Читать книгу Multimedia Security, Volume 1 - William Puech - Страница 42

In a JPEG-compressed image, the 8 × 8 blocks are created following a regular pattern starting at the pixel in the top left of the image and therefore coinciding with an original grid (0, 0).

The aim of the method is to find the stage of separation in 8 × 8 blocks of the JPEG algorithm. This leads to having the position of the grid by giving its origin (this can vary if the image is cropped). If a grid is present, among the 8 × 8 = 64 different original possibilities, only one is correct.

Here, two families of methods are presented: methods based on block artifacts and methods based on the impact of quantization on the DCT coefficients.