Given a color image previously compressed using JPEG, we estimate the image's JPEG compression history components including the color transformation, subsampling, and the quantization table employed during the previous JPEG operations. The compression history information, which is lost during image format conversion, can be used for improving JPEG recompression, as an authentication feature, or for covert messaging. The DCT coefficient histograms of previously JPEG-compressed images exhibit near-periodic behavior due to quantization. We propose a statistical approach to exploit this structure and thereby estimate the image's compression history. Using simulations, we first demonstrate the accuracy of our estimation. Further, we show that JPEG recompression performed by exploiting the estimated compression history strikes an excellent file-size versus distortion tradeoff.
Ramesh Neelamani, Ricardo L. de Queiroz, Zhigang F