Читать книгу Multimedia Security, Volume 1 - William Puech - Страница 41

In JPEG encoding, the division of the image into 8 × 8 blocks and the application of a quantization step lead to the appearance of discontinuities at the edges of these blocks in the decompressed image.

Figure 1.9 shows the blocking effect that appears after JPEG compression. Contrast enhancement allows us to clearly see the 8 × 8 blocks. The greatest loss of information is during the quantization step, explored in more detail in section 1.2.4. The blocking effect is due to quantization, depending on the Q parameter, applied on all 8 × 8 size blocks. Therefore, standard JPEG compression leaves two characteristic traces: the division into 8×8 non-overlapping blocks and the quantization, according to a quantization matrix, of the DCT coefficients. In other words, the two features to be detected from the image are:

 1) the origin of the 8×8 grids;

 2) the values of the quantization matrix.

Figure 1.9. Close-ups on an image before and after compression. The contrast has been enhanced to observe the JPEG artifacts, in particular the blocking effect, allowing us to see the edges of the 8 × 8 blocks

In order to authenticate an image, the previous detections must verify that (1) the origin of the grid is aligned with the top left of the image; and (2) the quantization matrix calculated from the image is similar to the one in the header of the JPEG file. If the image is not in the JPEG format (providing the header file giving the quantization matrix), then estimating this information from the image itself is even more useful as an initial analysis.

Methods by Pevny and Fridrich (2008) make it possible to detect if an image has undergone a double compression, which creates an immediate argument against the authenticity of the image. Indeed, this would imply a duplicate in the processing chain of the image.

In the following sections, grid detection and quantization matrix estimation methods are illustrated. When no detection is made, the image may be classified as not having undergone JPEG compression.