tracking an image in matlab

Question

How do you for example put a blue circle centered over an image you are tracking? Not a filled in circle, but more like a donut.

Answer 1

For example like this:

%# load image and display it
img = imread('autumn.tif');
figure,imshow(img);

%# make it such that plotting something doesn't erase 
%# the previous plot (i.e. the image)
hold on

%# plot a circle. MarkerSize controls the size of the circle
plot(200,100,'ob','MarkerSize',14)