Assigning values ta set of points in 2d image

After performing an operation on an image I get a set of points, the Cartesian coordinates are stored in a variable 'xy' of dimension n by 2. So xy(:,1) is x coordinate of each point and xy(:,2)is the y coordinate.

I want to create a new image of the same dimensions as the original image with these set of points being 1 and other points 0.

Let me create a problem to explain it further.

A = magic(10);
[x,y] = find(A<=20);
xy = [x,y];
B = zeros(size(A));

% now assign B as 1 for all points A<=20
Please help with the assignment command.

B(x,y) doesn't work i would like to know why ?

Answer 1

[Image Analyst]

Then just do it this way:

A = magic(10)
[y,x] = find(A<=20); 
% Note: y = rows, x = cols
B = zeros(size(A));
linearInd = sub2ind(size(A), y, x)
B(linearInd) = 1

Answer 2

[Image Analyst]

Why not just simply say

B = A<=20;

You can cast B from logical to another data type after that if you want.

[Suhas Deshpande]

No Because in the real problem it is not as easy as that. The problem i have put forward here is just a random one which will help you guys answer my question better. I have got coordinates using an algorithm i cannot discuss here. And i have to use the 'm by 2 matrix' to assign the new variable.

Answer 3

[Vishwajith Upendra]

for i=1:1:numel(x)

 B(x(i),y(i))=1

end

This should do it. Number of elements in x and y are gonna be the same. So, took one of them.

Hope, that helped.

[Suhas Deshpande]

I am trying to avoid any loops to improve the code performance, but this works. But I think it should be possible without a loop.

Thanks anyways