1. An image processing method, comprising: receiving an input image; referring to a first threshold to compare a reference background and the input image to determine a first foreground image within the input image; determining a second foreground image, which is within the input image and different from the first foreground image, according to the first foreground image; and referring to a second threshold different from the first threshold to compare the reference ground and the second foreground image to determine a third foreground image within the input image.