Although we wanted the moved object to disappear from the resulting image, it appears twice, at its old and at its new position. The reason is that the object has rather low pixel values (similar to a logical 0) whereas the background has a high values (similar to a logical 1). However, we normally associate an object with logical 1 and the background with logical 0, therefore we actually ANDed the negatives of two images, which is equivalent to NOR them. To obtain the desired result we have to invert the images before ANDing them, as it was done in