Adaptive block size image compression method and system

An image signal compression system comprising:

• transform means for, receiving an input block of pixel data, performing a discrete cosine transform (DCT) operation on said block of pixel data and on at least one predetermined level of constituent sub-blocks thereof, and providing corresponding block and sub-blocks of DCT coefficient values;
• block size assignment means for, receiving said block and each sub-block of DCT coefficient values, determining for said block and each corresponding group of sub-blocks of DCT coefficient values a bit count value corresponding to a number of bits required to respectively encode said block and corresponding group of sub-blocks of DCT coefficient values according to a predetermined coding format, determining from said bit count values ones of said block and corresponding group of sub-blocks of DCT coefficient values requiring a lesser number of bits to encode according to said coding format, and providing a corresponding selection value;
• selection means for, receiving said selection value and said block and each sub-block of DCT coefficient values, selecting ones of block and sub-blocks of DCT coefficients corresponding to said selection value, and providing an output of a corresponding composite block of DCT coefficient values formed from said selected ones of block and sub-blocks of DCT coefficient values;
• ordering means for, receiving said composite block of DCT coefficient values, ordering said composite block DCT coefficient values according to a predetermined ordering format, and providing a corresponding output of ordered DCT coefficient values;
• encoder means for, receiving said ordered DCT coefficient values, coding said ordered DCT coefficient values according to said predetermined coding format, and providing corresponding coded values;
• and
• assembler means for, receiving said selection value and said coded values, combining said selection value and said coded value as a coded image value representative of said input block of pixel data with said coded image value of a reduced bit count with respect to a bit count of said input block of pixel data, and providing an output of said coded image value.

A method for compressing an image signal corresponding to a block of pixel data, comprising the steps of:

• providing a block of pixel data capable of division into varying levels of constituent sub-blocks;
• performing a discrete cosine transform (DCT) operation on said block of pixel data so as to provide a corresponding block of DCT coefficients;
• performing a DCT operation on each sub-block of pixel data in at least one level of pixel data sub-blocks so as to provide corresponding sub-blocks of DCT coefficients;
• determining for each block and corresponding group of sub-blocks of DCT coefficients a bit count value corresponding to a number of bits necessary to encode each block and group of sub-blocks according to a predetermined coding format;
• determining, from said bit count values, ones of said block and corresponding groups of sub-blocks of DCT coefficients corresponding to a lower number of bits necessary to encode each block and corresponding group of sub-blocks of DCT coefficients so as to provide a corresponding block size assignment value;
• assembling, according to said block assignment value, a composite block of DCT coefficients from said ones of said block and corresponding groups of sub-blocks of DCT coefficients having a lower bit encoding number;
• ordering said composite block of DCT coefficients according to a predetermined ordering format so as to provide a serial string of ordered DCT coefficients;
• encoding said string of ordered DCT coefficients according to said predetermined coding format so as to provide a set of coded DCT coefficients;
• and
• assembling said block assignment value and said coded DCT coefficients into coded image data representative of said block of pixel data, said coded image data being of a reduced bit count with respect to a bit count of said block of pixel data.