In addition to the orientation image, another important parameter that is used
in the construction of the Gabor filter is the local ridge frequency. The frequency
image represents the local frequency of the ridges in a fingerprint. The first
step in the frequency estimation stage is to divide the image into blocks of size
W £W. The next step is to project the grey-level values of all the pixels located
inside each block along a direction orthogonal to the local ridge orientation.
This projection forms an almost sinusoidal-shape wave with the local minimum
points corresponding to the ridges in the fingerprint. An example of a projected
waveform is shown in Figure 2.2.