# Calculate the Frequency Response of a 2-D Filter

This example shows how to calculate and display the frequency response of a two-dimensional filter using the `freqz2` function. The frequency response of a filter describes the gain of a filter at different input frequencies.

With no output arguments, `freqz2` creates a mesh plot of the frequency response. For example, consider this FIR filter.

```h = [0.1667 0.6667 0.1667 0.6667 -3.3333 0.6667 0.1667 0.6667 0.1667];```

Calculate and display the 64-by-64 point frequency response of `h`.

`freqz2(h)`

To obtain the frequency response `H` and the frequency point vectors `f1` and `f2`, use output arguments.

`[H,f1,f2] = freqz2(h);`

`freqz2` normalizes the frequencies `f1` and `f2` so that the value 1.0 corresponds to half the sampling frequency, or π radians.

For a simple m-by-n response, as shown above, `freqz2` uses the two-dimensional fast Fourier transform function `fft2`. You can also specify vectors of arbitrary frequency points, but in this case `freqz2` uses a slower algorithm.