function convexprop(action)

%  get button descriptors
weakhandle = findobj(gcbf,'Tag','Weakbutton');
stronghandle = findobj(gcbf,'Tag','Strongbutton');

%  get the polygon data from the figure
polyhandle = findobj(gcbf,'Tag','Bpolygon');
pp = [get(polyhandle,'XData'); get(polyhandle,'YData')];

switch(action)
case 'strong',

   % find the order of the spline (default 3)
   t = get(polyhandle,'UserData');
   if isempty(t)
      k=4;
   else
      k=t(1);
   end
   for i=1:size(pp,2)-k+1
      index = convhull(pp(1,i:i+k-1),pp(2,i:i+k-1));
      fillhandle = fill(pp(1,i-1+index),pp(2,i-1+index),[.8 .8 .8]);
      set(fillhandle,'Tag','Strongpolygon');
      set(fillhandle,'ButtonDownFcn','drawbspline(''start'')')
   end
   % move polygon to front
   h = line(pp(1,:),pp(2,:),'Color','r','LineStyle','--');
   set(h,'Tag','Bpolygon2')
   set(h,'ButtonDownFcn','drawbspline(''start'')')
   % redefine pushbutton
   set(stronghandle,'String','Erase Strong CH')
   set(stronghandle,'Callback','convexprop(''clearstrong'')')  
case 'weak'
   index = convhull(pp(1,:),pp(2,:));
   fillhandle = fill(pp(1,index),pp(2,index),[.9 .9 .9]);
   set(fillhandle,'Tag','Weakpolygon');
   set(fillhandle,'ButtonDownFcn','drawbspline(''start'')')
  % move polygon to front
   h = line(pp(1,:),pp(2,:),'Color','r','LineStyle','--');
   set(h,'Tag','Bpolygon2')
   set(h,'ButtonDownFcn','drawbspline(''start'')')

   % redefine pushbutton
   set(weakhandle,'String','Erase Weak CH')
   set(weakhandle,'Callback','convexprop(''clearweak'')')  
case 'clearweak'
   delete(findobj(gcbf,'Tag','Weakpolygon'));
   delete(findobj(gcbf,'Tag','Bpolygon2'));
   set(weakhandle,'String','Weak Convex Hull')
   set(weakhandle,'Callback','convexprop(''weak'')')  
case 'clearstrong'
   delete(findobj(gcbf,'Tag','Strongpolygon'));
   delete(findobj(gcbf,'Tag','Bpolygon2'));
   set(stronghandle,'String','Strong Convex Hull')
   set(stronghandle,'Callback','convexprop(''strong'')')  
end