Enhanced method and apparatus for image compression

A method of image compression involving a variable-to-variable coding scheme, whereby a variable number of bits are used to encode for each rectangular block of 4 samples, herein identified as a quad:

• i) a significance of each sample in the quad, herein identified as a significance pattern, comprising 4 bits with one bit for each sample in the quad indicating whether the sample in the quad is zero or non-zero;
• ii) a variable amount of information from a 4 bit EMB pattern for the quad, said EMB pattern consisting of the magnitude bits from each of the quad's samples that are found within a bounding magnitude bit-plane for the quad, said bounding magnitude bit-plane having the property that no samples of the quad have non-zero magnitude bits in any higher bit-plane;
• iii) a 1-bit unsigned residual offset value that indicates whether said bounding magnitude bit-plane is greater than a reference value or equal to the reference value, wherein the 1-bit unsigned residual value for the quad being 0 indicates the said bounding magnitude bit plane is equal to the reference value and the 1-bit unsigned residual value for the quad being 1 indicates the said bounding magnitude bit plane is greater than the reference value, said reference value depending on the sample values associated with previously encoded quads,
• the method involving a lookup table that an encoder addresses in order to find a suitable codeword from a context dependent variable length (CxVLC) code, wherein the CxtVLC code includes codewords that communicate EMB pattern information for specific values of a significance pattern for the quad when an unsigned residual offset value for the quad is 0,
• the method comprising the steps of:
• determining for the quad a modified significance pattern and a modified EMB pattern, each consisting of 1 bit per sample in the quad, based on the original significance pattern for the quad, the original EMB pattern for the quad, and the 1-bit unsigned residual offset value for the quad,
• the modified EMB pattern including some or all of the original EMB pattern for the quad both when the unsigned residual offset value is 1 and, for specific values of the significance pattern for the quad, when the unsigned residual offset value for the quad is 0,
• whereby the 8 bits of the modified significance and the modified EMB pattern infer the residual offset value for the quad, in addition to the original significance pattern for the quad, through transformations that exploit the property that whenever any EMB bit is 1, the corresponding significance bit must be 1.