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--- a +++ b/wavelets/fwt.m @@ -0,0 +1,157 @@ +function c = fwt(f,h,J,varargin) +%FWT Fast Wavelet Transform +% Usage: c = fwt(f,h,J); +% c = fwt(f,h,J,...); +% +% Input parameters: +% f : Input data. +% h : Analysis Wavelet Filterbank. +% J : Number of filterbank iterations. +% a : Explicitly defined subsampling factors. +% +% Output parameters: +% c : Coefficients stored in a cell-array. +% +% `c=fwt(f,h,J)` returns wavelet coefficients *c* of the input signal *f* +% using *J* iterations of the basic wavelet filterbank defined by *h*. +% The fast wavelet transform algorithm (or Mallat's algorithm) is employed. +% If *f* is a matrix, the transformation is applied to each of *W* columns. +% +% The coefficents *c* are Discrete Wavelet transform of the input signal *f*, +% if *h* defines two-channel wavelet filterbank. The function can apply +% the Mallat's algorithm using basic filter banks with any number of the +% channels. In such case, the transform have different name. +% +% The basic analysis wavelet filterbank $h$ can be passed in several formats. +% The formats are the same as for the |fwtinit|_ function. The simplest +% is passing a cell array, whose first element is the name of the function +% defining the basic wavelet filters (`wfilt_` prefix) and the other elements are +% the parameters of the function. e.g. `{'db',10}` calls `wfilt_db(10)` internally. +% The second possible format of $h$ is to pass cell array of one dimensional +% numerical vectors directly defining the wavelet filter impulse responses. +% In this case, outputs of the filters are subsampled by a factor equal +% to the number of the filters. For creating completely custom filterbanks use the +% |fwtinit|_ function. +% The third option is to pass structure obtained from the |fwtinit|_ +% function. +% +% The coefficients in the cell array $c\{jj\}$ for $jj=1,\ldots,J\cdot(N-1)+1$, +% where $N$ is number of filters in the basic wavelet filterbank, are ordered +% with inceasing central frequency of the corresponding effective filter frequency +% responses or equivalently with decreasing wavelet scale. +% +% If the input *f* is matrix with *W* columns, each element of the cell +% array $c\{jj\}$ is a matrix with *W* columns with coefficients belonging +% to the appropriate input channel. +% +% The following flag groups are supported: +% +% 'per','zpd','sym','symw','asym','asymw','ppd','sp0' +% Type of the boundary handling. +% +% +% Boundary handling: +% ------------------ +% +% The default periodic extension considers the input signal as it was a one +% period of some infinite periodic signal as is natural for the transforms +% based on the FFT. The resulting wavelet representation is +% non-expansive, that is if the input signal length is a multiple of a +% $J$-th power of the subsampling factor, the total number of coefficients +% is equal to the input signal length. +% +% If the input signal length is not a multiple of a $J$-th power of the +% subsampling factor, the processed signal is padded internally by +% repeating the last sample at each step of the transform to the next +% multiple of the subsampling factor rather than doing the prior explicit +% padding. In addition, the periodic extension restrict the input signal +% length to be greater than a certain length. +% +% `fwt(f,h,J,ext)` with `ext` other than `'per'` computes a slightly +% redundant wavelet representation of the input signal *f* with the chosen +% boundary extension *ext*. +% +% The default periodic extension can result in "false" high wavelet +% coefficients near the boundaries due to the possible discontinuity +% introduced by the periodic extension. The custom extension can diminish +% this phenomenon. The extensions are done at each level of the transform +% internally rather than doing the prior explicit padding. The supported +% possibilities are: +% +% * `'zpd'` - zeros are considered outside of the signal (coefficient) support. +% +% * `'sym'` - half-point symmetric extension. +% +% * `'symw'` - whole-point symmetric extension +% +% * `'asym'` - half-point antisymmetric extension +% +% * `'asymw'` - whole point antisymmetric extension +% +% * `'ppd'` - periodic padding, same as `'ppd'` but the result is expansive representation +% +% * `'sp0'` - repeating boundary sample +% +% Note that the same flag have to be used in the call of the inverse transform +% function `ifwt`. +% +% Examples: +% --------- +% +% A simple example of calling the |fwt|_ function::: +% +% f = gspi; +% J = 10; +% c = fwt(f,{'db',8},J); +% plotfwt(c); +% +% See also: ifwt, fwtinit +% +% References: ma98 + + +if nargin<3 + error('%s: Too few input parameters.',upper(mfilename)); +end; + +if ~isnumeric(J) || ~isscalar(J) + error('%s: "J" must be a scalar.',upper(mfilename)); +end; + +if(J<1 || rem(J,1)~=0) + error('%s: J must be a positive integer.',upper(mfilename)); +end + +% Initialize the wavelet filters structure +h = fwtinit(h,'ana'); + +%% ----- step 0 : Check inputs ------- +definput.import = {'fwt'}; +[flags,kv]=ltfatarghelper({},definput,varargin); + + +%% ----- step 1 : Verify f and determine its length ------- +% Change f to correct shape. +% TO DO: if elements of h.a are not equal and the flag is 'per', +% do zero padding to the next multiple of 2^J +[f,Ls,W,wasrow,remembershape]=comp_sigreshape_pre(f,upper(mfilename),0); +if(Ls<2) + error('%s: Input signal seems not to be a vector of length > 1.',upper(mfilename)); +end + + +%% ----- step 2 : Check whether the input signal is long enough +% input signal length is not restricted for expansive wavelet transform (extension type other than the default 'per') +flen = length(h.filts{1}.h); +if(strcmp(flags.ext,'per')) + minLs = (h.a(1)^J-1)*(flen-1); % length of the longest equivalent filter -1 + if Ls<minLs + error('%s: Input signal length is %d. Minimum signal length is %d or use %s flag instead. \n',upper(mfilename),Ls,minLs,'''ppd'''); + end; +end + + +%% ----- step 3 : Run computation + c = comp_fwt_all(f,h.filts,J,h.a,'dec',flags.ext); + +