and has the effect of simplifying the code word. the following quantization step and improving compression through the Huffman stage. The quantization step reduces the 5.2LOSSLESS DECOMPRESSION number of possible DCT coefficient values by doing an Decompressing an image produced by the lossless integer divide. Higher frequencies are divided by higher compression algorithm follows the steps shown in figure 4.2 factors because the eye is less sensitive to quantization noise COPRESSION DATA in the higher frequencies. The quantization factor can vary RGPA DATA from 2 to 4096. Using a factor of 4096 produces zeros for all HUFFMA INVERSE YUV TO input values. Each color and alpha plane has its own N/RLE PREDICTI RGB quantization factor. Reducing the detail in the frequency domain by quantization leads to better compression and the 5.2.1the lossless decompression process expense of lost detail in the image. The quantized data is then Huffman encoded using the same process as was described for The encoded data is first decoded using a Huffman decoder lossless compression. using fixed code tables. The data from the Huffman decoder is then passed through the inverse of the prediction filter used in 5.4LOSSY DECOMPRESSION compression. For predicted pixel d(x, y) the output pixel The decompression process for images compressed values p(x, y) are given by: using the TREC lossy compression algorithm is shown in p(0, 0) = d(0, 0), p(0, y) = d(0, y-1) + d(0, y) figure for y > 0 Type and LOD Factors Parameters p(x, y) = d(x-1, y) + d(x, y) for x > 0 The final step is to convert the YUV-like data back to RGB using: R = Y + U, G = Y,B = Y + V 5.3LOSSY COMPRESSION Compressed Huffman/RLE Inverse Zig-zag Data Decoding Quantize Reordering The lossy compression algorithm is perhaps more interesting since it achieves much higher degrees of compression that the lossless algorithm and is used more extensively in compressing the 3D images we are interested in. Figure 3 shows the compression steps. RGBA RGB to YUV Forward Zig-zag Inverse YUV to RGB Conversion DCT Ordering DCT Conversion RGBA Data Data Fig. 4.4 the lossy decompression process The decompression process is essentially the reverse Quantize Huffman/RLE Encoding Compressed of that used for compression except for the inverse Data quantization stage. At this point a level of detail (LOD) parameter can be used to determine how much detail is required in the output image. Applying a LOD filter during Type and Factors decompression is useful when reducing the size of an image. The LOD filter removes the higher frequency DCT Fig. 4.3 the lossy compression process coefficients which helps avoid aliasing in the output image 144 http://sites.google.com/site/ijcsis/ ISSN 1947-5500 (IJCSIS) International Journal of Computer Science and Information Security, Vol. 9, No. 9, September 2011 when simple pixel sampling is being used to access the source [6] S. Martucci. Reversible compression of hdtv images using pixels. median adaptive prediction and arithmetic coding. Proc. IEEE Note that the level of detail filtering is not a part of International Symposium on Circuits and Systems, pages the TREC specification and not all TREC decompressions will 1310–1313, 1990. implement it. [7] M. Weinberger, G. Seroussi, and G. Sapiro. LOCO-I: A low complexity, context-based, lossless image compression 6.EXPERIMENTAL RESULTS algorithm. Proc. Data Compression Conference, pages 140– We present experimental results based on the steps 149, March 1996. [8] ITU-T Rec. T.84 - Information Technology. Digital Step1. Lossless Compression compression and coding of continuous-tone still images: Step2. Lossless Decompression Extensions, July 1996. [9] M. Weinberger, J. Rissanen, and R. Arps. Applications of Step3. Lossless image Compression using Huffman coding universal context modeling to lossless compression of gray- Step4. Lossless image Decompression using Huffman coding scale images. IEEE Trans. on Image Processing, 5(4):575– 586, April 1996. Step5. Lossless image Compression for transmitting Low [10] G. Wallace. The jpeg still picture compression standard. Bandwidth Line Comms. of the ACM, 34(4):30– 44, April 1991. 7.CONCLUSIONS [11] ISO/IEC 14495-1, ITU Recommendation T.87, “Information technology - Lossless and near-lossless This work has shown that the compression of image can be compression of continuous-tone still images,” 1999. improved by considering spectral and temporal correlations as well as spatial redundancy. The efficiency of temporal [12] M. J. Weinberger, G. Seroussi, and G. Sapiro, “LOCO-I: prediction was found to be highly dependent on individual A low complexity lossless image compression algorithm.” image sequences. Given the results from earlier work that ISO/IEC JTC1/SC29/WG1 document N203, July 1995. found temporal prediction to be more useful for image, we can conclude that the relatively poor performance of temporal prediction, for some sequences, is due to spectral prediction being more efficient than temporal. Another Conclusions and Future Work finding from this work is that the extra compression available from image can be achieved without necessitating a large increase in decoder complexity. Indeed the presented scheme has a decoder that is less complex than many lossless image compression decoders, due mainly to the use of forward rather than backward adaptation. Although this study considered a relatively large set of test image sequences compared to other such studies, more test sequences are needed to determine the extent of sequences for which temporal prediction is more efficient than spectral prediction.. 8.REFERENCES [1] N. Memon and K. Sayood. Lossless image compression: A comparative study. Proc. SPIE Still-Image Compression, 2418:8–20, March 1995. [2] N. Memon and K. Sayood. Lossless compression of rgb color images. Optical Engineering, 34(6):1711–1717, June 1995. [3] S. Assche, W. Philips, and I. Lemahieu. Lossless compression of pre-press images using a novel color decorrelation technique. Proc. SPIE Very High Resolution and QualityImaging III, 3308:85–92, January 1998. [4] N. Memon, X. Wu, V. Sippy, and G. Miller. An interband coding extension of the new lossless jpeg standard. Proc. SPIE Visual Communications and Image Processing, 3024:47–58, January 1997. [5] N. Memon and K. Sayood. Lossless compression of video sequences. IEEE Trans. on. Communications, 44(10):1340– 1345, October 1996. 145 http://sites.google.com/site/ijcsis/ ISSN 1947-5500

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