A neutron spectrometer is provided by a series of substrates covered by a solid-state detector stacked on an absorbing layer. The processor is arranged to receive the input image, convert the input image to a printable image and output the printable image to the printer for printing. Then the processor searches, as second searched rectangular pieces, the rectangular pieces that are traversed by the edges of the target polygon element, so as to register the element number of the target polygon element for the rows corresponding to the second searched rectangular pieces.