I is already a matrix, so what do you mean by obtain values in matrix form?
Matrix indexing in matlab starts at 1, so your loops should be
for x = 1:xmax
for y = 1:ymax
%whatever you want to do
end
end
Note that if indexing started at 0, then your loop would have to terminate at xmax-1, not xmax.
Also note that indexing is in the form (row, column), so if you use the standard convention that x is horizontal and y vertical, then your indexing should be:
I(y, x)
