An image forming apparatus includes a head, processing circuitry, and a reader. The head includes a first nozzle, a second nozzle, and a third nozzle at a position different from the first nozzle in a conveyance direction of a recording medium. The circuitry controls relative movement of the medium and the head along a main scanning direction and relative movement of a conveyance amount between the medium and the head along the conveyance direction. The circuitry controls the head to form a first test pattern with color ink discharged from the first nozzle, form a second test pattern with color ink discharged from the third nozzle, and form a mask image overlapping on or under the first pattern and overlapping on or under the second pattern with base ink discharged from the second nozzle. The reader reads positions of the first pattern and the second pattern on the medium.