Set value for elements within a polygon

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Hi all,
I am new to Matlab and trying to solve a problem with polygon.
I want to create a random 2-D matrix, let's say 100 by 100 as a domain.
domain=ones(100);
Then by enter the x & y coordinates of the polygon, I would like to draw that polygon and set all the elements inside the polygon to a different value.
xv=[40;50;60;50;40];
yv=[60;50;60;70;60];
plot(xv,yv) %plot the polygon
in=inpolygoon(xq,yq,xv,xv);
domain(in)=2;
I am stuck right here, I dont know how to define xq and yq to complete the inpolygon syntax. Please help :(.
Thank you all.

Accepted Answer

Matt J
Matt J on 7 Aug 2021
Edited: Matt J on 7 Aug 2021
If you have the Image Processing Toolbox,
domain=ones(100);
xv=[40;50;60;50;40];
yv=[60;50;60;70;60];
in = roipoly(domain,xv,yv);
domain( in )=2;
imshow(domain); caxis([0,2])
  3 Comments
Matt J
Matt J on 8 Aug 2021
Edited: Matt J on 8 Aug 2021
That will be very slow. Sean's answer does essentially the same thing, but much faster. My answer will be even faster than that, if you have the required toolbox.
Anh Mai
Anh Mai on 9 Aug 2021
Yup, that's true. That's why I said it might look amateur :)). Thanks again for the solution.

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More Answers (1)

Sean de Wolski
Sean de Wolski on 7 Aug 2021
Something along this line will work too. Note, you may need to deal with x/y v. row/col indexing (swap meshgrid for ndgrid).
domain = ones(100);
[xx,yy] = meshgrid(1:100);
xv=[40;50;60;50;40];
yv=[60;50;60;70;60];
% plot(xv,yv) %plot the polygon
xvec = xx(:);
yvec = yy(:);
in=inpolygon(xx(:),yy(:),xv,yv);
domain(sub2ind([100 100],xvec(in),yvec(in)))=2;
imshow(domain); caxis([0,2])
  1 Comment
Matt J
Matt J on 8 Aug 2021
Edited: Matt J on 8 Aug 2021
No need for sub2ind that I can see:
domain( inpolygon(xx(:),yy(:),xv,yv) )=2;

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