spcrvdem.m

演示matlab曲线拟和与插直的基本方法

%%        More Spline Curves
% Use of SPMAK, SPCRV or CSCVN to generate spline curves.

%   Copyright 1987-2005 C. de Boor and The MathWorks, Inc.
%   $Revision: 1.18.4.2 $

%% A simple spline curve
% The Spline Toolbox can handle  v e c t o r - v a l u e d  splines.
% A d-vector valued univariate spline provides a curve in d-space.
% In this mode,  d = 2  is most common, as it gives plane curves.
%
% Here is an example, in which a spline with 2-dimensional coefficients
% is constructed and plotted.

knots = [1,1:9,9];
curve = spmak( knots, repmat([ 0 0; 1 0; 1 1; 0 1 ], 2,1).' );

t = linspace(2,8,121); values = fnval(curve,t);
plot(values(1,:),values(2,:),'linew',2)
axis([-.2 1.2 -.2 1.2]), axis equal, grid off
title('A spline curve')

%% Add some tangent vectors
% We also draw the tangent vector to the curve at some points.

t = 3:.4:6.2; lt = length(t);
cv = fnval( curve, t );
cdv = fnval( fnder(curve), t );
hold on
quiver(cv(1,:),cv(2,:), cdv(1,:),cdv(2,:))
xlabel('... and its tangent vector at selected points.')
hold off

%% A word of caution
% You may have noticed that, in this example, I did not use FNPLT to plot the
% curve, but instead plotted some point on the curve obtained by FNVAL:
%     t = linspace(2,8,121); values = fnval(curve,t);
%     plot(values(1,:),values(2,:),'linew',2)
% Using FNPLT directly with this particular curve gives the red curve above.
% The explanation?
% The spline is of order 4, yet the end knots in the knot sequence
%      knots = [1,1:9,9];
% have only multiplicity 2. Therefore, all the B-splines of order 4 for this
% knot sequence are 0 at the endpoints of the basic interval. This makes the
% curve start and stop at (0,0).
hold on
fnplt(curve,'r',.5)
xlabel('... and its plot (red) via FNPLT.')
hold off
%% A remedy
% Since, in this case, we are really interested only in the curve segment
% corresponding to the parameter interval [2 .. 8], we can use FNBRK to extract
% that part, and then have no difficulty plotting it with FNPLT:
mycurve = fnbrk(curve,[2 8]);
hold on, fnplt(mycurve,'y',2.5)
xlabel('... and its interesting part (yellow) plotted via FNPLT.'), hold off
%% SPCRV: control polygon ...
% I use spline curves extensively in the generation of illustrations in
% which nothing more than a smooth curve of a certain roughly imagined
% shape is required. For this, the toolbox contains a special M-file called
% SPCRV which is independent of the rest of the setup. Given a sequence
% of points in the plane and, optionally, an order  k , it generates (by
% repeated midpoint knot insertion) the spline curve (of order  k ) whose
% control polygon is specified by the given sequence.
% The above picture shows such a control polygon. The next slide shows the
% corresponding spline curve of order 3.

points = [0 0; 1 0; 1 1; 0 2; -1 1; -1 0; 0 -1; 0 -2].';
plot(points(1,:),points(2,:),'k'), axis([-2 2 -2.1 2.2]),  grid off
title('Control polygon')

%% ... and corresponding spline curve
% We have added the corresponding spline curve of order 3 provided by SPCRV.

hold on
values = spcrv(points,3);
plot(values(1,:),values(2,:),'r','linew',1.5)
xlabel(' ... and the corresponding quadratic spline curve')

%%
% You notice that the curve touches each segment of the control polygon
% at its midpoint, and follows the shape outlined by the control polygon.

%% Raising the order
% Raising the order  k  will pull the curve away from the control polygon
% and make it smoother, but also shorter.
% Here we added the corresponding spline curve of order 4.

value4 = spcrv(points,4);
plot(value4(1,:),value4(2,:),'b','linew',2)

xlabel(' ... and the corresponding spline curves, quadRatic and cuBic')

%% CSCVN
% On the other hand, the command CSCVN provides an interpolating curve.
% Here is the resulting parametric `natural' cubic spline curve:

fnplt( cscvn(points), 'g',1.5 )

xlabel(' ... and now with the interpolating spline curve (in green) added')

%%
% By adding the point  (.95,-.05)  near  the second control point, (1,0), we
% can make this curve turn faster there:

[d,np] = size(points);
fnplt( cscvn([ points(:,1) [.95; -.05] points(:,2:np) ]), 'm',1.5)
xlabel(['The interpolating curve (magenta) is made to ',...
      'turn faster by addition of another point'])
plot(.95,-.05,'*')
legend('control polygon','quadratic spline curve','cubic spline curve',...
       'interpolating spline curve','faster turning near (1,0)',4)
title('')
hold off


displayEndOfDemoMessage(mfilename)