|Image Processing Toolbox User's Guide|
Creating the Desired Frequency Response Matrix
The filter design functions
fwind2 all create filters based on a desired frequency response magnitude matrix. Frequency response is a mathematical function describing the gain of a filter in response to different input frequencies.
You can create an appropriate desired frequency response matrix using the
freqspace returns correct, evenly spaced frequency values for any size response. If you create a desired frequency response matrix using frequency points other than those returned by
freqspace, you might get unexpected results, such as nonlinear phase.
For example, to create a circular ideal lowpass frequency response with cutoff at 0.5, use
[f1,f2] = freqspace(25,'meshgrid'); Hd = zeros(25,25); d = sqrt(f1.^2 + f2.^2) < 0.5; Hd(d) = 1; mesh(f1,f2,Hd)
Ideal Circular Lowpass Frequency Response
Note that for this frequency response, the filters produced by
fwind2 are real. This result is desirable for most image processing applications. To achieve this in general, the desired frequency response should be symmetric about the frequency origin (
|Windowing Method||Computing the Frequency Response of a Filter|
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