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A simple and efficient method for lossy colour image compression is proposed. Here, the discrete cosine transform (DCT) is applied to the YCbCr image obtained from the original RGB image. The bisection method is used to define the required threshold for a prefixed user peak signal to noise ratio as a controlled quality criterion. The thresholded and quantised DCT coefficients are encoded with a new technique. The proposed technique uses the difference of the indexes of the retained coefficients in coordination with DCT block adaptive scanning to encode efficiently the coefficients. The difference of the indexes is stored in a lookup table called (dLUT). When compared with recent methods, the obtained results show that the proposed algorithm achieves high performance. Introduction: The compression can lead to an efficient transmission and storage of images. It reduces the size of the images by decreasing the redundancy that contains. In general, there are two kind of compression: lossless and lossy compression. The lossless compression guaranties that the original and reconstructed images are identical. The efficiency and the compression ratios are weak compared with those of the lossy compression. In this last, the quality of the reconstructed image is degraded to an acceptable level in the benefit of high compression ratios. The lossy compression of the colour image has been widely studied and it can be further categorised in two principle methods [1]: † The direct methods act directly on the image samples as in block truncation [2] and vector quantisation based techniques [3]. † The transform methods use transforms to concentrate the energy of the image in a few number of coefficients. Many transforms are used such as principal component analysis [4], discrete cosine transform (DCT) [5] and wavelets [6].

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