Research On The Image Compression Coding Of The Set Partitioning In Hierarchical Tress Based On Wavelet Packet Transform

SPHIT is a kind of image compression which compresses the wavelet-coefficient. It is recognized as one of the most efficient algorithm in the field of image compression. But it still has a few defects: Firstly, the algorithm uses the wavelet transform only to analyse the sub-image with low frequency carefully, and it is lack of analysing the sub-image with high frequency, so it is easy to lost signalof high-frequency; Secondly, after the wavelet transform, wavelet-coefficient'entropy of sub-image with low frequency is much higher than the wavelet-coefficient'entropy of sub-image with high frequency , so it is not suitable for using the same algorithm to process ; thirdly, the coding process uses a lot of memory and it is a waste of time to scan the unimportant elements repeatedly.Based on the analysis of the wavelet packet transform and the classical SPIHT , a new method has been studied out. The method used the fixed four-level wavelet packet transform to displace the wavelet transform for ameliorating the limitation "high-frequency low-resolution";It uses different processing methods for the every sub-image.The sub-image with low frequency is compressed by DPCM method,and the improved SPIHT coding method is used to compress the sub-images with high frequency. It takes full advantage of the features of wavelet-coefficient. The improved algorithm modifies the SPIHT algorithm by using two ideas—"minimum threshold"and"minimum exported bit"to reduce the memory requirement,and establishes"the max value table"in order to lower the repeated calculations. The result of experimental data's analysis and visual effects's comparison shows that the method improves the quality of restored-image and reduces the time of encoding/ decoding .The validity of the method has been validated too.