1. A computing device, the computing device 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 that captures a digital image of an object using the image capturing device, and obtains a standard image of the object from the storage system, wherein the standard image is stored in the storage system and is a reference image of the object having a determined level of brightness and contrast; an image processing module that selects a first portion of the digital image and selects a second portion of the standard image, generates a first gray picture by calculating gray values of the first portion and replacing RGB values of the first portion with the gray values of the first portion, and generates a second gray picture by calculating gray values of the second portion and replacing RGB values of the second portion with the gray values of the second portion; an threshold generating module that calculates a first square value of all pixel values of the first gray picture and a second square value of all pixel values of the second gray picture, creates a threshold range between the first square value and the second square value, and generates a threshold value by selecting a medial value from the threshold range; and an image comparing module that extracts a plurality of first feature points from the first picture according to the threshold value, extracts a plurality of second feature points from the second gray picture according to the threshold value, determines a first feature area of the first gray picture according to the first feature points, determines a second feature area of the second gray picture according to the second feature points, and identifies a difference between the digital image and the standard image by comparing the first feature area with the second feature area.