Correcting back side image data based on positions of four corner detection marks specified using ground color luminance

An image forming apparatus comprising:

• an image forming device configured to form a front side image and a back side image on a front side and a back side of a medium, respectively;
• and
• an image scanner disposed downstream from the image forming device in a medium conveyance direction,
• the image scanner including:
• a conveyance path;
• a conveyor configured to convey the medium through the conveyance path, the medium bearing a detection mark that is spaced from a leading edge of the medium in the medium conveyance direction;
• a background member disposed opposite the conveyance path;
• a luminance sensor disposed opposite the background member via the conveyance path, the luminance sensor configured to detect a luminance value of each of the background member and the medium;
• and
• a controller configured to control the luminance sensor to detect the luminance value at a plurality of times while the conveyor conveys the medium so as to obtain a plurality of detected luminance values,
• the controller configured to determine a ground color luminance value of a ground color area on the medium based on a determination range including a first peak luminance value among the plurality of detected luminance values.

An image forming method comprising:

• forming a detection mark in each of four corners on a front side of a medium at a position spaced from one of a leading edge and a trailing edge of the medium in a medium conveyance direction so as to form four detection marks on the medium;
• forming a front side image in an enclosed area enclosed by the four detection marks;
• determining a ground color luminance value of a ground color area on the medium formed with the four detection marks and the front side image;
• determining a detection threshold used to detect the four detection marks based on the determined ground color luminance value;
• specifying positions of the four detection marks formed in the four corners on the front side of the medium, respectively, by using the determined detection threshold;
• correcting back side image data for a back side image to be formed on a back side of the medium based on the specified positions of the four detection marks, respectively;
• and
• forming the back side image on the back side of the medium according to the corrected back side image data.