MATLAB: How to draw a circle in an image

Question

i want to draw a circle on an image using this equation x^2+y^2=r

Answer 1

img   = rand(100, 100, 3) * 0.2;  % Test data
value = bsxfun(@plus, ((1:100) - 50) .^ 2, ((1:100).' - 50) .^ 2);
r     = 30;
mask  = value < (r + 1)^2 & value > (r - 1)^2;
mask3 = cat(3, mask, mask, mask);
img(mask3) = 1.0;
image(img);