Method and apparatus for encoding image data, and computer-readable recording medium in which program for encoding image data is recorded

An apparatus for encoding image data representing a source image, the image data having a multiplicity of blocks each of which includes a number of pixels whose tone values are to be transformed into transform coefficients by an orthogonal transformation, said apparatus comprising:

• sub-area defining means for defining a plurality of sub-areas so that each of the plural sub-areas has one or more of the multiple blocks;
• quantization means for quantizing the transform coefficients of each of the multiple blocks to obtain quantization coefficients;
• number counting means for counting the number of valid coefficients among the quantization coefficients of each of the plural sub-areas;
• adjusting means for adjusting the quantization coefficients of the multiple blocks in accordance with a distribution of the numbers of valid coefficients over the plural sub-areas (that is, in accordance with a degree of appearance of the valid coefficients of each said sub-area), the numbers being counted by said number counting means, in such a manner that the quantization coefficients are to be encoded to a desired data volume;
• and
• encoding means for encoding the adjusted quantization coefficients.

A computer-readable recording medium in which a program for encoding image data representing a source image, the image data having a multiplicity of blocks, each of which includes a number of pixels whose tone values are to be transformed into transform coefficients by an orthogonal transformation, said program instructing a computer to function as:

• sub-area defining means for defining a plurality of sub-areas so that each of the plural sub-areas has one or more of the multiple blocks;
• quantization means for quantizing the transform coefficients of each of the multiple blocks to obtain quantization coefficients;
• number counting means for counting the number of valid coefficients among the quantization coefficients of each of the plural sub-areas;
• adjusting means for adjusting the quantization coefficients of the multiple blocks in accordance with a distribution of the numbers of valid coefficients over the plural sub-areas (that is, in accordance with a degree of appearance of the valid coefficients of each said sub-area), the numbers being counted by said number counting means, in such a manner that the quantization coefficients are to be encoded to a desired data volume;
• and
• encoding means for encoding the adjusted quantization coefficients.