Question about for loops and immultiply

I wanted to edit a zeros matrix with a certain pattern of some various function, for this argument lets say that the function is a square (any geometrical pattern should be the same method?) I also wanted to implement immultiply, except i get a whitespace... Without using a for loop as such:

Z = zeros(800,600);
for m=275:325
    for n=375:425
        Z(m,n,:)=1
    end
end
Is = immultiply(Id,Z); imshow(Is);

How do i go about it? because the for loop takes like 15 minutes..

for example I can make a diamond looking square but idk know a regular square equation at the center

[M N]= size(Id);
X=0:N-1;
Y=0:M-1;
[X Y]=meshgrid(X,Y);
Cx=0.5*N;
Cy=0.5*M;
square = (abs(X-Cx)/25+abs(Y-Cy)/25)
imshow(square)
figure
Is = immultiply(Id, square); imshow(Is);

Thanks, Benjy

[KSSV]

What is Id in the above code?

Loop is taking much time because you are taking the output on the screen. Terminate the line Z(m,n,:)=1 ( Z(m,n,:)=1;). Off-course this can be done without loop.

Answer 1

[Thorsten]

A fast method to create a white square is

 Z(800,600)= 0; 
 Z(275:325, 375:425) = 1;