Method and apparatus for compressing and expanding image data

A compression method for compressing image data comprising:

• transforming original image data partitioned into first blocks, each of which is composed of a plurality of pixels, to reduced-image data composed of a smaller number of pixels than that of the original image data;
• applying a fluency transform to the reduced-image data to generate expanded-image data partitioned into second blocks corresponding to the first blocks, the fluency transform having a plurality of modes;
• selecting one mode from the plurality of modes, the expanded-image data being generated in accordance with the selected one mode;
• calculating an error in each of the plurality of modes, the error representing, a difference between the original image data and the expanded-image data;
• determining an optimum mode, by which the error becomes minimum, among the plurality of modes;
• and
• recording the reduced-image data and the optimum mode in a recording medium.

A memory medium that stores a program for compressing image data, the program comprising:

• transforming original image data partitioned into first blocks, each of which is composed of a plurality of pixels, to reduced-image data composed of a smaller number of pixels than that of the original image data;
• applying a fluency transform to the reduced-image data to generate expanded-image data partitioned into second blocks corresponding to the first blocks, the fluency transform having a plurality of modes;
• selecting one mode from the plurality of modes, the expanded-image data being generated in accordance with the selected one mode;
• calculating an error in each of the plurality of modes, the error representing, a difference between the original image data and the expanded-image data;
• determining an optimum mode, by which the error becomes minimum, among the plurality of modes;
• and
• recording the reduced-image data and the optimum mode in a recording medium.

A compression apparatus for compressing image data comprising:

• a reduced-image generating processor that transforms original image data partitioned into first blocks, each of which is composed of a plurality of pixels, to reduced-image data composed of a smaller number of pixels than that of said original image data;
• a fluency transform processor that applies a fluency transform to said reduced-image data to generate expanded-image data partitioned into second blocks corresponding to said first blocks, said fluency transform having a plurality of modes;
• a contrast coefficient calculating processor that obtains a contrast coefficient in such a manner that a contrast of said expanded-image data becomes close to a contrast of said original image data by applying said contrast coefficient to said expanded-image data;
• a contrast transform processor that applies said contrast coefficient to said expanded-image data to modify the contrast of said expanded-image data so that modified expanded-image data is obtained;
• a mode setting processor that selects one mode from said plurality of modes, said modified expanded-image data being generated in accordance with said selected one mode;
• an error calculating processor that calculates an error, which represents a difference between said original image data and said modified expanded-image data, said error calculating processor calculating in each of said plurality of modes;
• and
• an optimum mode/contrast determining processor that determines an optimum mode, by which said error becomes minimum, among said plurality of modes, and then sets a contrast coefficient corresponding to said optimum mode as an optimum contrast coefficient.