% thresh (Thresholding function) by Dan Piehl (2006)
%
% This will set all values to zero which are within a threshold value.
% It can accept one, two, or three dimensional inputs, and requires the
% specification of a threshold parameter.
% Thresholds can be "soft" if an exponent is supplied (e.g. 1).
% The third dimension (for color) may be added if desired.

function b = thresh(a,threshold,softexponent)

  % 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 < 2)
      error('MATLAB:wavelet:NoInputs',['Not enough input arguments specified.'])
  end
  if (numDimsA<1 || numDimsA>3) % Only 1/2/3 dimensional arrays are allowed
      error('MATLAB:wavelet:NoInputs',['Invalid array dimensions.']) 
  end
  if not ( S1>=1 && S2>=1 && S3>=1 )
      error('MATLAB:wavelet:NoInputs',['Invalid array dimensions.'])
  end

  % Set everything below threshold to zero (or softened value)
  b=a;                            % Reset the output array
  if (threshold>0)                % If a threshold is provided
    if (S3==1)
      n=abs(b)/threshold;
    else
      n=sqrt(sum(b.^2,3))/threshold;  % Compute euclidean
    end
    n=min(cat(3,n,ones(S1,S2)),[],3); % Keep bounded at one
    if (nargin==3)                % For soft threshold... 
      n=n.^softexponent;          % Apply exponent
    else                          % Otherwise...
      n=floor(n);                 % Flip values less than one to zero
    end 
    for iteration=1:S3            % Apply to each color plane
      b(:,:,iteration)=b(:,:,iteration).*n;
    end
  end