An image forming apparatus comprises an output unit to output output data converted from input data by a halftone process with a first or second matrix in accordance with attribute information on pixels of the input data. The thresholds in the first matrix, whose thresholds are arranged to express the tone level of output data by the number of dots or dot area modulation for input data, and the second matrix, whose thresholds are arranged to express the tone level of output data by dot area modulation for the input data, are arranged such that the average dot size of output data obtained by the halftone process with the second matrix on input data with the first density is smaller than the average dot size of output data obtained by the halftone process with the first matrix on the input data with the first density.