[O]=drawcircle(I,C,r,n)

%function to generate the pixels on the boundary of a regular polygon of n sides
%the polygon approximates a circle of radius r and is used to draw the circle
%INPUTS:
%1.I:Image to be processed
%2.C(x,y):Centre coordinates of the circumcircle
%Coordinate system :
%origin of coordinates is at the top left corner
%positive x axis points vertically down
%and positive y axis horizontally and to the right
%3.n:number of sides
%4.r:radius of circumcircle
%OUTPUT:
%O:Image with circle
%Author:Anirudh S.K.
%Department of Computer Science and Engineering
%Indian Institute of Techology,Madras
% 
% -------------------------------------------------------------------------
% Copyright (c) 2007, Anirudh Sivaraman
% All rights reserved.
% 
% Redistribution and use in source and binary forms, with or without 
% modification, are permitted provided that the following conditions are 
% met:
% 
%     * Redistributions of source code must retain the above copyright 
%       notice, this list of conditions and the following disclaimer.
%     * Redistributions in binary form must reproduce the above copyright 
%       notice, this list of conditions and the following disclaimer in 
%       the documentation and/or other materials provided with the distribution
%       
% THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" 
% AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 
% IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 
% ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE 
% LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 
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% CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 
% ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 
% POSSIBILITY OF SUCH DAMAGE.
% -------------------------------------------------------------------------
function [O]=drawcircle(I,C,r,n)
if nargin==3
    n=600;
end
theta=(2*pi)/n;% angle subtended at the centre by the sides
%orient one of the radii to lie along the y axis
%positive angle ic ccw
rows=size(I,1);
cols=size(I,2);
angle=theta:theta:2*pi;
%to improve contrast and help in detection
x=C(1)-r*sin(angle);%the negative sign occurs because of the particular choice of coordinate system
y=C(2)+r*cos(angle);
if any(x>=rows)|any(y>=cols)|any(x<=1)|any(y<=1)%if circle is out of bounds return image itself
    O=I;
    return
end
for i=1:n
I(round(x(i)),round(y(i)))=1;
end
O=I;