  We report results on the scaling properties of changes in contrast of natural
images in different visual environments. This study confirms the existence, in
a vast class of images, of a multiplicative process relating the variations in
contrast seen at two different scales, as was found in
\cite{NIPS97,TuMaPaNa98}. But it also shows that the scaling exponents are not
universal: Even if most images follow the same type of statistics, they do it
with different values of the distribution parameters. Motivated by these
results, we also present the analysis of a generative model of images that
reproduces those properties and that has the correct power spectrum. Possible
implications for visual processing are also discussed.
