what is Octave equivalent of "freqspace" function of Matlab?

Question

In Matlab we can write,

[u, v] = freqspace(size(I),'meshgrid');

What is Octave's equivalent of "freqspace" function of Matlab?

Answer 1

As you have found out, freqspace is not implemented in Octave:

>> help freqspace
error: help: the 'freqspace' function is not yet implemented in Octave

Please read <http://www.octave.org/missing.html> to learn how you can
contribute missing functionality.

Looking at the documentation page for the MATLAB function, you get:

Syntax

[f1,f2] = freqspace(n) [f1,f2] = freqspace([mn]) [x1,y1] = freqspace(...,'meshgrid') f = freqspace(N) f = freqspace(N,'whole')

Description freqspace returns the implied frequency range for equally spaced frequency responses. freqspace is useful when creating desired frequency responses for various one- and two-dimensional applications.

[f1,f2] = freqspace(n) returns the two-dimensional frequency vectors f1 and f2 for an n-by-n matrix.

For n odd, both f1 and f2 are [-n+1:2:n-1]/n .

For n even, both f1 and f2 are [-n:2:n-2]/n .

[f1,f2] = freqspace([mn]) returns the two-dimensional frequency vectors f1 and f2 for an m -by- n matrix.

[x1,y1] = freqspace(...,'meshgrid') is equivalent to

[f1,f2] = freqspace(...); [x1,y1] = meshgrid(f1,f2);

f = freqspace(N) returns the one-dimensional frequency vector f assuming N evenly spaced points around the unit circle. For N even or odd, f is (0:2/N:1) . For N even, freqspace therefore returns (N+2)/2 points. For N odd, it returns (N+1)/2 points.

f = freqspace(N,'whole') returns N evenly spaced points around the whole unit circle. In this case, f is 0:2/N:2*(N-1)/N .

Based on this, I put together the following function. It was done very quickly though and doesn't cover all the cases, but the cases implemented appear to work fine in Octave. Hopefully this should give you an idea to get started:

function varargout = freqspace(varargin)

  if nargin==1 && nargout==2 % [f1,f2] = freqspace(n)
    n = varargin{1};

    if mod(n,2)==0 % n is even
      varargout{1} = [-n:2:n-2]/n;
      varargout{2} = [-n:2:n-2]/n;
    else % n is odd
      varargout{1} = [-n+1:2:n-1]/n;
      varargout{2} = [-n+1:2:n-1]/n;
    end

  elseif nargin==1 && nargout==1 % f = freqspace(N)
    N = varargin{1};
    varargout{1} = (0:2/N:1);

  elseif nargin==2 && nargout==1 % f = freqspace(N,'whole')
    N = varargin{1};
    if ~ischar(varargin{2}) || ~strcmpi(varargin{2},'whole')      
      error('The correct syntax is f = freqspace(N,''whole'')');
    else
      varargout{1} = 0:2/N:2*(N-1)/N;
    end

  else  
    disp('Case not yet implemented.')
    return

  end