# Get y from x on the cardinal spline curve

by mishalkk
Tags: spline
 P: 4 Hi, Thank you for your reply :) http://algorithmist.wordpress.com/20...plines-part-2/ Cardinal splines specify the tangents at interior points based on the vector from previous point to subsequent point. Each tangent is parallel to this vector and some multiple of its length. For example, the tangent direction at point P1 is parallel to the vector P2 – P0, or we could simply write something like T1 = s(P2 – P0) where s is a real number. Here is the pat of the code. xtarget is the input value X, here in my case 1.5  for (Double t = 0; t<=1; t += 0.01) { s = (1 - t) / 2; P(t)x = s(-t3 + 2t2 – t)P1X + s(-t3 + t2)P2X + (2t3 – 3t2 + 1)P2X + s(t3 – 2t2 + t)P3X + (-2t3 + 3t2)P3X + s(t3 – t2)P4X P(t)y = s(-t3 + 2t2 – t)P1Y + s(-t3 + t2)P2Y + (2t3 – 3t2 + 1)P2Y + s(t3 – 2t2 + t)P3Y+ (-2t3 + 3t2)P3Y + s(t3 – t2)P4Y if(P(t)x=>xtarget) { return P(t)y; } ----------------- I get P(t)y which is approximate. But it is not the exact point on the curve(sometimes it is not on the curve). I need to get the exact points on the curve..ie P(t)y which is exactly on the curve.Any help is appreciated. Thank you :)