closestpt.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 [xpt,ypt,inode,d]=closestpt(x,y,xnot,ynot)

% [xpt,ypt,inode,d]=closestpt(x,y,xnot,ynot)
% 
% Given a piecewise linear curve whose nodes are defined by the vectors x & y
% CLOSESTPT returns the x,y coordinates of the point on the curve which is
% closest to (xnot,ynot) (which is not necessarily on the curve). This
% algorithm works well if (xnot,ynot) is close to the curve, however, if
% it is far away, then the returned point is just the closest node.
% xpt,ypt= coordinates of the closest point
% inode = closest point either is (x(inode),y(inode)) or occurs between 
% 		(x(inode),y(inode)) & (x(inode+1),y(inode+1))
% d = euclidean distance from (xnot,ynot) to (xpt,ypt)   
%
% 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 
% project may be contacted via email at:  [e-mail]
% 
% The term 'SOFTWARE' refers to the Matlab source code, translations to
% any other computer language, or object code
%
% Terms of use of this SOFTWARE
%
% 1) Use of this SOFTWARE by any for-profit commercial organization is
%    expressly forbidden unless said organization is a CREWES Project
%    Sponsor.
%
% 2) A CREWES Project sponsor may use this SOFTWARE under the terms of the 
%    CREWES Project Sponsorship agreement.
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% 3) A student or employee of a non-profit educational institution may 
%    use this SOFTWARE subject to the following terms and conditions:
%    - this SOFTWARE is for teaching or research purposes only.
%    - this SOFTWARE may be distributed to other students or researchers 
%      provided that these license terms are included.
%    - reselling the SOFTWARE, or including it or any portion of it, in any
%      software that will be resold is expressly forbidden.
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xhat1=[]; xhat2=[]; yhat1=[]; yhat2=[];
if length(x) ~= length(y)
	error(' Vectors x and y must have same length');
end

%t=clock;
n=length(x);

% test for equality
ind= find(x==xnot);
if(~isempty(ind))
	ind2=find(y(ind)==ynot);
	if(~isempty(ind2))
			xpt=x(ind(ind2));
			ypt=y(ind(ind2));
			inode=ind(ind2);
			d=0.0;
			return;
	end
end

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

it=[];
for k=1:length(indx)
	if isempty(indy)
		it=find(isempty(indx));
	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);
	d1=d(1);%closest node
	d2=d(2);%next closest
	% we hunt for a minimum at on the line segments going into and outof the closest
	% point 
	i1=ind(1);
	i2=i1+1;
	if( i2<=length(x) )
		[dp1,xhat1,yhat1]=pdist([x(i1) x(i2)],[y(i1) y(i2)],xnot,ynot);
		if( ~oncurve(x,y,xhat1,yhat1) )
			dp1=inf;
		end
	else
		dp1=inf;
	end
	i2=ind(1)-1;
	if( i2>=1 )
		[dp2,xhat2,yhat2]=pdist([x(i1) x(i2)],[y(i1) y(i2)],xnot,ynot);
		if( ~oncurve(x,y,xhat2,yhat2) )
			dp2=inf;
		end
	else
		dp2=inf;
	end
	% test for dp1 & dp2 finding the same point
	
	if( ~isempty(xhat1) & ~isempty(xhat2) & ~isempty(yhat1) & ~isempty(yhat2) )
		if( xhat1==xhat2 & yhat1==yhat2 )
			dp1=inf;
		end
	elseif( isempty(xhat1) & isempty(xhat2) & ~isempty(yhat1) & ~isempty(yhat2) )
		if( yhat1==yhat2 )
			dp1=inf;
		end
	elseif( ~isempty(xhat1) & ~isempty(xhat2) & isempty(yhat1) & isempty(yhat2) )
		if( xhat1==xhat2 )
			dp1=inf;
			
		end
	elseif( isempty(xhat1) & isempty(xhat2) & isempty(yhat1) & isempty(yhat2) )
			dp1=inf;
	end
	
	
	indd=find([dp1 dp2 d1]==min([dp1 dp2 d1]));
	
	if( indd==1 )
			xpt=xhat1;
			ypt=yhat1;
			inode=i1;
			d=dp1;
	elseif( indd==2 )
			xpt=xhat2;
			ypt=yhat2;
			inode=i2;
			d=dp2;
	else	
		xpt=x(ind(1));
		ypt=y(ind(1));
		inode=ind(1);
		d=d(ind(1));
	end
		
	return;
end
	
%compute perpedicular distance

if( x(itest+1)~=x(itest) )
	m=(y(itest+1)-y(itest))./(x(itest+1)-x(itest));
	b=y(itest)-m.*x(itest);

	dtest=abs(m*xnot-ynot+b)./sqrt(m.*m+1);

	d=min(dtest);

	it=find(d==dtest);
	ind=itest(it);
	xpt=(m*(ynot-y(ind))+xnot+m*m*x(ind))/(m*m+1);
	ypt=m*(xpt-x(ind))+y(ind);
	inode=ind;
else
	xpt=x(itest);
	ypt=ynot;
	d=abs(x(itest)-xnot);
	inode=itest(1);
end