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extrema.m, extrema2.m

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14 Sep 2006 (Updated )

Search all extrema points of a time series or a surface with/without NaN's.

Editor's Notes:

This file was selected as MATLAB Central Pick of the Week

extrema2.m
function [xymax,smax,xymin,smin] = extrema2(xy,varargin)
%EXTREMA2   Gets the extrema points from a surface.
%   [XMAX,IMAX,XMIN,IMIN] = EXTREMA2(X) returns the maxima and minima 
%   elements of the matriz X ignoring NaN's, where
%    XMAX - maxima points in descending order (the bigger first and so on)
%    IMAX - linear indexes of the XMAX
%    XMIN - minima points in descending order
%    IMIN - linear indexes of the XMIN.
%   The program uses EXTREMA.
% 
%   The extrema points are searched only through the column, the row and
%   the diagonals crossing each matrix element, so it is not a perfect
%   mathematical program and for this reason it has an optional argument.
%   The user should be aware of these limitations.
%
%   [XMAX,IMAX,XMIN,IMIN] = EXTREMA2(X,1) does the same but without
%   searching through the diagonals (less strict and perhaps the user gets
%   more output points).
%
%   DEFINITION (from http://en.wikipedia.org/wiki/Maxima_and_minima):
%   In mathematics, maxima and minima, also known as extrema, are points in
%   the domain of a function at which the function takes a largest value
%   (maximum) or smallest value (minimum), either within a given
%   neighbourhood (local extrema) or on the function domain in its entirety
%   (global extrema). 
%
%   Note: To change the linear index to (i,j) use IND2SUB. 
%
%   Example:
%      [x,y] = meshgrid(-2:.2:2,3:-.2:-2);
%      z = x.*exp(-x.^2-y.^2); z(10,7)= NaN; z(16:19,13:17) = NaN;
%      surf(x,y,z), shading interp
%      [zmax,imax,zmin,imin] = extrema2(z);
%      hold on  
%       plot3(x(imax),y(imax),zmax,'bo',x(imin),y(imin),zmin,'ro')
%       for i = 1:length(zmax)
%        text(x(imax(i)),y(imax(i)),zmax(i),['  ' num2str(zmax(i))])
%       end
%       for i = 1:length(zmin)
%        text(x(imin(i)),y(imin(i)),zmin(i),['  ' num2str(zmin(i))])
%       end
%      hold off 
%
%   See also EXTREMA, MAX, MIN

%   Written by
%   Lic. on Physics Carlos Adrin Vargas Aguilera
%   Physical Oceanography MS candidate
%   UNIVERSIDAD DE GUADALAJARA 
%   Mexico, 2005
%
%   nubeobscura@hotmail.com

% From       : http://www.mathworks.com/matlabcentral/fileexchange
% File ID    : 12275
% Submited at: 2006-09-14
% 2006-11-11 : English translation from spanish. 
% 2006-11-17 : Accept NaN's.
% 2006-11-22 : Fixed bug in INDX (by JaeKyu Suhr)
% 2007-04-09 : Change name to MAXIMA2, and definition added.

M = size(xy);
if length(M) ~= 2
 error('Entry must be a matrix.')
end
N = M(2);
M = M(1);

% Search peaks through columns:
[smaxcol,smincol] = extremos(xy);

% Search peaks through rows, on columns with extrema points:
im = unique([smaxcol(:,1);smincol(:,1)]); % Rows with column extrema
[smaxfil,sminfil] = extremos(xy(im,:).');

% Convertion from 2 to 1 index:
smaxcol = sub2ind([M,N],smaxcol(:,1),smaxcol(:,2));
smincol = sub2ind([M,N],smincol(:,1),smincol(:,2));
smaxfil = sub2ind([M,N],im(smaxfil(:,2)),smaxfil(:,1));
sminfil = sub2ind([M,N],im(sminfil(:,2)),sminfil(:,1));

% Peaks in rows and in columns:
smax = intersect(smaxcol,smaxfil);
smin = intersect(smincol,sminfil);

% Search peaks through diagonals?
if nargin==1
 % Check peaks on down-up diagonal:
 [iext,jext] = ind2sub([M,N],unique([smax;smin]));
 [sextmax,sextmin] = extremos_diag(iext,jext,xy,1);

 % Check peaks on up-down diagonal:
 smax = intersect(smax,[M; (N*M-M); sextmax]);
 smin = intersect(smin,[M; (N*M-M); sextmin]);

 % Peaks on up-down diagonals:
 [iext,jext] = ind2sub([M,N],unique([smax;smin]));
 [sextmax,sextmin] = extremos_diag(iext,jext,xy,-1);

 % Peaks on columns, rows and diagonals:
 smax = intersect(smax,[1; N*M; sextmax]);
 smin = intersect(smin,[1; N*M; sextmin]);
end

% Extrema points:
xymax = xy(smax);
xymin = xy(smin);

% Descending order:
[temp,inmax] = sort(-xymax); clear temp
xymax = xymax(inmax);
smax = smax(inmax);
[xymin,inmin] = sort(xymin);
smin = smin(inmin);


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

function [smax,smin] = extremos(matriz)
% Peaks through columns or rows.

smax = [];
smin = [];

for n = 1:length(matriz(1,:))
 [temp,imaxfil,temp,iminfil] = extrema(matriz(:,n)); clear temp
 if ~isempty(imaxfil)     % Maxima indexes
  imaxcol = repmat(n,length(imaxfil),1);
  smax = [smax; imaxfil imaxcol];
 end
 if ~isempty(iminfil)     % Minima indexes
  imincol = repmat(n,length(iminfil),1);
  smin = [smin; iminfil imincol];
 end
end


function [sextmax,sextmin] = extremos_diag(iext,jext,xy,A)
% Peaks through diagonals (down-up A=-1)

[M,N] = size(xy);
if A==-1
 iext = M-iext+1;
end
[iini,jini] = cruce(iext,jext,1,1);
[iini,jini] = ind2sub([M,N],unique(sub2ind([M,N],iini,jini)));
[ifin,jfin] = cruce(iini,jini,M,N);
sextmax = [];
sextmin = [];
for n = 1:length(iini)
 ises = iini(n):ifin(n);
 jses = jini(n):jfin(n);
 if A==-1
  ises = M-ises+1;
 end
 s = sub2ind([M,N],ises,jses);
 [temp,imax,temp,imin] = extrema(xy(s)); clear temp
 sextmax = [sextmax; s(imax)'];
 sextmin = [sextmin; s(imin)'];
end


function [i,j] = cruce(i0,j0,I,J)
% Indexes where the diagonal of the element io,jo crosses the left/superior
% (I=1,J=1) or right/inferior (I=M,J=N) side of an MxN matrix. 

arriba = 2*(I*J==1)-1;

si = (arriba*(j0-J) > arriba*(i0-I));
i = (I - (J+i0-j0)).*si + J+i0-j0;
j = (I+j0-i0-(J)).*si + J;


% Carlos Adrin Vargas Aguilera. nubeobscura@hotmail.com

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