# Specify Desired 2-D Frequency Response Matrix

The 2-D filter design functions `fsamp2`, `fwind1`, and `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` 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 might get unexpected results, such as nonlinear phase.

For example, create a circular ideal low-pass frequency response with cutoff at 0.5.

```[f1,f2] = freqspace(25,"meshgrid"); Hd = zeros(25,25); d = sqrt(f1.^2 + f2.^2) < 0.5; Hd(d) = 1; mesh(f1,f2,Hd)```

Note that for this frequency response, the filters produced by `fsamp2`, `fwind1`, and `fwind2` are real. Filters with real coefficients are 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).