thresh.m

linear time－frequency toolbox

function [xo,N]=thresh(ttype,xi,lambda);
%THRESH   Threshold (hard/soft)
%   Usage:  x=thresh(ttype,x,lambda);
%           [x,N]=thresh(ttype,x,lambda);
%
%   THRESH('hard',x,lambda) or THRESH(1,x,lambda) will perform
%   hard thresholding on x, i.e. all element with absolute value
%   less than lambda will be set to zero.
%
%   THRESH('soft',x,lambda) or THRESH(2,x,lambda) will perform
%   soft thresholding on x, i.e. lambda will be subtracted from
%   the absolute value of every element of x.
%
%   [x,N]=THRESH(ttype,x,lambda) additionally returns a number N specifying
%   how many numbers where kept.
%
%   The function WTHRESH in the Matlab Wavelet toolbox implements the same
%   functionality.
%

if nargin~=3;
  error('Wrong number of input arguments.');
end;

if (prod(size(lambda))~=1 || ~isnumeric(lambda))
  error('lambda must be a scalar.');
end;

if ischar(ttype)
  ttype=lower(ttype);
end;

switch ttype
  case {'hard',1}
    xo=xi.*(abs(xi)>=lambda);
  case {'soft',2}
    xa=abs(x)-lambda;

    % In the following line, the +0 is significant: It turns
    % -0 into +0, oh! the joy of numerics.
    xo=((xa>0)*xa+0).*sign(xi);
  otherwise
    error('Unknown thresholding type.');
end;

if nargout==2
  work=(abs(xi)>=lambda);
  N=sum(work(:));
end;