The image formation is based on the X-rays, which are not absorbed in the body and therefore reach the imaging medium.
The absorption of X-rays through the body depends on the tissues they pass (tissue density and thickness) and the X-ray quality. Bone surfaces having a high X-ray absorption appear light, and air, which does not absorb radiation, is black. Fatty tissue has lower density (is darker) than other soft tissues. Fat occurs in the bone marrow between bone trabeculae and contributes to the clear delineation of internal bone trabeculae. They present white due to their relative high density against the dark fatty tissue, e.g. in the heel bone (calcaneus).