1. A method of computing an intermediate function for use in cone-beam (CB) reconstruction used to reconstruct 3D images, the intermediate function evaluated at a selected point in three dimensions where image reconstruction will be performed, the method comprising: obtaining CB projection data from a vertex path segment, wherein the vertex path segment comprises a path taken by an x-ray source in a reference frame of a target object to be reconstructed and wherein a chord connecting first and second endpoints of the vertex path segment does not include the selected point; and determining a difference between first and second values; the first value comprising a first Hilbert transform of a density function along a straight line passing through the first endpoint of the vertex path segment and the selected point, the first Hilbert transform evaluated at the selected point, the second value comprising a second Hilbert transform of the density function along a straight line passing through the second endpoint of the vertex path segment and the selected point, the second Hilbert transform evaluated at the selected point; and storing the difference values.