Compute,store, and find min of 3d distances between two different matrices.
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Hello, I have trouble using the built in functions of matlab to do this because what I want is different from what I think those functions do. I have two matrices of 3d points, say A and B. They do not have the same number than elements A<<B. I want to compute distances between each point in A and all points in B, and then be able to store it with the distance and index of the point in B. Then repeat for the next point in A until the end. I was thinking to try to use nested loops reiterating through A and the inner one through B, I just am unsure how to finely put the storage of the distance and index of the min into another vector or matrix (size A x 1)
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Matt J
2013-8-16
编辑:Matt J
2013-8-16
Here's what I use, but there are better optimized alternatives on the FEX.
[distances,index]=min(interdists(A,B),[],2);
function Graph=interdists(A,B)
%Finds the graph of distances between point coordinates
%
% (1) Graph=interdists(A,B)
%
% in:
%
% A: matrix whose columns are coordinates of points, for example
% [[x1;y1;z1], [x2;y2;z2] ,..., [xM;yM;zM]]
% but the columns may be points in a space of any dimension, not just 3D.
%
% B: A second matrix whose columns are coordinates of points in the same
% Euclidean space. Default B=A.
%
%
% out:
%
% Graph: The MxN matrix of separation distances in l2 norm between the coordinates.
% Namely, Graph(i,j) will be the distance between A(:,i) and B(:,j).
%
%
% (2) interdists(A,'noself') is the same as interdists(A), except the output
% diagonals will be NaN instead of zero. Hence, for example, operations
% like min(interdists(A,'noself')) will ignore self-distances.
%
% See also getgraph
noself=false;
if nargin<2
B=A;
elseif ischar(B)&&strcmpi(B,'noself')
noself=true;
B=A;
end
N=size(A,1);
B=reshape(B,N,1,[]);
Graph=l2norm(bsxfun(@minus, A, B),1);
Graph=permute(Graph,[2,3,1]);
if noself
n=length(Graph);
Graph(linspace(1,n^2,n))=nan;
function out=l2norm(X,varargin)
%out=l2norm(X,varargin)
%
%Takes the L2 norm along desired dimensions of the array X.
%
%VARARGIN can be any of the additional arguments of the sum.m function
%and follows the same conventions as this and similar functions. That is,
%if VARARGIN={}, the function operates along the first non-singleton
%dimension, etc...
%
%SEE ALSO sumsq
X=X.^2;
out=sum(X,varargin{:});
out=sqrt(out);
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