Real-time linefeed measurement of inkjet printer

A method of determining line feed error in an image forming apparatus for printing an input image by overprinting multiple times on the same area of a print medium, wherein the image forming apparatus includes a print head having ink ejection nozzles within nozzle banks and two head sensors configured to sense information from the input image printed on a print medium, wherein a first one of the head sensors acts as a front head sensor while the second head sensor act as a trailing head sensor when the print head moves in a scan direction, and the first head sensor acts as the trailing head sensor while the second head sensor act as the front head sensor when the print head moves in a direction opposite to the scan direction,

• the method comprising:
• obtaining swaths by half-toning the input image, wherein each swath includes correlatable noise pattern in the bottom area of the swath and height of the bottom area is equivalent to line feed distance;
• printing a first swath of the swaths on the print medium while the print head moves across the print medium in a scan direction;
• repositioning the print head relative to the print medium on which the first swath is printed by at least one line feed distance in a feed direction which intersects the scan direction;
• and
• after the repositioning, sensing the bottom of the first swath on the repositioned print medium by the preceding head sensor, determining the line feed error based upon information sensed from the printed first swath by the preceding head sensor, and then printing a second swath of the swaths while the print head moves across the print medium in the scan direction after the repositioning,
• wherein the line feed error is determined from a vertical shift represented by a position of the head sensor before the repositioning, the line feed distance, and a position of the printed first swath in the feed direction, based upon information sensed from the printed first swath by the head sensor when the print head reaches an edge of the printed first swath in the scan direction, and
• wherein the second swath is printed on top of the printed first swath on the print medium while compensating for the determined line feed error by controlling the ejection of ink from the ink ejection nozzles within the nozzle banks of the image forming apparatus.