GetContourNormals2D.m

function N=GetContourNormals2D(P, Closed)
% This function calculates the normals, of the contour points
% using the neighbouring points of each contour point
%
% N=GetContourNormals2D(P)
%
% inputs,
%  P : List with contour coordinates M x 2
%
% outputs,
%  N : List with contour normals M x 2
%
% Function is written by D.Kroon University of Twente (July 2010)

% Use the n'th neighbour to calculate the normal (more stable)
if Closed
    a=4;
    
    % From array to separate x,y
    xt=P(:,1); yt=P(:,2);
    
    % Derivatives of contour
    n=length(xt);
    f=(1:n)+a; f(f>n)=f(f>n)-n;
    b=(1:n)-a; b(b<1)=b(b<1)+n;
    
    dx=xt(f)-xt(b);
    dy=yt(f)-yt(b);
    % Normals of contourpoints
    l=sqrt(dx.^2+dy.^2);
    nx = -dy./l;
    ny =  dx./l;
    N(:,1)=nx; N(:,2)=ny;
else
    N = zeros(size(P));
end