The present invention discloses fiducial marker systems or tag systems and methods to detect and decode a tag. The tag comprises a plurality of cells arranged in a grid pattern. The plurality of cells forms a square or rectangular shape. Each cell is in square or rectangular shape and has either black, white, or gray grayscale. Any two cells that share a common borderline should have different grayscales; and the four corner cells of the tag are all in black. The method for detecting and decoding such a tag comprises detecting a plurality of connected cells in an image, determining whether the plurality of connected cells in the image have all three grayscales, determining whether total number of cells in the plurality of connected cells is greater than a predetermined threshold, determining whether any two cells in the plurality of connected cells that share a common borderline in the image have different grayscales, and decoding the tag based on the grayscales of the plurality of connected cells in the image.