Encoding device and encoding method for changing code amount...

Details Encoding device and encoding method for changing code amount... Encoding device and encoding method for changing code amount...

2001-04-25

2002-05-14

Tokar, Michael (Department: 2819)

Coded data generation or conversion

Digital code to digital code converters

To or from variable length codes

C341S050000, C341S059000, C341S065000, C341S067000, C382S244000, C382S245000, C382S246000, C382S250000, C382S251000, C382S282000

Reexamination Certificate

active

06388588

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to an encoding device and an encoding method. More specifically, the present invention relates to an encoding device and an encoding method for changing a code amount of already compressed image data.
JPEG (Joint Photographic Experts Group) is an international image compression/expansion standard that is efficient for still images. In recent years, compressed image files compliant to the JPEG standard (hereinafter referred to as “JPEG files”) have been widely used on the Internet, and the Internet users can easily download JPEG files. The Internet users can download the JPEG files and store them in a storage medium such as a hard disk or a memory.
In some cases, a user may wish to further reduce the size (code amount) of a downloaded JPEG file in view of the storage capacity of the storage medium. However, if the JPEG file has not been created by the user but has been obtained from a medium such as the Internet, the user does not have the uncompressed original image file. In such a case, it is necessary to first (1) produce recovered image data by performing an image expansion operation on the downloaded JPEG file, and then (2) compress the recovered image data at an increased compression rate.
Such an operation can be performed by using an image compression/expansion device as illustrated in
FIG. 43 through a
procedure as illustrated in FIG.
44
.
First, compressed image data JPGDI stored in a code memory
7
is read out (ST
201
).
The read-out compressed image data JPGDI is separated by a separation circuit
8
into quantization step values DQT(D), code table information DHT(D) and variable-length codes CODE(D) (ST
202
).
The quantization step values DQT(D) are set in a quantization table
12
, and the code table information DHT(D) is set in a code table
13
(ST
203
).
A variable-length decoding section
9
c
produces a run length RRRR(D)/size SSSS(D) combination from each variable-length code CODE(D), while extracting an additional bit string ADBIT(D) therefrom, based on the code table information set in the code table
13
(ST
204
).
A coefficient generation section
9
b
extracts an effective coefficient Sq(D) by using the size SSSS(D) and the additional bit string ADBIT(D) (ST
205
).
A run-length decoding section
9
a
produces a DCT coefficient ZZ(D) from the run length RRRR(D)/effective coefficient Sq(D) combination (ST
206
).
An inverse quantization section
10
inverse-quantizes each DCT coefficient ZZ(D) according to a corresponding quantization step value set in the quantization table
12
(ST
207
), and an inverse Discrete Cosine Transform (hereinafter referred to as “DCT”) section
11
further performs a two-dimensional inverse DCT on the obtained data (ST
208
). The results of the inverse DCT are sequentially written into an image memory
2
as recovered image data PXLDO (ST
209
).
After the recovered image data PXLDO is obtained by performing an image expansion operation on the compressed image data as described above, the obtained data is compressed again at an increased compression rate. In order to produce compressed image data of a high compression rate, the quantization step values are read out from the quantization table
12
through an external reading terminal
21
(ST
211
), changed to larger values, and then re-set in the quantization table
12
(ST
212
).
Then, recovered image data PXLDI is read out from the image memory
2
and input to a DCT section
3
(ST
214
).
The DCT section
3
performs a two-dimensional DCT operation to obtain DCT coefficients (ST
215
).
A quantization section
4
quantizes the DCT coefficients according to the quantization step values set in the quantization table
12
in step ST
212
(ST
216
).
The quantized DCT coefficients ZZ(E) are sequentially supplied to, and run-length-encoded in, a run-length encoding section
5
a
in the order of zigzag scanning, thereby obtaining run length RRRR(E)/effective coefficient Sq(E) combinations (ST
217
).
A size/additional bit string generation section
5
b
assigns a size SSSS(E) and an additional bit string ADBIT(E) to each effective coefficient Sq(E) (ST
218
).
A variable-length encoding section
5
c
assigns a variable-length code to each run length RRRR(E)/size SSSS(E) combination based on the information set in the code table
13
. The additional bit string ADBIT(E) assigned by the size/additional bit string generation section
5
b
is attached to the end of the variable-length code, thereby producing a variable-length code CODE(E) (ST
219
).
The variable-length code CODE(E) is assigned to the run length RRRR(E)/size SSSS(E) combination as described above.
A multiplexing circuit
6
multiplexes together the variable-length codes CODE(E), the quantization step values DQT(E) and the code table information DHT(E) to produce new compressed image data JPGDO (ST
220
).
If the code amount of the obtained compressed image data JPGDO is not of a desired size, the above-described image compression operation (ST
221
-ST
212
-ST
221
) is repeated while changing the quantization step values. Thus, it is possible to obtain compressed image data JPGDO having a desired code amount.
As described above, when changing the code amount of a compressed image with the conventional image compression/expansion device, it is necessary to perform all of the various image expansion operations (such as variable-length decoding, run-length decoding, inverse quantization, and inverse DCT) to produce a recovered image, change the quantization step values, and then perform all of the various image compression operations (such as DCT, quantization, run-length encoding, and variable-length encoding).
SUMMARY OF THE INVENTION
An object of the present invention is to provide an encoding device capable of efficiently changing the code amount(compression rate) of compressed image data.
According to one aspect of the present invention, there is provided an encoding device for changing a code amount of compressed image data which has been produced by performing a Discrete Cosine Transform (hereinafter referred to as “DCT”) operation, a quantization operation, a run-length encoding operation and a variable-length encoding operation on original image data, the encoding device including separation means, decoding means, a size conversion table, size conversion means, variable-length encoding means, and multiplexing means.
The separation means separates the compressed image data into variable-length codes, quantization step values and code table information. The code table information represents a correspondence between combinations of run length and size and code words. The decoding means decodes each of the variable-length codes from the separation means into a run length, a size and an additional bit string based on the code table information. A frequency region and a size conversion amount are set in the size conversion table. The size conversion means performs a conversion operation on the run length, the size and the additional bit string obtained by the decoding means based on the frequency region and the size conversion amount set in the size conversion table. The variable-length encoding means performs a variable-length encoding operation on the run length, the size and the additional bit string converted by the size conversion means based on the code table information. The multiplexing means multiplexes together variable-length codes obtained by the variable-length encoding means, the quantization step values and the code table information.
In the encoding device, the size conversion means changes the value of each size belonging to the frequency region set in the size conversion table among the sizes obtained by the decoding means by the size conversion amount set for the frequency region. Moreover, the size conversion means also changes the additional bit string and the run length according to the change in the size. Therefore, the code amount of a variable-length code obtained by the variable-length encoding means is different from the code amount of

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