Find matching points from two coordinate systems

Asked by Yangfan Peng on 7 Oct 2019

Latest activity Commented on by Matt J on 10 Oct 2019

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Hello,

I have 10 points with x and y coordinate (cell locations imaged with microscope). I have used two different methods to image these cells, so I have two sets of coordinates that could be rotated, sheared, shifted, mirrored to each other. I would like to assign each point from one coordinate system to the matching point in the other coordinate system. How can I do that? In theory, some kind of transformation optimizing minimum distance between the points would be desirable.

In the end I would like to assign properties I obtained from the cells in one method to properties obtained with the other method.

Thank you.

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Matt J on 8 Oct 2019

Generally speaking, it is an impossible problem. For example, suppose there were 3 points in each image with the A-points forming an equilateral triangle and the B-points an equilateral triangle in a different position. Then there is no unique matching between the points that can be determined purely from their relative spacing - the symmetry among the points is too perfect. There would have to be some a priori known asymmetry among the points in order to match them.

darova on 8 Oct 2019

What if just find closest pairs of points? Using pdist2() for example?

Yangfan Peng on 8 Oct 2019

I always have 7-10 points and they are mostly asymmetric. I would like to rotate/mirror the coordinates in a way that the points can be closest to each other. How can I combine pdist2() with the transformation part?

2 Answers

Link

Answer by Bruno Luong on 8 Oct 2019

Look for the literature of image registration.

For simple rotation/scaling/translation you can use Matt J's submission

For more complex deformation, you need to apply spline deformation

There are a bunch of intermediate method for camera which take into account for camera cushion distortion or higher order. Pick one that is suitable for your need.

2 Comments

Matt J on 8 Oct 2019

See also, fitgeotrans

https://www.mathworks.com/help/images/ref/fitgeotrans.html

Yangfan Peng on 8 Oct 2019

Thank you for the suggestions. I used the fitgeotrans function and got the transformation object. But how do I go from there to determine which point matches to which other point? See above for detailed description of the problem.

Link

Answer by Matt J on 8 Oct 2019

Edited by Matt J on 8 Oct 2019

If the shear component of the deformation isn't too strong, the matchpoints function defined below might work. It will sort the rows of B to correspond with A and also return the corresponding permutation indices. It relies on absor, mentioned by Bruno, which you will have to download

https://www.mathworks.com/matlabcentral/fileexchange/26186-absolute-orientation-horn-s-method

function [permIndices,Bsorted]=matchpoints(A,B)
%                 
%IN:              
%                 
%    A: an Nx2 matrix of points         
%    B: an Nx2 matrix of points from A transformed and unordered          
%                 
%OUT:             
%                 
%    permIndices: permutation indices of rows of B thought to match A 
%        Bsorted: the Nx2 permuted version of B
    La=landmarks(A);
    Lb=landmarks(B);
    B3=B.'; B3(3,:)=0;
    reg=absor( Lb,La,'doScale',1);
    C3=(reg.s*reg.R)*B3+reg.t;
    C=C3(1:2,:).';
    [~,permIndices]=pdist2(C,A,'euclidean','Smallest',1);
    if nargout>1
     Bsorted=B(permIndices,:);
    end
            function L=landmarks(P)
 
                G=pdist2(P,P); G(~G)=nan;
                
                [i,j]=find( G==min(G(:)) ,1);
                I=P(i,:);
                J=P(j,:);
                K=mean(P,1);
                
                if norm(I-K)<norm(J-K)
                    [I,J]=deal(J,I);
                end
                L=[I;J;K].';
                L(3,:)=0;
                
            end
end

Applying it to your example data, I obtain,

A = [376 455;421 489;465 537;353 512;355 535;329 571;377 593;417 598;482 575;355 634];
B = [168 88;107 138;69 194;126 229;163 232;199 267;272 239;228 210;235 155;215 68];
[permIndices,Bsorted]=matchpoints(A,B)
plot(graph(1:10,1:10),'EdgeColor','none','XData',A(:,1),'YData',A(:,2));
hold on
plot(graph(1:10,1:10),'EdgeColor','none','XData',Bsorted(:,1),'YData',Bsorted(:,2));
hold off

2 Comments

darova on 8 Oct 2019

Why so many times edited? Do you have some problems?

Matt J on 10 Oct 2019

Just making improvements.

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