1. A computer implemented method for applying a numerical approximation of a convolution of image I as represented by hierarchical signals a l of level l with filter f, said method comprising: generating a forward transformation by applying a convolution between a l and kernel h 1 for each level of hierarchy l and by down-sampling the result of convolved a l and h 1 , generating a backward transformation by applying, for each level of hierarchy l, a convolution between kernel h 2 and an up-sampled forward transformation and combining the result with a convolution of a l with kernel g; and combining the forward transformation with the backward transformation, to yield â 0 being an approximation of a convolution between a and f, wherein kernels h 1 , h 1 , and g are optimized kernels of filter f.