1. An image forming apparatus comprising: an image forming device that forms, on an image bearing member, an image with a plurality of materials, a patch image for detecting density of the image, and a trigger image to be detected to generate a trigger to read the patch image, wherein the patch image is formed with a first material among the plurality of materials and the trigger image is formed with a second material, which is different from the first material, among the plurality of materials; a reading device that reads the patch image based on the trigger generated by detecting the trigger image; and a controlling device that controls the density of the image formed by said image forming device based on the patch image read by said reading device, wherein a brightness of the trigger image formed with the second material is lower than that of the patch image formed with the first material.