function letsroll(option)
%LETSROLL Plots a cycloid in the making...
% Just type  letsroll  at the MATLAB prompt to see the movie.
% Written for MATLAB v5.

%Peter Dunn (dunn@romulus.sci.usq.edu.au)
%17 March 1999

if nargin==0,

   %Plot the `ground'
   LRFig=figure('tag','tagMain','Name','Cycloids');
   hold on;
   plot([-1 9],[0 0],'g-'); %the `ground'
   axis('equal');
   axis('off');

   %plot circle
   rad = 1.0;
   [xpt, ypt, hc, hp, hl] = plotcirc(0,rad);
   set(hc,'tag','tagCircle');
   set(hp,'tag','tagPoint');
   set(hl,'tag','tagLine');

   set(LRFig,'Visible','on');
   uicontrol('Style','Frame','Units','normalized',...
             'Position',[0.05,0.1,0.9,0.2]);

   %GO button
   uicontrol('Style','Pushbutton','Units','normalized',...
             'Position',[0.1 0.15 0.3 0.1],...
             'String','LET''S ROLL...','Callback','letsroll(1);');

   %QUIT Button
   uicontrol('Style','Pushbutton','Units','normalized',...
             'Position',[0.6 0.15 0.3 0.1],...
             'String','QUIT...','Callback','delete(gcf);');

else

   %delete current path if it's there
   if ~isempty(findobj('tag','tagPath') ),
      delete(findobj('tag','tagPath'));
   end

   %preparation
   maxtheta = 1.25*pi;
   mintheta = 0;
   numcirc=25;
   theta = linspace(mintheta, maxtheta, numcirc);
   theta = [theta, -sort(-theta)];

   rad = 1.0;
   xpt=rad;
   ypt=rad;

   %ilist = [ 1:numcirc, numcirc-1:-1:1];
   firsti1 = 1;
      

%   for i=ilist,
   for i = theta,

      l = i;

      %NOTE: This circle has radius 0.5, and so theta=2l;

      %Keep old points for joining path
      xold=xpt;
      yold=ypt;


      [xpt, ypt, hc, hp, hl] = plotcirc(l, rad);
      set(hc,'tag','tagCircle');
      set(hp,'tag','tagPoint');
      set(hl,'tag','tagLine');


      hpath=plot([xpt,xold],[ypt,yold],'r:');        %cycloid path
      set(hpath,'tag','tagPath');
      
      drawnow
   end;

end


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% SUBFUNCTIONS
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

function [xpt, ypt, hc, hp, hl] = plotcirc(l, rad)
%PLOTCIRC Plots the circle
%USE:  [xp, yp, hc, hp, hl] = plotcirc(l, rad) where l  is the distance 
%      travelled, rad is the radius, hc is the handle for the circle,
%      hp is the handle for the point and hl is the handle for the line.

t = linspace(0, 2*pi);

xt = 2*l + rad*cos(t); 
yt = rad + rad*sin(t);

xpt = 2*l + rad*cos(-2*l); 
ypt = rad + rad*sin(-2*l);

[handles] = plot( xt, yt, 'b-', xpt, ypt, 'ro', [2*l xpt],[rad, ypt], 'b-');

hc = handles(1);
hp = handles(2);
hl = handles(3);

set(hp, 'MarkerFaceColor', 'r');

set(hc,'EraseMode','xor');
set(hp,'EraseMode','xor');
set(hl,'EraseMode','xor');

%delete old stuff that we don't want.
delete( findobj('tag','tagCircle') );
delete( findobj('tag','tagPoint') );
delete( findobj('tag','tagLine') );

return;