Wärmeübertragungs-Bilddrucker

A thermal transfer image forming apparatus comprises: a recording head (6) having a plurality of heating resistor elements arranged in a line; a mechanism (3,10,11) for carrying a sheet (1) of recording paper and an ink sheet (9) at a predetermined pitch; a microcomputer including an image data reading means (31) for continuously reading N-lines' pixel data (N being an integer of N >/= 2) constituting input pixel data, a sampling means (33) for sampling the N-lines' pixel data to a matrix of N x N pixels, a gamma conversion means (34) having a first gamma characteristic conversion function for outputting print data to form main dots by performing gamma conversion proportionally in a range of from a low density to a high density, a second gamma characteristic conversion function for outputting print data being lower in density than the first gamma characteristic conversion function to form subsidiary dots, and third and fourth gamma characteristic conversion functions to form no dot in a large part of the density range, to thereby express the pixel density as dot size for each of the N x N pixels, and a conversion characteristic selection means (35) for assigning the gamma characteristic conversion functions to the sampled N x N pixels to arrange dots based on the first gamma characteristic conversion function at a predetermined angle; and a drive circuit (38) for supplying electric energy corresponding to the print data to the heating resistor elements of the recording head (6).