1. A method for recognition of a sky portion, a vertical object portion and a ground portion in an image executed in an electronic system, comprising: dividing the image into a plurality of pixel sets by the electronic system; calculating expected values of each pixel sets with a sky distribution function, a vertical object distribution function and a ground distribution function by the electronic system, the distribution probability of the sky distribution function decreasing from the top of the image, the distribution probability of the ground distribution function decreasing from the bottom of the image and the distribution probability decreasing from a vision horizon line to upper side and to lower side of the image; and comparing expected values of each pixel set in the sky distribution function, the vertical object distribution function and the ground distribution function to determine whether each pixel set belongs to a sky object, a vertical object or a ground object.