1. An image processing method, comprising: obtaining an image, the image having marker images and a background image; identifying presence of an object in the background image using a processor; and providing a signal for stopping a procedure if the presence of the object is identified; wherein the method further comprises reducing gradient variation across the image by (1) sampling a set of points in the image, (2) generating a gradient image with grayscale gradient that corresponds with the sampled set of points, and (3) subtracting the gradient image from the image.