Читать книгу Multimedia Security, Volume 1 - William Puech - Страница 35

Another method proposes to directly detect the CFA pattern used in the image (Kirchner 2010). In order to do this, the image is remosaiced and demosaiced in the four possible positions, with a simple algorithm such as bilinear interpolation. The reasoning is that demosaicing should produce an image closer to the original when it is remosaiced in the correct position. They then compare the residuals to detect which position of the CFA has been used. Since CFA artifacts are generally more visible in the green channel, they decide first the position of the sampled green pixels, before deciding between the remaining two positions with the red and blue channels, a paradigm that has been used in most publications since then. Their use of the bilinear algorithm limits them in the same way (Popescu and Farid 2005; González Fernández et al. 2018) due to the linearity and chromatic independence of the bilinear algorithm, which is not shared by most modern demosaicing algorithms. However, their method does not depend on the choice of algorithm, and could therefore provide very good results should the originally used demosaicing algorithm be known.