关键词: 图像压缩, 分形, 分形图像编码, 四分位数特征

Abstract: Although fractal image coding has excellent visual quality at high compression ratio，it has not been widely applied due to exhaustive inherent encoding time.The time is mostly spent on searching for the best-matched block to every range in a usually large domain pool;therefore，a fast algorithm is proposed to limit the search space in this paper.The search scope of best-matched block for an input range block is local against full on the basis of an inequality linking the root-mean-square and newly-defined quartiles feature of normalized block.In detail，it can effectively confine the search space to the vicinity of the initial-matched block （i.e.，the domain block having the closest quartiles feature to the input range block being encoded）.Besides，an error threshold is also used to control the size of search neighbourhood automatically.Simulation results demonstrate that，for three standard test images，the proposed scheme averagely obtain the speedup of 51 times or so by error threshold set 10，while can accomplish good quality of the reconstructed images against the full search method.Moreover，its performance is better than the newly cross trace feature of normalized block algorithm.

Key words: image compression, fractal, fractal image coding, quartiles feature