| 摘要 |
Methods and apparatus are provided for compressing and decompressing image data by producing two sets of reduced size image data and generating a modulation value for each elementary of the area from the image data, the modulation value encoding information about how to combine the sets of reduced size image data to generate an approximation to the image. In one arrangement, a set of index values is generated corresponding to a set of modulation values for each of the respective elementary areas of a group of elementary areas and these are assigned to each respective group and a second set of index values corresponding to one of the set of first index values for each elementary areas is assigned to each first group of elementary areas. These index values are then stored for use in deriving modulation data more accurately when decompressing the image data. In another arrangement a modulation mode is selected which includes a plurality of modulation values and one of the modulation values is assigned to each elementary area in each block of elementary areas of the image for use in recombining the reduced size image data to generate an approximation to the image. The modulation value is selected with a plurality of bits of index data for each elementary area. In another arrangement, a modulation from a set of possible modulation values is assigned to each elementary area in a group of elementary areas and is represented by a plurality of bits of modulation data. This arrangement is varied by providing modulation values which relate to each alternate elementary area and then either interpolating modulation values vertically or horizontally for each other elementary area or by interpolating either vertically or horizontally image data once sets of reduced sized image data have been recombined with the modulation data. |
