# Catmull Rom splines, scaling the input.

1. Jan 20, 2010

### Aeroengie

Hi. I've tried doing some camull-rom interpolation for the following data.
x y
0.2 1.221402861
0.3 1.349864712
0.4 1.491929555
0.5 1.649697833
0.6 1.828165418
0.7 2.042000232
0.8 2.332915111
0.9 2.808281551
1 3.718281828
1.1 5.597908484
1.2 9.511853345
1.3 17.45514585
1.4 32.98066546
1.5 62.14672813
1.6 114.9041952
1.7 207.0733374
1.8 363.0963701
1.9 619.7925202
2 1031.389056

I chose to interpolate for point 1.1 (i'm assuming this point is non-existent).
I am using the following method:

f(x) = [1, x, x^2, x^3] * M * [p0, p1, p2, p3]

And M is the matrix,

0.0 1.0 0.0 1.0
-0.5 0.0 0.5 0.0
1.0 -2.5 2.0 -0.5
-0.5 1.5 -1.5 0.5

The "tension" value (greek letter tau) is 0.5.

Thus, the 4 points chosen were 0.9, 1, (2 before 1.1), 1.2, 1.3 (the 2 after 1.1).

I did it in matlab and got wrong values (using an input of 1.1 and expecting to get a value close to 5.5979). I have since learnt that the input needs to be between 0 and 1. How do I scale this value of 1.1 to be between 0 and 1 ? And does anything else change , in the M matrix, matrix of points [p0, p1, p2, p3] ?

Thankyou.