1. A method for separating a foreground image, the method comprising: obtaining an input image, and color information and depth information of the input image; roughly dividing, based on the depth information of the input image, the input image to obtain an initial three-color image; reducing or expanding, based on the color information of the input image, an unknown region in the initial three-color image to obtain an optimized three-color image; and separating the foreground image from the optimized three-color image.