Let the time reversed version of be denoted .This notation is consistant with an idea from Chapter that the adjoint of a filter matrix is another filter matrix with a reversed filter. In engineering it is conventional to use the asterisk symbol ``'' to denote convolution. Thus, the idea that the autocorrelation of a signal is a convolution of the signal with its time reverse (adjoint) can be written as .
Wind the signal around a vertical-axis helix to see its two-dimensional shape :
Since the autocorrelation of is is a second derivative, the operator must be something like a first derivative. As a geophysicist, I found it natural to compare the operator with by applying them to a local topographic map. The result shown in Figure is that enhances drainage patterns whereas enhances mountain ridges.
The operator has curious similarities and differences with the familiar gradient and divergence operators. In two-dimensional physical space, the gradient maps one field to two fields (north slope and east slope). The factorization of with the helix gives us the operator that maps one field to one field. Being a one-to-one transformation (unlike gradient and divergence) the operator is potentially invertible by deconvolution (recursive filtering).
I have chosen the name ``helix derivative'' or ``helical derivative'' for the operator .A telephone pole has a narrow shadow behind it. The helix integral (middle frame of Figure ) and the helix derivative (left frame) show shadows with an angular bandwidth approaching .
Our construction makes have the energy spectrum kx2+ky2, so the magnitude of the Fourier transform is .It is a cone centered and with value zero at the origin. By contrast, the components of the ordinary gradient have amplitude responses |kx| and |ky| that are lines of zero across the (kx,ky)-plane.
The rotationally invariant cone in the Fourier domain contrasts sharply with the nonrotationally invariant function shape in (x,y)-space. The difference must arise from the phase spectrum. The factorization (11) is nonunique in that causality associated with the helix mapping can be defined along either x- or y-axes; thus the operator (11) can be rotated or reflected.
This is where the story all comes together. One-dimensional theory, either the old Kolmogoroff spectral factorization, or the new Wilson-Burg spectral-factorization method produces not merely a causal wavelet with the required autocorrelation. It produces one that is stable in deconvolution. Using in one-dimensional polynomial division, we can solve many formerly difficult problems very rapidly. Consider the Laplace equation with sources (Poisson's equation). Polynomial division and its reverse (adjoint) gives us which means that we have solved by using polynomial division on a helix. Using the seven coefficients shown, the cost is fourteen multiplications (because we need to run both ways) per mesh point. An example is shown in Figure .
Figure contains both the helix derivative and its inverse. Contrast them to the x- or y-derivatives (doublets) and their inverses (axis-parallel lines in the (x,y)-plane). Simple derivatives are highly directional whereas the helix derivative is only slightly directional achieving its meagre directionality entirely from its phase spectrum.
In practice we often require an isotropic filter.
Such a filter is a function of .It could be represented as a sum of helix derivatives to integer powers.