Encoding device and encoding method for changing code amount of compressed image data

An encoding device for changing a code amount of compressed image data which has been produced by performing a DCT operation, a quantization operation, a run-length encoding operation and a variable-length encoding operation on original image data, the encoding device comprising:

• separation means for separating the compressed image data into variable-length codes, quantization step values and code table information representing a correspondence between combinations of run length and size and code words;
• decoding means for decoding 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;
• coefficient generation means for specifying an effective coefficient based on the size and the additional bit string obtained by the decoding means;
• a coefficient conversion table in which a frequency region and a coefficient conversion amount are set;
• coefficient conversion means for performing a conversion operation on the run length obtained by the decoding means and the effective coefficient specified by the coefficient generation means based on the frequency region and the coefficient conversion amount set in the coefficient conversion table;
• size/additional bit string generation means for assigning a size and an additional bit string to the effective coefficient converted by the coefficient conversion means;
• variable-length encoding means for performing a variable-length encoding operation on the run length converted by the coefficient conversion means and the size and the additional bit string assigned by the size/additional bit string generation means based on the code table information;
• and
• multiplexing means for multiplexing together variable-length codes obtained by the variable-length encoding means, the quantization step values and the code table information.

An encoding method for changing a code amount of compressed image data which has been produced by performing a DCT operation, a quantization operation, a run-length encoding operation and a variable-length encoding operation on original image data, the encoding method comprising:

• a step of separating the compressed image data into variable-length codes, quantization step values and code table information representing a correspondence between combinations of run length and size and code words;
• a step of decoding each of the variable-length codes into a run length, a size and an additional bit string based on the code table information;
• a step of setting a frequency region and a size conversion amount;
• a step of performing a conversion operation on the run length, the size and the additional bit string obtained in the decoding step based on the frequency region and the size conversion amount which have been set;
• a step of performing a variable-length encoding operation on the run length, the size and the additional bit string converted in the conversion step based on the code table information;
• and
• a step of multiplexing together variable-length codes obtained in the variable-length encoding step, the quantization step values and the code table information,
• wherein if a code amount of compressed image data obtained in the multiplexing step is not a desired value, the frequency region and/or the size conversion amount set in the setting step are/is changed, and the conversion step, the variable-length encoding step and the multiplexing step are performed again.

An encoding method for changing a code amount of compressed image data which has been produced by performing a DCT operation, a quantization operation, a run-length encoding operation and a variable-length encoding operation on original image data, the encoding method comprising:

• a step of separating the compressed image data into variable-length codes, quantization step values and code table information representing a correspondence between combinations of run length and size and code words;
• a step of decoding each of the variable-length codes into a run length, a size and an additional bit string based on the code table information;
• a step of specifying an effective coefficient based on the size and the additional bit string obtained in the decoding step;
• a step of setting a frequency region and a coefficient conversion amount;
• a step of converting the run length obtained in the decoding step and the effective coefficient specified by the specification step based on the frequency region and the coefficient conversion amount which have been set;
• a step of assigning a size and an additional bit string to the effective coefficient converted in the conversion step;
• a step of performing a variable-length encoding operation on the run length converted in the conversion step and the size and the additional bit string assigned in the assignment step based on the code table information;
• and
• a step of multiplexing together variable-length codes obtained in the variable-length encoding step, the quantization step values and the code table information,
• wherein if a code amount of compressed image data obtained in the multiplexing step is not a desired value, the frequency region and/or the coefficient conversion amount set in the setting step are/is changed, and the conversion step, the assignment step, the variable-length encoding step and the multiplexing step are performed again.