A computing device reads an entire image of an object. The entire image is spliced by a plurality of part images. A user selects an area on the entire image. The computing device determines a first number of first pixel points between a center point of the selected area and a center point of each covered image. The converted images are part images that the selected area covers. The coordinate values of the center point of the selected area are calculated according to the first number of pixel points and a size of each pixel point of the entire image. The computing device calculates coordinate values of each point of a selected area according to the size of each pixel point and the coordinate values of the center point of the selected area.