Image compression device, compressing method, storage medium, image decompression device, decompressing method, and storage medium

An image compression device comprising a prediction unit for predicting a value of a decompressing target pixel from a value for peripheral values relative to the decompressing target pixel, and a quantizer for quantizing a prediction error calculated from the prediction value calculated by the prediction unit and an actual pixel value, wherein

• the quantizer comprises a plurality of quantization tables for quantizing the prediction error in different quantizing steps, wherein
• the image compression device comprises:
• a quantization table switching unit for outputting a control signal for switch of the quantization table to the quantizer;
• and
• a specific pixel level difference evaluation unit sequentially calculating a difference value between level values as a sum of the prediction value corresponding to each of the compressed pixel immediately before the compressing target pixel and peripheral compressed pixels adjacent to the pixel and a quantization value of the prediction error, and evaluating sameness of the difference value, wherein
• the specific pixel level difference evaluation unit outputs a instruction signal to switch a quantization table to perform quantization in a smaller quantizing step to the quantization table switching unit when the device detects the same difference value continuously.

A computer-readable storage medium on which is recorded a program, which is used to operate an image compression device having a prediction unit for predicting a value of a compressing target pixel from peripheral values relative to the compressing target pixel, and quantizing prediction error between a prediction value of the prediction unit and a value of an actual pixel in a predetermined quantizing step, used to direct the image compression device to function as:

• a specific pixel level difference evaluation unit reading from a buffer holding at least one line of a level value as a sum of the prediction value corresponding to each pixel and the prediction error quantization value the level value for a first pixel as a compressed pixel immediately before the compressing target pixel to be compressed on the same line and the level value for a second pixel in the same column position as the first pixel on the line on which a compressing process is performed immediately before the line, sequentially calculating a difference value between the level values, and evaluating sameness of the difference values;
• and
• a quantization switching device outputting control information to perform the quantization in a smaller step than the quantizing step used when the difference value continuously indicates the same value as a result of evaluation by the specific pixel level difference evaluation unit.

An image decompression device comprising a prediction unit for predicting a value of a decompressing target pixel from for peripheral values relative to the decompressing target pixel, and an inverse quantizer for inverse quantizing a prediction value calculated by the prediction device and a quantization number calculated in advance by the image compression device and calculating a prediction error quantization value, wherein

• the inverse quantizer comprises a plurality of inverse quantization tables for inverse quantization in different quantizing steps, wherein
• the image decompression device comprising:
• a quantization table switching unit for outputting a control signal for switch of the inverse quantization table to the inverse quantizer;
• and
• a specific pixel level difference evaluation unit sequentially calculating a difference value between the decompression values as a sum of the prediction value corresponding to each of decompressed pixel immediately before the decompressing target pixel and peripheral decompressed pixels adjacent to the pixel and a quantization value of the prediction error, and evaluating sameness of the difference value, wherein
• the specific pixel level difference evaluation unit outputs a instruction signal to switch a inverse quantization table to perform inverse quantization in a smaller quantizing step to the quantization table switching unit when the device detects the same difference value continuously.

A computer-readable storage medium on which is recorded a program, which is used to operate an image decompression device having a prediction unit for predicting a value of a decompressing target pixel from a value of peripheral decompressed pixel and calculating a sum of a prediction value of the prediction unit and a prediction error quantization value obtained by inverse quantizing a quantization number calculated in advance by an image compression device in a predetermined quantizing step, wherein

• the image decompression device operates as:
• a specific pixel level difference evaluation unit reading from a buffer holding at least one line of a decompression value as a sum of the prediction value corresponding to each pixel and the prediction error quantization value the decompression value for a first pixel as a decompressed pixel immediately before the decompressing target pixel on the same line and the decompression value for a second pixel in the same column position as the first pixel on the line on which a decompressing process is performed immediately before the line, sequentially calculating a difference value between the decompression values, and evaluating sameness of the difference values;
• and
• a quantization switching unit outputting control information to perform the inverse quantization in a smaller step than the quantizing step used when the difference value continuously indicates the same value as a result of evaluation by the specific pixel level difference evaluation unit.