Photomask patterns are represented using contours defined by level-set functions. Counters are used to track when the current CDR is identified as an expected CDR. When the electromagnet switch turns on, a magnetic force is produced to rotate the lens relative to the digital still camera so that a two- or multiple-level zooming can thus be achieved. Each second video frame is composed of a plurality of image regions, and a second object is present in an image region of at least one second video frame.