how to create a Image using matrix

I want to create a circle IMAGE using matrix . i dont know much of matrix but if anyone can help me will be great creating image of circle like black background and white circle using matlab :)

Answer 1

[jonas]

http://matlab.wikia.com/wiki/FAQ#How_do_I_create_a_circle.3F

Answer 2

[KSSV]

th = linspace(0,2*pi) ;      % theta 
r = 100 ;                    % radius of circle 
x = r*cos(th) ; 
y = r*sin(th) ;

N = 500 ;                      % dimensions of image 
I = zeros(N,N,3) ;

c = round(N/2) ;                % center of circle 

x = c+x ; y = y+c ;

[X,Y] = meshgrid(1:N,1:N) ;
idx = inpolygon(X(:), Y(:),x,y) ;
for i = 1:3
    T = I(:,:,i) ; 
    T(idx) = 255 ; 
    I(:,:,i) = T ;
end

[Der Turbanator]

i need that for 1:5 i have tried but its not getting into 1:5 ratio or to say meshgrid