An image data processor comprising an image signal receiver, a first processing unit, and a second processing unit, is provided. The image signal receiver receives a reference image signal and a laser image signal. The reference image and the laser image correspond to an optical image of an object respectively illuminated by a reference light and a laser light. An average wavelength of the laser light is shorter than that of the reference light. The first processing unit generates reference edge data and a laser edge data based on the reference image signal and the laser image signal respectively. The reference edge data and the laser edge data corresponds to a reference edge image and a laser edge image respectively. The reference edge image and the laser edge image show only edges in the reference image and the laser image respectively. The second processing unit detects a first partial edge based on the reference edge data and the laser edge data. The first partial edge is shown only in one of the reference edge image and the laser edge image.