Image forming apparatus and storage medium

A computer-readable non-transitory storage medium storing a program for a computer of an image forming apparatus that includes a plurality of image forming units for forming images with color materials based on image data and that forms, on a recording medium, a color image by superimposing the images of respective colors formed by the respective image forming units, the program causing the computer to:

• acquire a read image obtained by reading an image forming surface of the recording medium on which the color image has been formed;
• form a register patch outside a job image forming region of the recording medium;
• and
• correct image forming positions of the plurality of image forming units based on the acquired read image that includes the register patch,
• wherein the register patch includes
• a leading end patch and a rear end patch formed of a color material of a reference color at a predetermined interval in a sub scanning direction and
• one or more measurement target color patches formed of color materials of respective measurement target colors between the leading end patch and the rear end patch,
• wherein the computer corrects the image forming positions based on relative positions of the leading end patch, the rear end patch, and the one or more measurement target color patches.