Hi Azarm,
A good way to solve this problem will be to obtain a logical image that represents the boundaries of the first frame. Once you have this logical image, you can use logical indexing to fill the region inside the boundary with specified values.
a = rand(200);
% Create a logical image that defines a rectangular boundary.
mask = false(size(a));
mask(10:100,10:100) = true;
% Assign a fill value of 40 to all pixels within the boundary
a(mask) = 40;
figure, imagesc(a)
I'm not sure whether you have the Image Processing Toolbox. If you do, you might be interested in looking at the function poly2mask.
doc poly2mask
This function is useful in moving from X,Y definitions of boundaries to a logical image representation of the mask.
