Image forming apparatus having scanning optical system to form on photosensitive member N beam spots separated by M scan lines

An image forming apparatus, comprising:

• a photosensitive drum;
• a semiconductor laser having N laser emitters, N being a natural number larger than or equal to two;
• an incident optical system configured to convert light emitted from each laser emitter to a beam;
• a polygon mirror configured to deflect and cyclically scan each beam from the incident optical system;
• a scanning optical system configured to form on the photosensitive drum N beam spots by N beams deflected by the polygon mirror, each beam spot scanning along a scan line on the photosensitive drum to form a beam image along the scan line for one cycle, any neighboring two beam spots in the N beam spots having centers with a center-to-center distance of the neighboring two beam spots separated by M scan lines, M being a natural number larger than or equal to two;
• an optical sensor configured to detect a beam which is deflected to output a detection signal for each cycle;
• a page memory configured to store raster image data including a plurality of sets of line data;
• a line memory configured to store the sets of line data read from the page memory, the line memory having storage areas and addresses identifying respective ones of the storage areas;
• and
• a memory controller configured to perform:
• for each cycle based on the detection signal outputted from the optical sensor,
• reading adjacent sets of line data for scan lines from the page memory, the adjacent sets of line data for scan line successively arranged on the photosensitive drum;
• writing the adjacent sets of line data for scan lines to the storage areas identified by a plurality of successive addresses among the addresses of the line memory;
• selecting addresses identifying storage areas storing unadjacent sets of line data for N scan lines having non-successive two scan lines separated by M scan lines on the photosensitive drum;
• and
• outputting the unadjacent sets of line data stored in the storage areas identified by the selected addresses to the N laser emitters.