Context adaptive data encoding

A image data decoding method for decoding successive arithmetic-encoded data values, the method comprising:

• moving a read pointer back in an input data stream by a predetermined number of bit positions to avoid interpreting the predetermined number of bit positions from a next input data stream;
• defining a range of a set of code values as zero;
• reading, from the input data stream, a stream value indicating a parameter of a set of code values;
• setting the proportions of respective sub-ranges relative to the set of code values according to a context variable;
• and detecting by circuitry a current decoded output data value according to which sub-range the stream value lies in.

A image data encoding apparatus comprising:

• a selector configured to select one of a plurality of complementary sub-ranges of a set of code values according to the value of a current input data value, proportions of the sub-ranges relative to the set of code values being defined by a context variable associated with the input data value;
• a data assigning circuit configured to assign the current input data value to a code value within the selected sub-range;
• a data modifying circuit configured to modify the set of code values in dependence upon the assigned code value and a size of the selected sub-range;
• a detector configured to detect whether the set of code values is less than a predetermined minimum size and if so, successively increasing the size of the set of code values until reaching at least the predetermined minimum size;
• to output an encoded data bit in response to each such size-increasing operation;
• and to modify the context variable, for use in respect of a next input data value, to increase the proportion of the set of code values in the sub-range which was selected for the current data value;
• and after encoding a group of input data values, a data terminator to terminate the output data by:
• setting a value defining an end of the set of code values to a value having a plurality of least significant bits equal to zero to have the plurality of least significant bits ignored to reduce a number of bits to be outputted at the termination of the output data;
• increasing the size of the set of code values;
• and writing the value defining the end of the set of code values to the output data.

A image data decoding apparatus comprising:

• a pointer controller configured to move a read pointer back in an input data stream by a predetermined number of bit positions to avoid interpreting the predetermined number of bit positions from a next input data stream;
• a setting circuit configured to define a range of a set of code values as zero, to read, from the input data stream, a stream value indicating a parameter of a set of code values;
• to set the proportions of respective sub-ranges relative to the set of code values according to a context variable;
• and to detect a current decoded output data value according to which sub-range the stream value lies in.