Image forming apparatus

An image forming apparatus prints a first image data rendered at a first number of halftone levels based on a second image data rendered at a second number of halftone levels larger than the first number of halftone levels. A dot forming section (51, 451, 551, 651) forms lines of pixels on a print medium, the lines including a number of groups of sub lines of sub dots. The groups are aligned such that the sub lines of sub dots extend in traverse directions (X) substantially perpendicular to an advance direction. A controller (105) controls the dot forming section such that a selected number of sub dots in the advance direction (Y) are combined to form a pixel. Sub dots on the same sub line have the same exposure energy and sub dots on different sub lines in the same pixel have different exposure energies.