Image compression system and method of determining quantization parameters therefor

An image compression system comprising:

• a wavelet transform module having an input and an output, the input being coupled to an input terminal of the image compression system for receiving image data in the form of image blocks and the output providing wavelet transform coefficients for each image block;
• a quantisation module having an input and an output, the input being coupled to the output of the wavelet transform module and the output providing quantised wavelet transform coefficients for each image block;
• a coding module having an input and an output, the input being coupled to the output of the quantisation module and the output being coupled to an output terminal of the image compression system for providing compressed image data comprising coded quantised wavelet transform coefficients for each image block;
• an activity value generator with a memory, the activity generator having an input and an output, the input being coupled to the output of the wavelet transform module;
• and
• a quantisation control module having first and second inputs and an output, the first input being coupled to the output of the activity value generator, the second input being coupled to receive an available bit rate, and the output being coupled to a control input of the quantisation module,
• wherein the activity value generator generates activity values representing a statistical analysis of the wavelet transform coefficients, and in use the memory stores updated average activity values calculated using the activity values calculated for a current image block and previous average activity values stored in the memory, and wherein the updated average activity values are provided at the output of the activity value generator.