Signal and Data Processing

A lot of researchs have been performed in image compression and different methods have been proposed. Each of the existing methods presents different compression rates on various images. By identifing the effective parameters in a compression algorithm and strengthen them in the preprocessing stage, the compression rate of the algorithm can be improved. JPEG is one of the successful compression algorithm that various works have been done to improve its performance. Image contrast is one important factor affecting on JPEG compression rate. The lower the image contrast, the less detail is visible and JPEG will be able to compress such images with a higher rate. In this paper, a semi-lossless preprocessing method based on power operator is proposed that decreases the image contrast by reducing the range of graylevels in the image. Therefore, JPEG can compress the preprocess images with a higher compression ratio. To restore the image, after decoding the compressed image, by applying the inverse exponent value to the power operator on this image, an image similar to the original image will be achieved. The results show that the proposed preprocessing method substantially increases the JPEG compression ratio on natural images.