Image Processing Toolbox | ![]() ![]() |
Creating the Desired Frequency Response Matrix
The filter design functions fsamp2
, fwind2
, and fwind2
all create filters based on a desired frequency response magnitude matrix. You can create an appropriate desired frequency response matrix using the freqspace
function. 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 may 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)
Figure 7-9: Ideal Circular Lowpass Frequency Response
Note that for this frequency response, the filters produced by fsamp2
, fwind1
, and 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 (f1
=
0
, f2
=
0
).
![]() | Windowing Method | Computing the Frequency Response of a Filter | ![]() |