curve fitting: how to get the fitting curve?

Hi everyone~

I did some curve fitting using curve fitting toolbox. But I don't know how to get the fitting curve. Can somebody help?

Also, in the future I want to do this using code. I'm reading the manual for spline fitting. http://www.mathworks.com/help/curvefit/construction.html

There seems to be lots of kinds of it. I want it shape-conserve, and must pass all the points. Which one should I use?

Any help is appreciated. Thank you!

Answer 1

[John D'Errico]

Use pchip to create a shape preserving spline. ppval can evaluate it.

[Yuji Zhang]

Hi Thanks John~

I did testing and found "spline" is more shape-preserving. See the example.

Do you know how I can control the curve to go up or down at the side where there's no data? Any discussion's appreciated. Thank u~

Example: circles: data (4 points)

blue curve: pchip

red curve: spline

code:

x0 = [-10 -5 0 5 ];
y0 = [-50 -10 0 -10];

x = -50:0.1:50;

y1 = pchip (x0, y0, x);
y2 = spline (x0, y0, x);

plot(x0, y0, 'o');
hold on;
plot(x, y1,'b');
plot(x, y2,'r');

[Yuji Zhang]

Hi John~

Actually too bad. For my case, "pchip" doesn't work for the left end, and "spline" doesn't work for the right end. I just want a bell shape facing down.

See the plot:

Code:

x0 = [-10 -5 0 5 100];
y0 = [-50 -10 0 -10 -200];

x = -300:0.1:300;

y1 = pchip (x0, y0, x);
y2 = spline (x0, y0, x);

plot(x0, y0, 'o');
hold on;
plot(x, y1,'b');
plot(x, y2,'r');
ylim([-700 200]);

Let me know is there any way to fix it. Thanks a lot~