function U = xyhomproj (T)
    % given a 4xn matrix T result of a perspective 3D projection
    % determine 2xn matrix U by diving first 2 rows of T by 4th row
    % thus normalizing the homogenerous coordinate to 1
    U(1,:)=T(1,:) ./ T(4,:);
    U(2,:)=T(2,:) ./ T(4,:);