JP3088740B2 - Data compression and decompression method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Summary] Encoded data is divided into different sub-sequences and registered in a dictionary, and input data is designated by a registration number having a maximum length match among the sub-sequences in the dictionary and encoded. LZW, which is a kind of universal encoding that converts and decodes code words using a dictionary
The data compression and decompression method using codes is intended to increase the compression ratio by initial registration of the dictionary, and to assign a registration number in advance to a set of character strings consisting of not only one character but also one or more characters with high appearance frequency. It is configured to initially register with.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a data compression and decompression method using an LZW code known as an improvement of an incremental decomposition type, which is a kind of universal code.

In recent years, various types of data such as character codes, vector information, and images have been handled by computers, and the amount of data handled has rapidly increased. When dealing with large amounts of data, you can reduce the storage capacity by compressing the amount of data by eliminating redundant parts of the data,
It can be transmitted quickly.

Universal coding has been proposed as a method that can compress various data in one system.

Here, the field of the present invention is not limited to character code compression, and can be applied to various types of data. In the following, one word unit of data is called a character, following the name used in information theory, Data in which arbitrary words are connected is called a character string.

As a typical method of universal code, Ziv-Lempe
There is an l (Jib-Lempel) code (for example, for example,
Munakata "Ziv-Lempel Data Compression Method", Information Processing, Vol.2
6, No. 1, 1985).

 For Ziv-Lempel codes, two algorithms, a universal type and an incremental parsing type, have been proposed.

Furthermore, as an improvement of the universal algorithm,
LZSS code (TCBell, “Better OPM / L Text Comp
ressin ", IEEE Trans.on Commun., Vol.COM-34, No.12, De
c.1986).

In addition, as an improvement of the incremental decomposition algorithm, LZW
(Lempel-Ziv-Welch) code (TAWelch, "A Te
chnique for High-Performance Data Compressign ", Co
mputer, June 1984).

Among these codes, the LZW code is used for file compression of a storage device or the like because of its high speed processing and the simplicity of the algorithm.

[Prior Art] A conventional LZW code encoding algorithm is shown in FIG. 8, and a decoding algorithm is shown in FIG.

LZW encoding has a rewritable dictionary, divides input character code and data into different character strings, assigns numbers to the character strings in the order they appear, and registers them in the dictionary.
The character string currently input is represented and encoded by the number of the longest matching character string registered in the dictionary.

In the LZW encoding process shown in FIG. 8, first, in step S1 (hereinafter, "step" is omitted), a character string consisting of one character for all characters is registered in a dictionary as an initial value before encoding starts. In step S1, the dictionary is searched using the first character K that has been input to obtain a reference number ω.
tring).

Next, the next character K of the input data is read in S2, and it is checked in S3 whether or not the character input has been completed.
A search is performed to determine whether or not the dictionary has a character string ωK obtained by adding the character K read in S2 to the initial character string ω obtained in S1.

If the character string ωK is not found in the dictionary in S4, the process proceeds to S6, where the reference number ω of the character K obtained in S1 is output as a codeword code (ω), and a new reference number is added to the character string (ωK). Then, the input character K of S2 is replaced with the reference number ω, the dictionary address n is incremented, and the process returns to S2 to read the next character K.

On the other hand, if the character string (ωK) is found in the dictionary in S4, the character string (ωK) is replaced with the reference number ω in S5, and the process returns to S2 to search for the maximum matching length until the character string (ωK) cannot be found in the dictionary. Continue.

The LZW encoding will be specifically described with reference to FIGS.

First, the input data INPUT in FIG. 10 is read from left to right. When the first character a is input, there is no matching character string other than the character a in the dictionary, so OUTPUT CODE1 (reference number ω) is output as a code word. Then, the character a is set as a new initial character string ω.

Next, the second character b is input, and since the character string ωK = ab obtained by adding the input character b to the initial character string ω is not in the dictionary, the OUTPUT CODE2 of the character b is output as a code word. Then, the extended character string ab is assigned a reference number 4 and registered in the dictionary. The actual registration is a character string as shown on the right side of FIG.
Registered in the form of 1b. Then, the character b input second becomes the new initial character string ω.

Then, if the third character a is input, the extended character string ωK = ba = 2a obtained by adding the initial character string ω to the input character a is not in the dictionary, so the OUTPUT CODE2 of the character b is output as a code word. After that, the extended character string ωK = ba is represented by 2a, and is added to the reference number 5 and registered in the dictionary. Then, the third input character a becomes a new initial character string ω.

For the fourth input character b, the extended character string ωK = ab is already registered in the dictionary as the code word 4, so the character string ωK is set as a new initial character string ω, and the fifth character c is input. An extended character string ωK = 4c = abc is created. Since this extended character string ωK = abc is not registered in the dictionary, the character string a
OUTPUT CODE4 of b = 1b is output as a code word, and the extended character string ωK = abc is registered in the dictionary with reference number 6 in the form of 4c. Hereinafter, this process is similarly continued.

The decoding process of FIG. 9 performs the reverse operation of the encoding of FIG.

In the decoding shown in FIG. 9, similarly to the encoding, the decoding is started after a character string consisting of one character for every character is previously registered in the dictionary as an initial value.

First, the first code (reference number) is read in S1, and the current code is read.
CODE is OLDcode, and since the first code corresponds to one of the reference numbers of one character already registered in the dictionary, a character code (K) that matches the input code CODE is searched for and the character K is output. The output character (K) is set in FINchar for later exception processing.

Then, the process proceeds to S2, where the next code is read and set as CODE in INCODE. In S3, it is checked whether there is a new code, that is, whether or not the code input has been completed, and the process proceeds to S4, where it is checked whether the code CODE input in S3 is defined (registered) in the dictionary. Normally, the input code word is registered in the dictionary in the previous processing, so the process proceeds to S5 to execute the code CODE.
A character string code (ωK) corresponding to is read from the dictionary, and S6
Temporarily stacks the character string K with reference number code (ω)
Is returned to S5 again as a new CODE, and the steps of S5 and S6 are recursively repeated until the reference number ω reaches one character. Finally, the process proceeds to S7, where the characters stacked in S6 are LILO (Last In Fa).
output in pop-up format. At the same time, in S7, a new reference number is added to the character string represented as a set (ω, K) in which the character ω used last time and the character string restored this time are the first character K and registered in the dictionary.

The decoding process is specifically described below with reference to FIG.

First, in FIG. 12, the first input code is 1, and one character a, b,
Since c is already registered in the dictionary as reference numbers 1, 2, and 3 as shown in Table 2, the reference 1
Is replaced with the character string a of the reference number that matches with.
Similarly, the next code 2 is replaced with the character b and output. At this time, a new reference number 4 is added to 1b (= ab) obtained by combining the previously processed code and the first character b decoded this time and registered in the dictionary.

The third code 4 replaces 1b with ab by searching the dictionary and outputs a character string ab. At the same time, a character string 2a (= ba) obtained by combining the code 2 processed last time and the first character a of the character string decoded this time is added to the new reference number 5 and registered in the dictionary.

 Hereinafter, similarly, this processing is repeated.

 In the decoding of FIG. 12, there is the following exception processing.

This exception processing occurs when the sixth input code 8 is decoded. The code 8 is not defined in the dictionary at the time of decoding and cannot be decoded. In this case, a character string 5b is obtained by adding the first character b of the previously decoded character string ba to the previously processed code 5 and further replaced with 2ab and bab and output. Then, a reference number 8 is added to a character string 5b obtained by adding the character b of the character string decoded this time to the previous code word 5 to the output word of the character string and registered in the dictionary.

This exception processing is performed through the processing of S4 and S8 in the decoding processing flow of FIG. 9, and finally, the output of the character string and the addition of the reference number to the new character string in S7 are registered in the dictionary in S7. Done.

The encoding / decoding processing of FIGS. 8 and 9 is performed while creating the same dictionary.

[Problem to be Solved by the Invention] In the conventional LZW code, data that appeared in the past is registered in a dictionary, and compression is performed using the registered information. However, it cannot be asserted that registering all data that has appeared in the past in the dictionary is an effective dictionary registration method. For example, in the case of image data, it is known that the appearance frequency of all white and all black patterns is high, and if these data are registered sequentially, a high compression ratio can be obtained if dictionary registration does not progress to some extent. There is a problem that can not be.

The present invention has been made in view of such conventional problems, and has as its object to provide a data compression and decompression method using an LZW code that can increase a compression ratio by initial registration of a dictionary.

[Means for Solving the Problems] FIG. 1 is an explanatory view of the principle of the present invention.

First, the present invention divides the encoded data into different sub-sequences, registers the sub-sequences in the dictionary 10, and inputs data with a registration number of the sub-sequences in the dictionary 10 that matches the maximum length. The present invention is directed to a data compression method using an LZW code including an encoding unit 100 that performs designation and encoding.

In the present invention regarding such a data compression method,
A registration number is assigned to each input character in advance and is initially registered in the dictionary 10, and a registration number is assigned in advance to the dictionary 10 by assigning a registration number to a character string having two or more characters and having a high appearance frequency. The encoding means 100 is used to compress and encode the input data. A character string in which one character is continuous in the data to be compressed;
For example, a registration number is previously assigned to aa, aaa, aaaa, aaaaa, and the like, and is initially registered in the dictionary 10.

Further, according to the present invention, the encoded data is divided into different sub-sequences, the sub-sequences are registered in the dictionary 10, and the input data is registered by the registration number of the sub-sequences in the dictionary 10 which matches the maximum length. The present invention is directed to a data restoration method including a decoding unit 200 for restoring a designated and encoded codeword to original data using the dictionary 10.

In the present invention, such a data restoration method is also applied to a dictionary in which a registration number is assigned in advance to each input character.
In addition to the initial registration in the dictionary 10, a registration number is assigned in advance to a character string having two or more characters and having a high frequency of occurrence, and is initially registered in the dictionary 10, and the decoding unit 200 uses the dictionary 10 to restore the input data. It is characterized by the following. In a data string to be compressed, a character string in which one character is continuous plural times, for example,
A registration number is assigned in advance to aa, aaa, aaaa, aaaaa, and the like, and is initially registered in the dictionary 10.

[Operation] According to the data compression and decompression method of the present invention having such a configuration, a character string having a high appearance frequency, for example, a character string in which the same characters are consecutively registered in advance according to the number at the initialization of the dictionary. If a registered character string λ appears in advance when a character string is registered, a high compression can be realized by performing compression processing using the registered number.

[Embodiment] Fig. 2 is an embodiment configuration diagram showing one embodiment of the present invention.

In FIG. 2, reference numeral 12 denotes a CPU as control means,
The program memory 14 and the data memory 26 are connected to the PU 12.

The program memory 14 has control software 16 and LZW
Maximum match length search software 18 that performs maximum match length search using codes, encoding software 2 that converts input character strings to LZW codes
0, decoding software 22 for restoring the code converted to the LZW code by the encoding software 20 to the original character string, and 1 to be processed
In addition to each of the characters, a frequently used character string λ, for example, in this embodiment, the same character a
An initial registration software 24 is provided for initially registering a character string of up to 15 consecutive characters with a registration number.

On the other hand, the data memory 26 stores a character string to be encoded or a code string to be decoded from now on, and a data buffer 28 which is sequentially created at the time of encoding and decoding for the LZW code. It has a dictionary 10 which is used while using.

Initial registration of the dictionary according to the present invention in the embodiment of FIG. 2 is performed as follows.

First, the CPU 12 activates the initial registration software 24 under the control of the control software 16, and performs initial registration of the dictionary 10. That is, based on the initial registration software 24, the CPU 12 assigns a registration number (reference number) to each of the characters of the character type to be processed and performs initial registration in the dictionary 10.

Assuming that three characters a, b, and c are to be processed for simplicity of description, as shown in the dictionary configuration explanatory diagram of FIG. 5, characters a, b, and c are assigned reference numerals 1, 2, and 3, and Initial registration.

In addition to this, in the present invention, as a character string λ having a high appearance frequency, a character string in which 2 to 10 consecutive characters a are consecutive as shown in the dictionary configuration of FIG. Initially register in the dictionary by assigning registration numbers 4 to 12 to aaaaaaaaaa. The fifth
In the figure, they are shown as a × 2, a × 3,..., A × 10.

After the initial registration for such a dictionary 10,
Desired input data or encoded data is stored in the data buffer 28 of the data memory 26, and encoding processing by the encoding software 20 or decoding processing based on the decoding software 22 is performed.

FIG. 3 shows an encoding algorithm of the LZW code according to the present invention, and FIG. 4 shows a decoding algorithm.

Here, the encoding of the LZW code in FIG. 3 will be described with reference to the input characters in FIG. 5. First, in S1, the character code i is registered in the dictionary address i one character at a time. A character is λ, and not only one character λ but also a plurality of types of character strings are registered in the dictionary according to the number of λ. For example, in the example of the dictionary configuration of FIG. 6 for characters a and b, one to ten consecutive character strings of the character a are registered.

When the initial registration of S1 is completed, a process in the case where a plurality of characters λ initially registered by the processes of S2 to S7 are consecutive. Immediately, K = λ is determined in S3, the λ counter indicating the continuous number of characters λ is incremented by one in S4, and the process is repeated until the input character K is no longer a character λ. That is, since the first input character a in FIG. 5 is the character λ, the process proceeds from S3 to S4 and proceeds to λ.
Increment the counter by one. Since the next input character b is not the character λ, the process proceeds to S5.
Since it is 1, the flow advances to S6 to output the value of the λ counter as the code code CODE (λ) = CODE1, and the character b input second in S7 is set as a new initial character string ω.

The following S8, S9, S11, S12, S13 and S22 are the same as the conventional LZW encoding processing shown in FIG. 8, and the reference numerals in FIG. 8 are shown in parentheses.

In the present invention, during the processing of the LZW encoding that is the same as the conventional S, processing of a character λ with a high appearance frequency that newly branches from S10 to the processing of S14 to S21 is included.

Specifically, after the character b input second in S7 is converted into a multi-word character string ω, the third character a is read in S8, K = λ is determined in S10 through S9, and the processing in S14 is performed. Proceed to.

In S14, after the initial character string ω = b at that time is output as the code code CODE (ω) = CODE2, the λ counter 1 is incremented by one in S15, and the next character b is read in S16. K = λ is determined in S19 through S18. In this case, since the character b is not λ, code is used as a code in S20.
(Λ) = code1 is output, and after resetting to λ = 0, the process returns to S7 again.

On the other hand, from the 13th input character in FIG. 5, there are seven consecutive characters a as λ.
The repetition of the process of 9 yields the count value λ = 7 of the λ counter, and the code 9 is output as the code code in S20.

Here, in the case of a character string in which 10 or more characters a are continuous, for example, in the case where 15 characters a are continuous, they are represented using two code codes of reference numerals 12 and 7. In the data encoding method of the present invention, when a character corresponding to λ appears during the encoding process, the character string ω up to that point is output as a code code in S14, and the dictionary of the character string ωK = ωλ is registered. Absent.

 Next, the decoding process according to the present invention shown in FIG. 4 will be described.

In the decoding process of FIG. 4, S1 to S4, S8 and S9 are provided for decoding a code code indicating the continuous number of characters λ having a high appearance frequency, and the other processes are performed in FIG. Is the same as the conventional decoding processing shown in FIG.

Taking the case of restoring the input code code shown in FIG. 7 as an example, the decoding up to the twelfth code code is the same as the conventional decoding. However, for the code code code1, S4 or S9
Output one λ = a.

The 13th code code code9 in FIG. 7 is determined to be a λ code in S8, and the process advances to S9 to output seven characters a by referring to the dictionary with the code code value 9.

In the above-described embodiment, a character string in which a plurality of the same characters are consecutive is used as an example of a character string to be initially registered. However, the present invention is not limited to this. If so, an appropriate character string may be initially registered.

[Effects of the Invention] As described above, according to the present invention, character strings representing a plurality of numbers are registered and used in advance in a dictionary in consideration of the frequency of use of the character strings, and compressed using the dictionary. By performing the restoration, the compression ratio of the data compression of the LZW code can be further increased.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining the principle of the present invention; FIG. 2 is a block diagram of an embodiment of the present invention; FIG. 3 is a flowchart of an LZW code encoding process according to the present invention; FIG. 5 is an explanatory diagram of LZW code encoding according to the present invention; FIG. 6 is an explanatory diagram of dictionary configuration according to the present invention; FIG. 7 is an explanatory diagram of LZW code restoration according to the present invention; FIG. 9 is a conventional LZW encoding processing flow diagram; FIG. 9 is a conventional LZW decoding processing flowchart; FIG. 10 is a conventional LZW encoding explanatory diagram; FIG. 11 is an explanatory diagram of a conventional dictionary configuration example; The figure is an explanatory diagram of conventional LZW decoding. In the figure, 10: Dictionary 12: CPU 14: Program memory 16: Control software 18: Maximum matching length search software 20: Encoding software 22: Decoding software 24: Initial registration software 26: Data memory 28: Data buffer 100: Code Decryption means 200: decryption means

────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Yasuhiko Nakano 1015 Uedanaka, Nakahara-ku, Kawasaki-shi, Kanagawa Prefecture Inside Fujitsu Limited (56) References JP-A-60-116228 (JP, A) JP-A-61-232724 (JP, A) JP-A-63-209228 (JP, A) JP-A-63-209229 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H03M 7/40

Claims (4)

(57) [Claims] An encoded data is divided into different sub-sequences, the sub-sequences are registered in a dictionary (10), and input data is stored in a maximum length of the sub-sequences in the dictionary (10). In a data compression system including an encoding means (100) for designating and encoding a match with a registration number, a registration number is assigned in advance to each input character, and is initially registered in the dictionary (10). The dictionary (10) in which a registration number is previously assigned to a character string having a high appearance frequency of two or more characters.
A data compression method, wherein input data is compression-coded by the coding means (100) using the dictionary (10). 2. A data compression method according to claim 1, wherein a registration number is previously assigned to a character string in which a plurality of characters have a plurality of consecutive characters in the data to be compressed, and the character string is initially registered in said dictionary (10). Data compression method. 3. The encoded data is divided into different subsequences, the subsequences are registered in a dictionary (10), and the input data is stored in a maximum length of the subsequences in the dictionary (10). Decoding means (20) for restoring the code word specified and coded by the registration number to the original data using the dictionary (10)
0), a registration number is assigned to each input character in advance and is initially registered in the dictionary (10), and a registration number is assigned in advance to a character string having two or more characters and having a high appearance frequency. Allocate the dictionary (10)
Data decoding method, wherein input data is restored by the decoding means (200) using the dictionary (10). 4. The data restoration method according to claim 3, wherein a registration number is previously assigned to a character string in which a plurality of characters have a plurality of consecutive characters in the data to be compressed, and the character string is initially registered in the dictionary (10). Data restoration method.