We characterize some properties of the zero-crossings of the laplacian of signals - in particular images - filtered with linear filters, as a function of the scale of the filter (following recent work by A. Witkin, 1983). We prove that in any dimension the only filter that does not create zero crossings as the scale increases is gaussian. This result can be generalized to apply to level-crossings of any linear differential operator: it applies in particular to ridges and ravines in the image density. In the case of the second derivative along the gradient we prove that there is no filter that avoids creation of zero-crossings.
