T. A. Welch. A Technique for High-Performance Data Compression. IEEE Computer, 17(6):8--19, June 1984.  Home/Search   Document Not in Database   Summary   Related Articles   Check

....needs to build the same dictionary (the pair that defines the block r is read at the r th step of the algorithm) Many variations on LZ78 exist, which deal basically with the best way to code the pairs in the compressed file. A particularly interesting variant is from Welch, called LZW [21]. In this case, the extra letter (second element of the pair) is not coded, but it is taken as the first letter of the next block (the dictionary is started with one block per letter) LZW is used by Unix s Compress program. In this paper we do not consider LZW separately but just as a coding ....

T. Welch. A technique for high performance data compression. IEEE Computer Magazine, 17(6):8--19, June 1984.

....it is unlikely to found an algorithm approximating this problem within O(log n= log log n) The framework of the grammar based compression can uniformly describe the dictionarybased coding schemes which are widely presented for real world text compression. For example, LZ78 [16] including LZW [13]) and BISECTION [5] encodings are considered as algorithms to nd a straight line program, which is a very restricted CFG. Lehman and Shelat [9] also showed the lower bounds of the approximation ratio of almost dictionarybased encodings to the smallest CFG, and unfortunately, these lower bounds ....

T. A. Welch. A Technique for High Performance Data Compression. IEEE Comput., 17:8-19, 1984.

....the same. Therefore, identical coefficient strings appear frequently across block boundaries for simple images. Our method presented in [5] is to exploit the characteristic to increase the compression ratio in the entropy encoding using BWT described below. BWT was proposed by Burrows and Wheeler [9, 10, 11]. Consider an input string S, which has n data. The first step of the BWT is to form an n n matrix M, where the n th row is the data string obtained by performing (n 1) rotate left operations on the original string. Then the rows of the matrix are sorted by their magnitude. Let the transformed ....

....the original string S in the sorted matrix. The output of the transformation is a two tuple (T, I) Burrows and Wheeler showed that the original string S can be restored from (T, I) through a simple process. The inverse BWT does not require a sorting process. For details of BWT process, refer to [9, 10, 11]. To examine the effect of BWT, suppose that an image has two consecutive identical 8x8 pixel blocks in which the CS of each block is 117, 1, 4, 1 . BWT performed on the concatenation of the two CSs is illustrated in Figure 4. Figure 4 (a) and (b) show the matrix formed by rotating the original ....

. T.A. Welch. "A technique for high performance data compression," IEEE Computer, Vol. 17, No. 6, pp. 8-19, June 1984.

....compression, and Amir, Landau, and Vishikin [6] and Amir and Benson [2, 3] and Amir, Benson, and Farach [4] addressed its two dimensional version. Farach and Thorup [9] and Gasieniec, et al. 11] addressed the LZ77 compression [18] Amir, Benson, and Farach [5] addressed the LZW compression [16]. Karpinski, et al. 12]andMiyazaki,et al. 15] addressed the straight line programs. However, it seems that most of these studies were undertaken mainly from the theoretical viewpoint. Concerning the practical aspect, Manber [14] pointed out at CPM 94 as follows. It is not clear, for example, ....

T. A. Welch. A technique for high performance data compression. IEEE Comput., 17:8--19, June 1984.

.... matching in compressed text without decoding it, which is often referred to as compressed pattern matching , has been studied extensively [4] 18] 43] Along these lines, string search in compressed text was developed for the compression paradigm of LZ78 [52] and its subsequent variant LZW [50], as described in [30] 44] A more challenging problem is that of fully compressed pattern matching when both the pattern and text strings are compressed [21] 22] For the LZ78 LZW paradigm, compressed matching has been extended and generalized to that of approximate pattern matching ( ....

.... identi ed and stored in a dictionary, and whenever, later in the process, a phrase or concatenation of phrases is encountered again, this is compactly encoded by suitable pointers [34] 51] 52] Of the several existing versions of the method, we will use the ones which are denoted LZ78 family [50], 52] The main feature which distinguishes LZ78 factorization from previous LZ compression algorithms is in the choice of codewords. Instead of allowing pointers to reference any string that has appeared previously, the text seen so far is parsed into phrases, where each phrase is the longest ....

Welch, T.A., A Technique for High Performance Data Compression, IEEE Trans. on Computers, 17(6), 8-19 (1984).

....to find a pattern in a compressed text without decompressing it. The problem was first defined by Amir and Benson [3] and several researchers have tackled this problem for various compression methods (see an, excellent survey paper [23] Amir, Benson, and Farach[4] addressed the LZW compression[27] and presented a series of algorithms having various time and space complexities (O(n rn 2) time and space, O(nlogrn rn) time and O(n rn) space, and so on, where n is the length of compressed text and rn is the length of pattern) Among them, we focus on the O (n q rn 2) time and space ....

....and a suffix of u, respectively. Let Prefix(u) be the set of prefixes of a string u, and let Prefix(S) Ju Prefix(u) for a set S of strings. We also define the sets Suffix and Factor in a similar way. We denote the cardinality of a set V by IV I. 3. 1 LZW compression The LZW compression [27] is a very popular compression method. It is adopted as the compress command of UNIX, for instance. It parses a text into phrases and replaces them with pointers to a dictionary. The dictionary initially consists of the characters in E. The compression procedure repeatedly finds the longest match ....

T A. Welch. A technique for high performance data compression. IEEE Cornput., 17:8-19, June 1984.

....needs to build the same dictionary (the pair that de nes the block r is read at the r th step of the algorithm) Many variations on LZ78 exist, which deal basically with the best way to code the pairs in the compressed le. A particularly interesting variant is from Welch, called LZW [27]. In this case, the extra letter (second element of the pair) is not coded, but it is taken as the rst letter of the next block (the dictionary is started with one block per letter) LZW is used by Unix s Compress program. In this paper we do not consider LZW separately but just as a coding ....

T. Welch. A technique for high performance data compression. IEEE Computer Magazine, 17(6):8-19, June 1984.

....the strategies to overcome such wastage is to defer the encoding of innovation as the first symbol of the next parsing, thus it will become the first symbol of the next citation instead of being coded alone. Such strategy is called deferred innovation, as implemented in the UNIX LZW coder compress [8]. It is also known that a practical LZW coder contains redundancy in its encoding. Consider a LZW coder with flat code, which outputs the codewords for citations at 2 n# bits, where n denotes the dictionary size, as many as half of the code spaces are possibly unused. This can be amended by ....

T.A. Welch. A technique for high-performance data compression. IEEE Computer, pages 8--19, January 1984.

....and Lempel schemes [13] 23] 24] 25] are much more sophisticated and powerful, encoding message sequences of increasing length and coping with redundancies of higher order. Modified and augmented versions of the Ziv and Lempel schemes have been implemented by Miller and Wegman [15] and Welch [21] and seem to work very well for some sources, but analytic bounds are less simple. A variant by Rodeh, Pratt, and Even [18] which uses universal codewords as variable length backpointers is close in spirit to interval encoding but preserves the power (and complexity) of dealing with higher order ....

T.A. Welch, "A technique for high-performance data compression," IEEE Cornput. vol. 17, pp. 8-19, June 1984.

....generally helps to use asymmetric algorithms where the decompression speed is significantly higher than the compression speed. This property is particularly important to the implementation suggested in this document. Both LZ77 [28] and X match [13] have this property, but LZ78 (or its variant LZW [25]) do not. High order bits in basic data types often show a low entropy. This observation has been exploited in [26] which introduces a new compression scheme that examines 32 bits at a time and looks for redundancies in the 22 MSBs only. For certain types of data (e.g. integers, floating ....

T.A. Welch, "A technique for high-performance data compression ", IEEE Computer, pp 8-19, June 1984.

....method, the most popular LZ78 variant. Then we look at modifying the LZSS compression scheme, which appears to be the most popular method in the LZ77 family. 2 A Non Greedy Parsing Version of LZW LZC LZW was first presented as a compression method suitable for implementation in hardware [8]. Later, a software version was widely distributed as the compress program. Because the compress program contains some extra features not described in the original LZW paper, compress is sometimes referred to as the LZC algorithm. More recently, a modified version of LZW has become part of the ....

Welch, T. A. A Technique for High-Performance Data Compression. IEEE Computer 17,6 (June 1984), pp. 8-19.

....it turns out to be smaller than the number of bits of the embedded message. Despite the popularity gained by LZ 77 because of the family of compressors zip gzip and the image for mat PNG, several others textual compression methods are widely used. For example, LZ 78 [26] and its variant LZW [21]) are used in compress and GIF. The LZ 78 scheme appears less prone to the sort of treatment we did here. The only arbitrary choice seems to be the initial assignment of codes to the symbol of the alphabet, which could hide a secret message. The size of the message, however, would be limited by ....

T. A. Welch. A technique for high-performance data compression. IEEE Computer, 17(6):8--19, June 1984.

....Whiting [1991] 41] employed hashing to find matches in a sliding window. For LZ78 methods, a simple trie can be used to store the dictionary, and matching can be done in constant time per character read by traversing a root to leaf path in the trie as characters of the match are read (e.g. Welch [42], Storer [28,29] Miller and Wegman [22] Royals et al. 25] and Storer and Reif [31] describe a massively parallel implementation of a systolic LZ compression scheme also using pair wise recursive parsing. Storer and Reif [30] describe the adaptation of LZ techniques to insure error resiliency. ....

T.A. Welch, A technique for high-performance data compression, IEEE Computer 17 (6) (1984) 8 19.

....earlier in the text. This family of algorithms are generally derived from one of the two di#erent approaches published in [63] and [64] namely LZ77 and LZ78, respectively. Some variants in the LZ77 family are: LZR [50] LZSS [12] LZB [13] and LZH [16] Some variants in the LZ78 family are: LZW [59], LZC [54] LZT [55] LZMW [43] and LZJ [36] LZFG [29] is based on both LZ77 and LZ78. One particularly e#ective variant is that of Welch labeled LZW [59] and is derived from LZ78. In this algorithm, we first initialize a list of phrases (i.e. dictionary) which contains all the alphabet ....

....respectively. Some variants in the LZ77 family are: LZR [50] LZSS [12] LZB [13] and LZH [16] Some variants in the LZ78 family are: LZW [59] LZC [54] LZT [55] LZMW [43] and LZJ [36] LZFG [29] is based on both LZ77 and LZ78. One particularly e#ective variant is that of Welch labeled LZW [59], and is derived from LZ78. In this algorithm, we first initialize a list of phrases (i.e. dictionary) which contains all the alphabet symbols. Then we parse the input text into phrases, where each phrase is the longest matching phrase seen previously plus one symbol. We output the index to the ....

T. A. Welch. A technique for high-performance data compression. IEEE Computer, 17:8--19, 1984.

....heuristics than MTF to manage the list appear in [5] and [6] A less ad hoc approach would be to consider words as forming the symbols of an alphabet. Such an alphabet can, in principle, be used as the basis of any existing compression algorithm. For example, LZW (aka the UNIX compress command) [7] could work by encoding sequences of words instead of sequences of characters. If particular sequences tend to recur in the source text, compression would be achieved. However, we need to overcome a major problem with word based compression algorithms. The number of distinct words that the ....

....Huffman codes become so long that fewer bits would be needed to re transmit the word as a new word. Several pruning strategies based on an analogy with page replacement algorithms in virtual memory systems are suggested in [6] 3. 2 Word Based LZW The LZW (Lempel Ziv Welch) compression algorithm [7] is the basis of the UNIX compress program and of the compression strategies implemented in many commercial prod Figure 1 Word Based Adaptive Huffman Algorithm read one alphanumeric word, AW; if AW S A then output AHuffman[Escape] output text of AW; S A : S A AW ; AFreq[AW] 1; ....

[Article contains additional citation context not shown here]

Welch, T.A. "A Technique for High-Performance Data Compression." IEEE Computer 17, 6 (June 1984), pp. 8-19.

....Research Triangle Institute, RTP, NC 27709. 707 708 lnllJJ IMAGE Ire I TROFUI ENCODER . IMAGE image I TRANSFORM [ ooefficienlsl TABLE F . r . lCOU.Esso. DECODER Fig. 1. A typical data compression system. arithmetic coding [6] or Lempel Ziv type methods [7,8] to attain a lossless technique that exceeds the performance of stand alone lossless methods. 1.1 Vector quantization Vector quantization is a lossy block coding technique that is used extensively to compress data at low bit rates (high compression ratios) Vector quantizers compress image data ....

....of both multi resolution algorithms over their entire dynamic range. Figure 9 shows that the Huffman scheme experiences better performance compared to variable size quad tree approach. It should also be noted that in the latter case the quad tree codes were compressed using a LZW type algorithm [8]. 7.1 Performance evaluation of the DCVQ schemes The required vocabularies were created using the tree structured version of the k means algorithm. Each vocabulary had 256 entries and was trained using a set of satellite images containing Thermatic Mapper data obtained from the LANDSAT ....

Welch, T.A. A technique for high-performance data compression. Communications of ACM, 8-19; June, 1984.

.... to tester, software based compression approaches have been reported [3, 4] By performing Burrows Wheeler transformation on the sequence of the test vectors and then applying the run length coding algorithm, the test data can be compressed with better compression ratio than that of LZW method [5]. Also, to decrease the time for testing a system chip, several compression techniques were proposed in [6 8] Jas et al. 6] proposed a hybrid approach to compress precomputed test vectors. A test set consisting of # test vectors (# # , # # , ###, # # ) was transformed to # difference vectors (# ....

T. Welch, "A technique for high-performance data compression ", IEEE Computer, vol. 17, no. 6, pp. 8--19, 1984.

....pressed image. Statistical redundancy results from the fact that different symbols (e.g. gray values) occur with different frequency. This kind of redundancy is handled using coding methods such as Huffman coding [13] arithmetic coding [5] variable bit length coding [3] and LZW coding [14] [15]. In Huffman coding, for example, frequently occurring symbols are coded using codes that have a smaller number of bits than the codes used for less frequently occurring symbols. Naturally occurring images have coherence, smoothness, and correlation, which cause spatial redundancy. There are ....

T. Welch, "A technique for high-performance data compression," IEEE Computer, June 1994, 8-19.

....and Vishkin[6] addressed the run length compression, and Amir, Landau, and Vishikin[5] and Amir and Benson[2, 3] addressed its twodimensional version. Farach and Thorup[7] and Gasieniec, et al. 8] addressed the LZ77 compression[14] Amir, Benson, and Farach[4] addressed the LZW compression[13]. Karpinski, et al. 9] and Miyazaki, et al. 12] addressed the straight line programs. For a fast pattern matching when the text is compressed, we need algorithms which are faster than a decompression followed by a simple search. However, these studies seem to be done mainly from the theoretical ....

T. A. Welch. A technique for high performance data compression. IEEE Comput., 17:8--19, June 1984.

....entropy compression will not accomplish our goal. Spike Areas 3.4 2.07 Non Spike Areas 0.591 0.557 Table 1: Measured data entropy (in bits sample) 1.3 Standard Data Compression methods Many standard methods for lossless compression of generic data have been developed. LZW compression [2], a standard compression algorithm, compresses a difference encoding of the original signal to 0.56 bits sample. This is slightly better than the zero order entropy of 0.59 bits sample, but far from the 0.015 bits sample required to accomplish our goal (Note that the LZW algorithm can compress at ....

Welch, T.A., " A Technique For High Performance Data Compression," IEEE Computer, vol. 17, no. 6, pp.8--19, Jun. 1984.

....which gives a theoretical upper bound for compression schemes that compress each field independently. The entropy for the entire instruction is the sum of the entropies for each field. Note that by adding the space for a Huffman encoding tree, we get the Huffman bound. ffl Lempel Ziv Welch (LZW) [12]. We also measured the popular LZW algorithm used by the UNIX utility compress. LZW is unsuitable for our purposes, as the codebook, a string table, will be out of sync when the program branches. We measured LZW result only as a comparison point. 5.1 Results of Compression Methods Table 4 shows ....

Welch, T. A., "A Technique for High-Performance Data Compression," IEEE Computer , Jun. 1984, pp. 8-19.

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T. A. Welch, "A technique for high performance data compression, " IEEE Computer 17, 8 --19 #1984#.

....or unless the application has no immediate deadline to meet (as in overnight archiving of files) most users would prefer to trade some compression performance for faster rates of compression and decompression. At present, the de facto method of choice is the Lempel Ziv Welch algorithm (LZW) [2]. LZW was originally designed for implementation by special hardware, but it turned out to be highly suitable for efficient software implementations too. An enhanced variant is available on UNIX systems and many other systems as the compress command. We refer to this variant as LZC. The speed and ....

....decoding algorithm (executed by the uncompress command on UNIX systems) must maintain the same dictionary adding new strings to the dictionary in a manner that mimics the behaviour of the compression algorithm. We omit a full description of the decoding process, referring the reader instead to [2] or to [1] Decoding should, of course, execute faster than compression because hash table searching for strings in the dictionary is unnecessary. When the dictionary has filled up, LZW becomes non adaptive it compresses using an unchanging dictionary. For heterogenous files composed of ....

Welch, T.A. "A Technique for High-Performance Data Compression." IEEE Computer 17,6 (June 1984), pp. 8-19.

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T. A. Welch. A Technique for High-Performance Data Compression. IEEE Computer, 17(6):8--19, June 1984.

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T. Welch. A technique for high performance data compression. IEEE Computer, 17(6):8--19, June 1984.

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T. Welch. A technique for high-performance data compression. IEEE Computer, 17(6):8--19, 1984.

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T. A. Welch. A technique for high performance data compression. IEEE Computer, 17(6):8--19, 1984.

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Welch, T.A.: A technique for high performance data compression. IEEE Computer 17 (1984) 8--19

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T. Welch, "A technique for high-performance data compression", IEEE Computer, 17 (6), 8-19, June 1984.

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T. Welch, A technique for high-performance data compression, IEEE Computer 17 (6) (1984) 8 -- 19. June.

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T. Welch, "A technique for high-performance data compression", IEEE Computer, 17 (6), 819, June 1984.

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T. Welch, A technique for High Performance Data Compression, IEEE Computer, vol. 16, no. 6, pp. 8 19, 1984.

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Welch T.A., A technique for high-performance data compression, IEEE Computer 17 (1984) 8--19.

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Welch, T. A. A technique for high-performance data compression. IEEE Computer 17, 6 (June 1984), 8--19.

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T. Welch. A technique for high performance data compression. IEEE Computer Magazine, 17(6):8-19, June 1984.

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T. A. Welch. A technique for high-performance data compression. Computer Magazine of the Computer Group News of the IEEE Computer Group Society, 17(6), 1984.

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T. A. Welch. "A Technique for High-Performance Data Compression." IEEE Computer, pp. 8-19. June 1984.

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T. A. Welch. A technique for high performance data compression. IEEE Computer Magazine, 17(6):8{ 19, June 1984.

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Welch, T. A. 1984. A technique for highperformance data compression. Computer 17, 6 (June), 8-19.

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T. A. Welch. A technique for high-performance data compression. Computer Magazine of the Computer Group News of the IEEE Computer Group Society, 17(6), 1984.

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T. A. Welch. A technique for high-performance data compression. IEEE Computer, 17(6):8-19, June 1984.

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T. A. Welch. "A Technique for High-Performance Data Compression." IEEE Computer, pp. 8-19. June 1984.

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T. A. Welch. A Technique for High-Performance Data Compression. Computer Magazine of the Computer Group News of the IEEE Computer Group Society, 17(6),

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T. Welch. A technique for high performance data compression. IEEE Computer, 17(6):8--19, June 1984.

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T.A. Welch, A technique for high-performance data compression, in: Computer 17, 6 (June) 1984, 8-19.

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T. A. Welch. A technique for high performance data compression. IEEE Computer, 17(6):8--19, 1984.

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T. A. Welch. A Technique for High-Performance Data Compression. IEEE Computer, pages 8-19, June 1984.

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T. A. Welch. A technique for high performance data compression. IEEE Comput., 17:8-19, 1984.

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Welch, T. 1984. A technique for high-performance data compression. IEEE Comput. 17(6):8-19.

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T. A. Welch, `A technique for high-performance data compression', IEEE Computer, 17, (6), 8--19 (1984).

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