Problem with Creating Ellipse from Meshgrid on Image

I am interested in creating an ellipse from a meshgrid, which is centered on my image; however, it doesn't quite work. Can anyone assist me?

    [rows cols dims] = size(image)
    [x,y] = meshgrid(1:cols, 1:rows);
    f = sqrt((x-cols).^2/200 + (y-rows).^2/900) -1

I am not sure if my equation is correct?

Answer 1

[Image Analyst]

This is covered in the FAQ: http://matlab.wikia.com/wiki/FAQ#How_do_I_create_an_ellipse.3F