closestnode.m

基于matlab的反演程序,用于地球物理勘探中射线追踪及偏移成像程序.

function inode=closestnode(x,y,xnot,ynot)

% inode=closestnode(x,y,xnot,ynot)
% 
% Given a piecewise linear curve whose nodes are defined by the vectors x & y
% CLOSESTNODE returns the index of the node on the curve which is closest to
% the point (xnot,ynot)
%
% G.F. Margrave, December 1993
%
% NOTE: It is illegal for you to use this software for a purpose other
% than non-profit education or research UNLESS you are employed by a CREWES
% Project sponsor. By using this software, you are agreeing to the terms
% detailed in this software's Matlab source file.
 
% BEGIN TERMS OF USE LICENSE
%
% This SOFTWARE is maintained by the CREWES Project at the Department
% of Geology and Geophysics of the University of Calgary, Calgary,
% Alberta, Canada.  The copyright and ownership is jointly held by 
% its author (identified above) and the CREWES Project.  The CREWES 
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it=[];
n=length(x);

% test for equality
ind= find(x==xnot);
if(~isempty(ind))
	ind2=find(y(ind)==ynot);
	if(~isempty(ind2))
			inode=ind(ind2);
			return;
	end
end

% find the points which bracket xnot,ynot
indx = surround(x,xnot);
indy = surround(y,ynot);

for k=1:length(indx)
	if isempty(indy) 
		it=find(isempty(indx(k)));
	else
		it=find(indx(k)==indy);
	end
	if ~isempty(it)
		itest=indx(k);
		break;
	end
end

if(isempty(it))
	% if no points bracket (xnot,ynot) then it is way exterior to the curve
	% we have no choice but to compute the distances to the various nodes and find 
	% the minimum
	d=sqrt( (x-xnot).^2 + (y-ynot).^2 );
	[d,ind]=sort(d);
	inode=ind(1);
	return;
else
	itest=[itest itest+1];
	d=sqrt( (x(itest)-xnot).^2 + (y(itest)-ynot).^2 );
	ind=find(d==min(d));
	inode=itest(ind(1));% just in case we get two minima
	return;
end