Читать книгу Multimedia Security, Volume 1 - William Puech - Страница 29

The dynamic range remains unchanged after JPEG compression. However, noise is reduced after JPEG compression due to the quantization of the DCT coefficients, in particular those corresponding to high frequencies. Furthermore, the curve estimated by Ponomarenko et al.’s algorithm (Colom and Buades 2013) differs more from the temporal series curves, because the noise estimation method estimates noise at high frequencies, which are altered or even destroyed by compression.

The noise present in JPEG images is the result of several transformations on the initial noise model, which initially follows a Poisson distribution. In the end, the final image’s noise does not follow any predefined model, it instead depends on many unknown parameters that are set by each manufacturer. The only certainty we have is that noise is intensity dependent and frequency dependent. Therefore, it is preferable to use non-parametric models to estimate noise curves, so as to estimate the curves from the image itself.