Adaptive runlength coding

Runlength coding is the standard coding technique for block transform based image/video compression. A block of quantized transform coefficients is first represented as a sequence of RUN (number of consecutive zeros) / LEVEL (the value of the following nonzero coefficient) pairs which are then entropy coded. We point out in this paper the inefficiency of conventional runlength coding and introduce a novel adaptive runlength coding scheme that encodes RUN and LEVEL symbols separately using context based adaptive binary arithmetic coding. We aim to maximize compression efficiency by adaptively exploiting the characteristics of block transform coefficients and the dependency between RUN and LEVEL. Coding results show that, with the same level of complexity, the proposed adaptive runlength coding algorithm outperforms the conventional runlength coding scheme by a wide margin in the rate-distortion (R-D) sense.