Double compression. Every time a video sequence that has already been compressed is modified, it has to be re-compressed. Studying processing chains consisting of multiple compression steps is useful, e.g., for tampering detection or to recognize the original encoder being used. This is the typical situation that we face when video content is downloaded from video-sharing websites. Of course, it is possible to obtain the parameters used in the last compression step, as they can be read directly from the bitstream. However, it is much more challenging to extract information about the previous coding steps. For this reason some authors have studied the footprints left by double video compression. The solutions proposed so far in the literature are mainly focused on MPEG video, thus exploiting some of the properties already discussed in the case of JPEG double-compression. In [104] the authors address the problem of estimating the traces of double compression of an MPEG coded video. The problem is faced either when the Group of Pictures (GOP) structure is preserved or not. In the former situation, every frame is re-encoded in a frame of the same kind, so that I,B, or P frames remain, respectively, I,B, or P. The latter situation is typical in frame removal or insertion attacks. Since encoding I-frames is not dissimilar from JPEG compression, when an I-frame is re-encoded at a different bitrate, DCT coefficients are subject to two levels of quantization. Therefore, similarly to JPEG double compressed images, the histograms of DCT coefficients assume a characteristic shape that deviates from the original, e.g. Laplacian, distribution. In particular, when the quantization step size decreases from the first to the second compression, some bins in the histogram are left empty. Conversely, when the step size increases, some spikes appear in the histograms. When the GOP structure is changed, I-frames can be re-encoded into another kind of frame. However this gives rise to larger prediction residuals after motion-compensation. The authors show that by looking at the Fourier transform of the energy of the displaced frame difference over time, the presence of spikes reveals a change in the GOP structure, which is a cue of double-compression. In [133] the authors propose another method for detecting MPEG double compression based on blocking artifacts. A metric for computing the Block Artifact Strength (BAS) for each frame is defined. This score is inspired by the method in [97] and relies ...
