Set value for elements within a polygon

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.

Answer 1

[Matt J]

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])

[Anh Mai]

Thanks for your suggestion. I just realized that I also can do the following syntax, look amateur but it works. lol.

domain=ones(100);
count=1;
for i=1:100
    for j=1:100
        xq(count)=i;
        yq(count)=j;
        count=count+1;
    end
end
xv=[10;40;70;10];
yv=[50;30;50;50];
in=inpolygon(xq,yq,xv,yv);
domain(in)=2;
imagesc(domain);

[Matt J]

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]

Yup, that's true. That's why I said it might look amateur :)). Thanks again for the solution.

Answer 2

[Sean de Wolski]

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])

[Matt J]

No need for sub2ind that I can see:

domain( inpolygon(xx(:),yy(:),xv,yv)  )=2;