x(u) = (1-u)^3 x_0 + 3(1-u)^2 u x_1 + 3 (1-u) u^2 x_2 + u^3 x_3
y(u) = (1-u)^3 y_0 + 3(1-u)^2 u y_1 + 3 (1-u) u^2 y_2 + u^3 y_3

the parameter u runs from 0 to 1.