Abstract: A method of manipulating a train of electrical signals equal in binary terms to the density gradation levels of a plurality of image pixels of an original image over a range of density gradation levels for controlling the reproduction of the original image on a receiving sheet in a thermal printer having an array of controllable printing elements corresponding in number to the pixels in an image line and an actuating circuit for actuating the respective printing elements in individually accessible fashion. In the method, the electrical signals for a given line of the image pixels are stored in a line memory and are outputted one by one from the line memory to a non-zero detector and downcounter for identification as to zero or non-zero status and downcounting of all non-zero signals to reduce the level thereof by one level.