I solve this issue using improfile since it get an accurate index of non-square diagonal. this diagonal index will guide us thug translation process.

[col, row, ~] = improfile(fix_img, [1, size(fix_img, 2)], [1, size(fix_img, 1)]);

col = round(col);

row = round(row);

for i =1:10:length(col)

fix_t = fix_img(1:row(i),1:col(i));%overlap area of first image

flt_t = float_img(row(length(col)-i+1):end,col(length(col)-i+1):end);%overlap area of second image

% disp(['diag -> i1 = ',num2str(i),' float =',num2str(size(flt_t)),' fix =',num2str(size(fix_t))]);

% if ~isequal(size(flt_t),size(fix_t))

% error('the overlap is not correct');

% end

end

I hope it is clear :)