The present invention proposes a method for generating a security bi-level image used to form one of the inks of a banknote, said image comprising an original bi-level image and a security pattern, said security pattern being obtained in the spatial domain by the inverse Fourier transform of the combination in the frequency domain between the Fourier transform of an auxiliary image and a two-dimensional sweep, said two-dimensional sweep being a circularly symmetric, two-dimensional pattern created by sweeping a self-similar, one-dimensional function along a 360-degree arc, such as said security pattern being detectable from the maximum value of the cross- correlation of said one-dimensional function with the Fourier transform of one line of said banknote, said method comprising the step of: -determining a distance map of the original bi-level image, -generating a merged image by linearly interpolating at least a part of said distance map with said security pattern, -thresholding the merged image to obtain the security bi-level image, -applying the security bi-level image on a support.