% The following assumes you have read the help sections on voronoi and
% delaunay. Start with vectors x,y giving the contour a 2D shape:  get 
% their full voronoi diagram of edges vx(1,i),vy(1,i) to vx(2,i), vy(2,i).
[vx,vy] = voronoi(x,y);

% To cull the voronoi edges which are partly outside the curve,
% first find all Delaunay triangles, making their vertices increase:
TRI = sort(delaunay(x,y)')';

% The signed areas of the triangles, pos if inside, negative if outside:
s = [(-(x(TRI(:,1))-x(TRI(:,3))).*(y(TRI(:,2))-y(TRI(:,3))) + ...
      (x(TRI(:,2))-x(TRI(:,3))).*(y(TRI(:,1))-y(TRI(:,3)))) -1];

% Find the triangles containing the ends of each voronoi segment and
% determine those segments for which both ends are inside the contour.
t1 = tsearch(x,y,TRI,vx,vy); t1(isnan(t1))=size(s,2);
ind = s(t1(1,:))>0 & s(t1(2,:))>0;

% The medial axis is drawn by:
plot(vx(:,ind),vy(:,ind),'-k')