# How to Construct Splines

This example shows how to construct splines in various ways using the spline functions in Curve Fitting Toolbox™.

### Interpolation

You can construct a *cubic spline interpolant* that matches the cosine function at the following sites `x`

, using the `csapi`

command.

x = 2*pi*[0 1 .1:.2:.9]; y = cos(x); cs = csapi(x,y);

You can then view the interpolating spline by using `fnplt`

.

fnplt(cs,2); axis([-1 7 -1.2 1.2]) hold on plot(x,y,'o') hold off

### Checking the Interpolant

The cosine function is 2*pi-periodic. How well does our cubic spline interpolant do in that regard? One way to check is to compute the difference in the first derivative at the two endpoints.

diff( fnval( fnder(cs), [0 2*pi] ) )

ans = -0.1375

To enforce periodicity, use `csape`

instead of `csapi`

.

csp = csape( x, y, 'periodic' ); hold on fnplt(csp,'g') hold off

Now the check gives

diff( fnval( fnder(csp), [0 2*pi] ) )

ans = -2.2806e-17

Even the second derivative now matches at the endpoints.

diff( fnval( fnder(csp, 2), [0 2*pi] ) )

ans = -2.2204e-16

The *piecewise linear interpolant* to the same data is available via `spapi`

. Here we add it to the previous plot, in red.

pl = spapi(2, x, y); hold on fnplt(pl, 'r', 2) hold off

### Smoothing

If the data are noisy, you usually want to approximate rather than interpolate. For example, with these data

```
x = linspace(0,2*pi,51);
noisy_y = cos(x) + .2*(rand(size(x))-.5);
plot(x,noisy_y,'x')
axis([-1 7 -1.2 1.2])
```

interpolation would give the wiggly interpolant shown below in blue.

hold on fnplt( csapi(x, noisy_y) ) hold off

In contrast, smoothing with a proper tolerance

tol = (.05)^2*(2*pi)

tol = 0.0157

gives a smoothed approximation, shown below in red.

hold on fnplt( spaps(x, noisy_y, tol), 'r', 2 ) hold off

The approximation is much worse near the ends of the interval, and is far from periodic. To enforce periodicity, approximate to periodically extended data, then restrict the approximation to the original interval.

noisy_y([1 end]) = mean( noisy_y([1 end]) ); lx = length(x); lx2 = round(lx/2); range = [lx2:lx 2:lx 2:lx2]; sps = spaps([x(lx2:lx)-2*pi x(2:lx) x(2:lx2)+2*pi],noisy_y(range),2*tol);

This gives the more nearly periodic approximation, shown in black.

hold on fnplt(sps, [0 2*pi], 'k', 2) hold off

### Least-Squares Approximation

Alternatively, you could use least-squares approximation to the noisy data by a spline with few degrees of freedom.

For example, you might try a cubic spline with just four pieces.

spl2 = spap2(4, 4, x, noisy_y); fnplt(spl2,'b',2); axis([-1 7 -1.2 1.2]) hold on plot(x,noisy_y,'x') hold off

### Knot Selection

When using `spapi`

or `spap2`

, you usually have to specify a particular spline space. This is done by specifying a *knot sequence* and an *order*, and this may be a bit of a problem. However, when doing spline interpolation to `x,y`

data using a spline of order `k`

, you can use the function `optknt`

to supply a good knot sequence, as in the following example.

k = 5; % order 5, i.e., we are working with quartic splines x = 2*pi*sort([0 1 rand(1,10)]); y = cos(x); sp = spapi( optknt(x,k), x, y ); fnplt(sp,2,'g'); hold on plot(x,y,'o') hold off axis([-1 7 -1.1 1.1])

When doing least-squares approximation, you can use the current approximation to determine a possibly better knot selection with the aid of `newknt`

. For example, the following approximation to the exponential function isn't all that good, as can be seen from its error, plotted in red.

x = linspace(0,10,101); y = exp(x); sp0 = spap2( augknt(0:2:10,4), 4, x, y ); plot(x,y-fnval(sp0,x),'r','LineWidth',2)

However, you can use that initial approximation to create another one with the *same* number of knots, but which are better distributed. Its error is plotted in black.

sp1 = spap2( newknt(sp0), 4, x, y ); hold on plot(x,y-fnval(sp1,x),'k','LineWidth',2) hold off

### Gridded Data

All the spline interpolation and approximation commands in the Curve Fitting Toolbox can also handle gridded data, in any number of variables.

For example, here is a bicubic spline interpolant to the Mexican Hat function.

x =.0001+(-4:.2:4); y = -3:.2:3; [yy,xx] = meshgrid(y,x); r = pi*sqrt(xx.^2+yy.^2); z = sin(r)./r; bcs = csapi({x,y}, z); fnplt(bcs) axis([-5 5 -5 5 -.5 1])

Here is the *least-squares* approximation to noisy values of that same function on the same grid.

knotsx = augknt(linspace(x(1), x(end), 21), 4); knotsy = augknt(linspace(y(1), y(end), 15), 4); bsp2 = spap2({knotsx,knotsy},[4 4], {x,y},z+.02*(rand(size(z))-.5)); fnplt(bsp2) axis([-5 5 -5 5 -.5 1])

### Curves

Gridded data can be handled easily because Curve Fitting Toolbox can deal with *vector-valued* splines. This also makes it easy to work with parametric curves.

Here, for example, is an approximation to infinity, obtained by putting a cubic spline curve through the points marked in the following figure.

t = 0:8; xy = [0 0;1 1; 1.7 0;1 -1;0 0; -1 1; -1.7 0; -1 -1; 0 0].'; infty = csape(t, xy, 'periodic'); fnplt(infty, 2) axis([-2 2 -1.1 1.1]) hold on plot(xy(1,:),xy(2,:),'o') hold off

Here is the same curve, but with motion in a third dimension.

roller = csape( t , [ xy ;0 1/2 1 1/2 0 1/2 1 1/2 0], 'periodic'); fnplt( roller , 2, [0 4],'b' ) hold on fnplt( roller, 2, [4 8], 'r') plot3(0,0,0,'o') hold off

The two halves of the curve are plotted in different colors and the origin is marked, as an aid to visualizing this two-winged space curve.

### Surfaces

Bivariate tensor-product splines with values in R^3 give surfaces. For example, here is a good approximation to a torus.

x = 0:4; y = -2:2; R = 4; r = 2; v = zeros(3,5,5); v(3,:,:) = [0 (R-r)/2 0 (r-R)/2 0].'*[1 1 1 1 1]; v(2,:,:) = [R (r+R)/2 r (r+R)/2 R].'*[0 1 0 -1 0]; v(1,:,:) = [R (r+R)/2 r (r+R)/2 R].'*[1 0 -1 0 1]; dough0 = csape({x,y},v,'periodic'); fnplt(dough0) axis equal, axis off

Here is a crown of normals to that surface.

nx = 43; xy = [ones(1,nx); linspace(2,-2,nx)]; points = fnval(dough0,xy)'; ders = fnval(fndir(dough0,eye(2)),xy); normals = cross(ders(4:6,:),ders(1:3,:)); normals = (normals./repmat(sqrt(sum(normals.*normals)),3,1))'; pn = [points;points+normals]; hold on for j=1:nx plot3(pn([j,j+nx],1),pn([j,j+nx],2),pn([j,j+nx],3)) end hold off

Finally, here is its projection onto the (x,y)-plane.

```
fnplt(fncmb(dough0, [1 0 0; 0 1 0]))
axis([-5.25 5.25 -4.14 4.14]), axis off
```

### Scattered Data

It is also possible to interpolate to values given at ungridded data sites in the plane. Consider, for example, the task of mapping the unit square smoothly to the unit disk. We construct the data values, marked as circles, and the corresponding data sites, marked as x's. Each data site is connected to its associated value by an arrow.

n = 64; t = linspace(0,2*pi,n+1); t(end) = []; values = [cos(t); sin(t)]; plot(values(1,:),values(2,:),'or') axis equal, axis off sites = values./repmat(max(abs(values)),2,1); hold on plot(sites(1,:),sites(2,:),'xk') quiver(sites(1,:),sites(2,:), ... values(1,:)-sites(1,:), values(2,:)-sites(2,:)) hold off

Then use `tpaps`

to construct a bivariate interpolating vector-valued thin-plate spline.

st = tpaps(sites, values, 1);

The spline does indeed map the unit square smoothly (approximately) to the unit disk, as its plot via `fnplt`

indicates. The plot shows the image of a uniformly-spaced square grid under the spline map in `st`

.

hold on fnplt(st) hold off