1 ## Copyright (C) 2008 Carlo de Falco <carlo.defalco@gmail.com>
3 ## This program is free software; you can redistribute it and/or modify it under
4 ## the terms of the GNU General Public License as published by the Free Software
5 ## Foundation; either version 3 of the License, or (at your option) any later
8 ## This program is distributed in the hope that it will be useful, but WITHOUT
9 ## ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
10 ## FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
13 ## You should have received a copy of the GNU General Public License along with
14 ## this program; if not, see <http://www.gnu.org/licenses/>.
17 ## @deftypefn {Function File} {@var{x0}} = zerocrossing (@var{x}, @
19 ## Estimates the points at which a given waveform y=y(x) crosses the
20 ## x-axis using linear interpolation.
21 ## @seealso{fzero, roots}
24 function retval = zerocrossing (x,y)
27 crossing_intervals = (y(1:end-1).*y(2:end) <= 0);
28 left_ends = (x(1:end-1))(crossing_intervals);
29 right_ends = (x(2:end))(crossing_intervals);
30 left_vals = (y(1:end-1))(crossing_intervals);
31 right_vals = (y(2:end))(crossing_intervals);
32 mid_points = (left_ends+right_ends)/2;
33 zero_intervals = find(left_vals==right_vals);
34 retval1 = mid_points(zero_intervals);
35 left_ends(zero_intervals) = [];
36 right_ends(zero_intervals) = [];
37 left_vals(zero_intervals) = [];
38 right_vals(zero_intervals) = [];
39 retval2=left_ends-(right_ends-left_ends).*left_vals./(right_vals-left_vals);
40 retval = union(retval1,retval2);
45 %! x = linspace(0,1,100);
46 %! y = rand(1,100)-0.5;
47 %! x0= zerocrossing(x,y);
48 %! y0 = interp1(x,y,x0);
49 %! assert(norm(y0,inf), 0, 100*eps)
52 %! x = linspace(0,1,100);
53 %! y = rand(1,100)-0.5;
55 %! x0= zerocrossing(x,y);
56 %! y0 = interp1(x,y,x0);
57 %! assert(norm(y0,inf), 0, 100*eps)
60 %! x = linspace(0,1,100);
61 %! y = rand(1,100)-0.5;
62 %! x0= zerocrossing(x,y);
63 %! y0 = interp1(x,y,x0);
64 %! plot(x,y,x0,y0,'x')
67 %! x = linspace(0,1,100);
68 %! y = rand(1,100)-0.5;
70 %! x0= zerocrossing(x,y);
71 %! y0 = interp1(x,y,x0);
72 %! plot(x,y,x0,y0,'x')