1. A foreground identification method, comprising steps of: (a) respectively performing MSOR and MSCR operations to an original image by using plural values of 3-D structure element for determining a 3-D opening-by-reconstruction modest structure element and a 3-D modest closing-by-reconstruction structure element; (b) comparing the original image and an image obtained by performing MSOR operation with the 3-D opening-by-reconstruction modest structure element to the original image so as to generate an enhanced top-hat image, and comparing the original image and an image obtained by performing MSCR operation with the 3-D modest closing-by-reconstruction structure element to the original image so as to generate an enhanced bottom-hat image; (c) locating an overlap region between the enhanced top-hat image and the enhanced bottom-hat image, where the overlap region forms a foreground identification screen; (d) simulating the variation of pixel colors in background areas extracted by the foreground identification screen for generating an interpolated background mesh; (e) dividing the original image into a plurality of regions and indexing the plural regions so that an image with indexed regions is obtained; (f) comparing the interpolated background mesh and the image with indexed regions for determining a refined foreground identification screen according to a refinement calculation; and (g) identifying and extracting foregrounds from the original image by utilizing the refined foreground identification screen.