Find zero crossings for a given 2D curve.

The code is very simple and needs only one line for basic results.

PURPOSE:

Find zero crossings of a given 2D curve.

If the curve is noisy, you might consider to smooth it beforehand,

e.g. with https://mathworks.com/matlabcentral/fileexchange/66099

Full call:

ZeroX=FindZeroCrossSimple(x,y,Interpolate)

INPUTS:

1 x: x-values. Vector with numbers.

2 y: y-values. Vector with numbers.

Optional value:

You may omit it.

3 Interpolate: 0: x zero position is nearest sample left of zero crossing

1: x zero position is at nearest sample

2: linear interpolation(default)

Output:

Zerox: Vector with zero x-positions

Empty vector for no zeros at all.

Processing time on my PC for 10^6 samples and 3.4*10^5 zero crossings is:

0) x zero position left of zero crossing: 9 ms

1) x zero position is at nearest sample: 21 ms

2) linear interpolation (default): 21 ms

If you process always in the same manner, you can delete many code lines.

E.g. you want allways all x zero position left of zero crossing:

 ZeroX=x(diff(sign(y))~=0);

Or you want allways all x zero position with linear interpolation:

   Zindx = find(diff(sign(y)));
   y1=y(Zindx);
   ZeroX=(y1*(x(1)-x(2)))./(y(Zindx+1)-y1)+x(Zindx); 

Or you want only ascending zero crosses with linear interpolation:

  Zindx = find(diff(sign(y))>0);%ascending zero crossings. 
  % For desending: Zindx = find(diff(sign(y))<0);%descending zero crossings.  
  y1=y(Zindx);
  ZeroX=(y1*(x(1)-x(2)))./(y(Zindx+1)-y1)+x(Zindx);