nt_mmx

[y,abscissa]=nt_mmx(x, N) - calculate min-max pairs

  y: array or matrix of min-max pairs
  
  x: data 
  N: target number of pairs (default: 1000 (or size(x,1) if smaller)

  To plot x cheaply: plot(mmx(x)).
  To get only maxima: y=mmx(x); y(2:2:end);

0001 function [y,abscissa]=nt_mmx(x,N)
0002 %[y,abscissa]=nt_mmx(x, N) - calculate min-max pairs
0003 %
0004 %  y: array or matrix of min-max pairs
0005 %
0006 %  x: data
0007 %  N: target number of pairs (default: 1000 (or size(x,1) if smaller)
0008 %
0009 %  To plot x cheaply: plot(mmx(x)).
0010 %  To get only maxima: y=mmx(x); y(2:2:end);
0011 
0012 if nargin<2; N=1000; end
0013 
0014 if ndims(x)==3; 
0015     y=nt_mmx(x(:,1,1)); % to get size
0016     y=zeros(size(y,1),size(x,2),size(x,3));
0017     for k=1:size(x,3)
0018         [y(:,:,k),abscissa]=nt_mmx(x(:,:,k),N);
0019     end
0020     return
0021 end
0022 
0023 [m,n]=size(x);
0024 
0025 if m<2*N
0026     y=zeros(2*m,n);
0027     for k=1:n
0028         xx=x(:,k);
0029         xx=[xx,xx]';
0030         xx=xx(:);
0031         y(:,k)=xx;
0032     end
0033 else
0034     N=ceil(m/floor(m/N));
0035     K=ceil(m/N);
0036     y=zeros(2*N,n);
0037     for k=1:n
0038         xx=x(:,k);
0039         xx(m:K*N)=xx(m);
0040         xx=reshape(xx,K,N);
0041         xx=[min(xx,[],1);max(xx,[],1)];
0042         xx=xx(:);
0043         y(:,k)=xx;
0044     end
0045 end
0046 
0047 abscissa=linspace(0, m, 2*N);
0048