Keywords: Image Compression, Difference Pulse Code Modulation, Wavelet Transform

Abstract: This paper present a hybrid difference pulse code modulation (DPCM) and wavelet transform (WT) approach to lossy compression of multispectral remotely sensed images. First, the Karhunen-Loeve Transform removes the interband correlation to produce the principal components of the multispectral images. Each component is decomposed into 16 subbands by the wavelet transform using the uniform subband decomposition. Then, the lowest-fequency subband is coded with the lossless DPCM, and all other subbands coded is designed for each subband. The experimental results showed that the proposed method provides low bit-rate and high quality of the reconstructed image.

1.Introduction
Nowadys, the important research fields for multispectral remotely sensed images are image viewing, image analyzing over many spectral bands, image classification and image storing. All requires efficient method to reduce the redundant information. The fundamental of image compressionis to reduce the bit rate for transmission or storage while maintaining an acceptable image fidelityor image quality. The image compression must be save in the memory needed for image storage or in channel capacity required for image transmissin (Rabbani, 1991). The compression techniques must be able to reduce the number of bits required to represent the image by removing the image redundancy. There are many techniques to compress the image. The well-known compression techniques are wavelet transform and different pulse code mudulation (DPCK). However, a more efficient compression algorithm can be obtained by combining wavelet transform and DPCM together. Wavelet transforms using in copression techniques are described by (Antonini, 1992). And the DPCM, the prediction is subtracted from the actual pixel value to form a differential image. The differential image is then quantized and encoded. In this paper, we proposed an efficient technique based on median-predictor DPCM and wavelet transform.

2.Our Approach
Lossy compression of multispectral images is usually performed by applying the KLT which removes the interband correlation and produces the principal components of the by a lossy compression algorithm, which is a hybrid of median predictive coding and wavelet transform techniques. We describes the detail of these techniques in the following subsections.

2.1 Karhunen-Loeve Transform
The KLT is the optimal transform in an energy-packing sense; the KLT coefficients will contain a larger fractinof total energy as compared to any other transform. Unfortunately, the KLT basis functions are image-dependent and require and estimation of the image covariance function for their computation. Usually in remotely sensed multispectral images, there is a large amount of interband correlation due to the co-located sensors and the spectral overlap of the bands. The most effective technique to exploit this correlation is the application of the KLT that produces the principal components of the image.

2.2 Wavelet Transform
A two-dimensional wavelet transform can be implemented as a multi-level transformation. At the first level, the rows of the image being processed are low-pass (L) and high pass (H) filtered and downsampled. Next each column of the row filtered image are again low-pass and high-pass filtered and downsampled. The output of each level is then four subband images including: approximation coefficients (CA), horizontal detail coefficients (CH), vertical detail coefficinents (CV), and diagonal detail coefficients (CD). With uniform subband decompositon, the same process is repreated on every subband to form the next level of the structure. As a result, each of the four subband images is a quarter of the size of its initial image. For J-level decompositions, the image is thus decomposed into 4j subbands (Vetterli, 1995).

Several methods are presented in literature for wavelet-based image compression. A classical technique is based on uniform quantization of wavelet coefficients.