I was reading a article about about Hermite interpolation on this link. In cardinal spline curve section they give a formula to calculate tangents at end point given as
$T_i = a ( P_{i+1} - P_{i-1} ) . $
Doubt:if i have two point $P_1$ AND $P_2$ then to find $T_1$
$T_1=a(P_2-P_0)$..i need to calculate this but what should be my $P_0$ point?
similarly to find $T_2$ i will need to KNOW $P_3$