How do I make a rectangular mask of 1's?

[aushah]

Hi. I have a matrix of 768x1024. Now, within this matrix, I want to have a rectangle such that it is centered around the middle column of the matrix and has dimension (k)x1024, where k is a variable. This (k)x1024 rectangular portion within the 768x1024 matrix should have all entries 1 and the rest of the matrix entries should all be zero. So, it creates sort of a mask so that when I multiply it with another 768x1024 matrix, the result is only the rectangular part of the other matrix. All help is appreciated. Thanks.
ps: I am attaching a sketch of what I mean by what I said above.

 

[IRstuff]

poly2mask

TTFN
faq731-376

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[flash3780]

Could you do it like this?

function main
totrow = 768;
totcol = 1024;  
k      = 10;    %Number of rows of ones
D      = maskmatrix(totrow,totcol,k)
end

function D = maskmatrix(totrow,totcol,k)
startones = floor((totrow-k)/2);
endones   = startones+k;

A = zeros(startones,totcol);
B = ones(k,totcol);
C = zeros(totrow-k-startones,totcol);
D = [A;B;C];
end