Locate elements of a vector inside a meshgrid

Question

Alberto Belvedere 2020-10-6

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https://ww2.mathworks.cn/matlabcentral/answers/606296-locate-elements-of-a-vector-inside-a-meshgrid

评论： Alberto Belvedere 2020-10-6

I'm trying to build a matrix RES, with the same dimensions of X and Y, that has a '1' in the position pointed (with a tollerance tollX and tollY) by the i-th couple contained in V.

[X,Y]=meshgrid(0:1:3,-2:1:2);  
V=[2.1 1.2;
     0.2 0.7;
     3.1 1.9;
     1.6 -1];
tollX=0.5; 
tollY=0.5;
RES=0;
for i=1:length(V)
    RES=(V(i,1)<X+tollX).*(V(i,1)>X-tollX).*(V(i,2)<Y+tollY).*(V(i,2)>Y-tollY)+RES;
end

This "rough" solution works well with small meshgrids and V, but since i have to manage far bigger data sets i would like to vectorize and refine the code to get better performances.

P.S. I expected '&' operator to be faster in general than ' .* ', but this doesn't seem to be true, at least for my case.

To test this I simply changed the statement inside the for loop with this one:

RES=((V(i,1)<X+tollX)&(V(i,1)>X-tollX)&(V(i,2)<Y+tollY)&(V(i,2)>Y-tollY))|RES;

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Answer 1

Rik 2020-10-6

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https://ww2.mathworks.cn/matlabcentral/answers/606296-locate-elements-of-a-vector-inside-a-meshgrid#answer_506486

I would suggest using ismembertol, or consider functions like normxcorr2 from the image processing toolbox.

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Rik 2020-10-6

I'm not quite sure how implicit expansion (with bsxfun) would increase performance. I suspect ismembertol would be more efficient. I also don't necessarily see how find would be useful.

Alberto Belvedere 2020-10-6