Video forensics on block detection with deblocking filter in H.264/AVC

Video forensics is an emerging research area that is still in need of studies, especially with regards to aspect of the advancement in video compression. De-blocking filters, firstly introduced in H.264/AVC, provide a means to reduce the blocking artifacts in compressed videos. These filters, in the forensic perspective, could induce a number of forensics threat, especially in the area of authentication, as it reduces the traces of block boundaries, which, in the past, often used in forensic studies. This paper proposed a spatial structural analysis (SSA) method to extract the de-blocking filter traces, by examining pixel intensity changes. At first, the SSA computes the intensity differences along the horizontal and vertical direction on the U channel of the compressed video. The SSA then locates all perpendicular turning points that were formed by a pair of intersected and perpendicular edges that appear to continuously share the same intensity difference in the same direction. Finally, a common spatial distance will be determined based on the highest occurrence of spatial distance between the horizontal and vertical lines formed along the perpendicular turning points detected. The distance will be used to determine if compression has been conducted for the video, where the common spatial distance computed is identical to the size of the macroblocks on both horizontal and vertical direction. The experimental results showed that the determined common spatial distance is 8 in the videos compressed with H.264/AVC that matches the size of the 8×8 macroblocks. Videos compressed without de-blocking filter showed stronger peak in the accumulated spatial distance count compared to those with de-blocking filters applied.