Similar to the filtering stage applied by Hong et al.: after the orientation image
has been computed, the raw image is then filtered using a set of bandpass filters
tuned to match the ridge orientation. The image is firstly converted from the
spatial domain into the frequency domain by application of the two-dimensional
discrete Fourier transform. The Fourier image is then filtered using a set of 16
Butterworth filters with each filter tuned to a particular orientation. The number
of directional filters corresponds to the set of directions used to calculate the
orientation image. After each directional filter has been independently applied
to the Fourier image, the inverse Fourier transform is used to convert each image
back to the spatial domain, thereby producing a set of directionally filtered images
called prefiltered images.