A method of characterizing the topography of the surface of a creped material, comprising: directing light onto a first surface of a creped material; obtaining at least two images of an identical portion of the first surface of the creped material, each image capturing the first surface illuminated from different directions; approximating from the images at least two surface normal vectors corresponding, respectively, to at least two portions of the first surface of the creped material; converting the at least two surface normal vectors to gradient image data; and analysing the gradient image data to characterize the topography of the first surface of the creped material, wherein analysing comprises: computing a two dimensional Fourier transform from the gradient image data; and computing a two dimensional power spectrum from the two dimensional Fourier transform, wherein computing a two dimensional power spectrum comprises: smoothing the two dimensional power spectrum, wherein smoothing the two dimensional power spectrum includes: obtaining a two dimensional filtered power spectrum; determining the ratio of an initial power spectrum to the filtered power spectrum for each point in the spectrum; comparing each point in the spectrum to a threshold value, wherein points above the threshold value are a marking spot; estimating the locations of spectral peak by fitting a second order two dimensional polynomial around the maximum value of peaks, wherein at least one of the spectral peaks corresponds to the marking spot; and replacing the values around the marking spot with the values of power spectrum in its neighbourhood. directing light onto a first surface of a creped material; obtaining at least two images of an identical portion of the first surface of the creped material, each image capturing the first surface illuminated from different directions; approximating from the images at least two surface normal vectors corresponding, respectively, to at least two portions of the first surface of the creped material; converting the at least two surface normal vectors to gradient image data; and analysing the gradient image data to characterize the topography of the first surface of the creped material, wherein analysing comprises: computing a two dimensional Fourier transform from the gradient image data; and computing a two dimensional power spectrum from the two dimensional Fourier transform, wherein computing a two dimensional power spectrum comprises: smoothing the two dimensional power spectrum, wherein smoothing the two dimensional power spectrum includes: obtaining a two dimensional filtered power spectrum; determining the ratio of an initial power spectrum to the filtered power spectrum for each point in the spectrum; comparing each point in the spectrum to a threshold value, wherein points above the threshold value are a marking spot; estimating the locations of spectral peak by fitting a second order two dimensional polynomial around the maximum value of peaks, wherein at least one of the spectral peaks corresponds to the marking spot; and replacing the values around the marking spot with the values of power spectrum in its neighbourhood. computing a two dimensional Fourier transform from the gradient image data; and computing a two dimensional power spectrum from the two dimensional Fourier transform, wherein computing a two dimensional power spectrum comprises: smoothing the two dimensional power spectrum, wherein smoothing the two dimensional power spectrum includes: obtaining a two dimensional filtered power spectrum; determining the ratio of an initial power spectrum to the filtered power spectrum for each point in the spectrum; comparing each point in the spectrum to a threshold value, wherein points above the threshold value are a marking spot; estimating the locations of spectral peak by fitting a second order two dimensional polynomial around the maximum value of peaks, wherein at least one of the spectral peaks corresponds to the marking spot; and replacing the values around the marking spot with the values of power spectrum in its neighbourhood. obtaining a two dimensional filtered power spectrum; determining the ratio of an initial power spectrum to the filtered power spectrum for each point in the spectrum; comparing each point in the spectrum to a threshold value, wherein points above the threshold value are a marking spot; estimating the locations of spectral peak by fitting a second order two dimensional polynomial around the maximum value of peaks, wherein at least one of the spectral peaks corresponds to the marking spot; and replacing the values around the marking spot with the values of power spectrum in its neighbourhood.