The main purpose of image compression is to reduce the memory space or transmission time and that of cryptography is to keep the security of the data. These two technologies are separate. But in some cases an image can be compressed if necessary, and then encrypted. Once the data are decrypted, all secrets will be leaked. Hence compression is performed after encryption of the image if there is a need to transmit the redundant data over an insecure and bandwidth constrained channel. The reverse system is also used now a days and it ensures more security. In both cases there is no compromise in compression efficiency. For compression and encryption several techniques are used. Some of these techniques are applicable to grayscale images, some others to color images and others do both. To encrypt the compressed data and for transformation function many techniques are used in several areas. In this work, A Discrete Wavelet Transform based method, Resolution Progressive Compression is used. In Resolution Progressive Compression three decomposition levels are used. Greater compression performance can be achieved by increasing the number of levels. The one fundamental problem that is coming in encryption before compression is, there is no utilization of the pixel redundancy for subsequent compression. In other words encryption masks the pixel redundancy that is there in the image. Therefore before encrypting the original image it can reduce the dimensionality of the gray scale image and obtain a decorrelated image using PCA and its variants. If the compression efficiency is poor PCA and encryption will be suffice the needs. In the decoding stage it can project the principal directions back to get the original image.

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Slepian Wolf coding RPC