% lowpass (Low pass filter function) by Dan Piehl (2006)
%
% This will determine perform a basic low pass filter operation
% on one or two dimensional data.  The filter is a simple
% three-point filter for 1-D arrays, or a 9-point for 2-D and 3-D arrays.
% The third dimension is always interpreted as a color subscript.
%
% Note: This function reflects boundary values, so that filtering assumes
%       values outside the array are equal to values on the array boundary

function b = lowpass(a)

  % Examine parameters and return error messages, if needed
  numDimsA = ndims(a);          % Get the number of input dimensions
  S1=size(a,1);                 % Get the number of rows
  S2=size(a,2);                 % Get the number of columns
  S3=size(a,3);                 % Get the number of color planes
  if (nargin < 1)
      error('MATLAB:wavelet:NoInputs',['No input arguments specified.'])
  end
  if (numDimsA<1 || numDimsA>3) % Only 1-3 dimensional arrays are allowed
      error('MATLAB:wavelet:NoInputs',['Invalid array dimensions.']) 
  end
  if not ( (S1==1 && S2>1) || (S1>1 && S2==1) || (S1>1 && S2>1) )
      error('MATLAB:wavelet:NoInputs',['Invalid array dimensions.'])
  end


  % Perform lowpass filtering
  b=a;
  for iteration=1:S3 
    if (S1==1)                 % For a single row...
      t=conv([1 2 1]/4,cat(2,a(:,1,iteration),a(:,:,iteration),a(:,S2,iteration)));
      b(:,:,iteration)=t(1,3:S2+2);
    elseif (S2==1)             % For a single column...
      t=conv([1 2 1]/4,cat(1,a(1,:,iteration),a(:,:,iteration),a(S1,:,iteration)));
      b(:,:,iteration)=t(3:S1+2,1);
    else                       % For 2-D or 3-D array...
      t=cat(1,a(1,:,iteration),a(:,:,iteration),a(S1,:,iteration));
      t=convn([1 2 1; 2 4 2; 1 2 1]/16,cat(2,t(:,1),t,t(:,S2)));
      b(:,:,iteration)=t(3:S1+2,3:S2+2);
    end
  end