6. The object monitoring apparatus of claim 5 , wherein the relative difference is in proportion to the square of differences between the pixels of the edge image and the pixels of the reference image and is in inverse proportion to a maximum value of the pixels of the edge image and of the reference image.
6. The object monitoring apparatus of claim 5 , wherein the relative difference is in proportion to the square of differences between the pixels of the edge image and the pixels of the reference image and is in inverse proportion to a maximum value of the pixels of the edge image and of the reference image. 7. The object monitoring apparatus of claim 6 , wherein the candidate object extractor is operative, when the relative difference is maintained above a predetermined threshold for longer than a preset time, to extract the candidate object pixels and group the extracted candidate object pixels into the candidate object.