Image forming apparatus and method for controlling image forming apparatus

A method for controlling an image forming apparatus that includes one or more processors and one or more memories storing program instructions, the method being implemented by the one or more processors when the program instructions are executed by the one or more processors, the method comprising:

• generating a print image to be printed on a sheet of paper and a position correcting mark for front and back side registration having a brightness lower than a minimum brightness of the print image;
• controlling printing of the print image and the position correcting mark on a first side of the sheet of paper;
• controlling reading of the print image and the position correcting mark from the first side and controlling generating of read image data;
• and
• detecting from the read image data the position correcting mark on the first side using a brightness difference.