1. A computing device, the computing device being connected to an image capturing device, the computing device comprising: a storage system; at least one processor; and one or more programs stored in the storage system and executable by the at least one processor, the one or more programs comprising: an image obtaining module operable to capture a digital image of an object using the image capturing device, and obtain a standard image of the object from the storage system; an image calculation module operable to calculate a first difference value between an image average energy density (IAED) value of the digital image and an IAED value of the standard image based on RGB (red, green, blue) channels, divide the digital image into N pixel blocks A 1 , A 2 ,. .. An, divide the standard image into N pixel blocks B 1 , B 2 ,. .. Bn, and calculate a second difference value between an IAED value of each pixel block An of the digital image and an IAED value of each corresponding pixel block Bn of the standard image; a coefficient generation module operable to generate a threshold coefficient T for comparing the digital image and the standard image; an image comparison module operable to determine whether image data of the pixel block An is the same with image data of the pixel block Bn by comparing each pixel block An of the digital image with each pixel block Bn of the standard image according to the first difference value, the second difference value, and the threshold coefficient T; and an image generation module operable to eliminate the pixel block An from the digital image when the image data of the pixel block An is different from the image data of the pixel block Bn, and generate a discrepant image according to remaining pixel blocks of the digital image when all of the pixel blocks of the digital image are compared.