1. A method for generating a projection image projected on a two-dimensional frame from image data of three or more dimensions that includes a plurality of first pixels of three or more dimensions, wherein each first pixel has a value and a coordinate, and the projection image includes a plurality of second pixels, the method comprising: setting a plurality of rays that pass through some of the first pixels, with each ray corresponding to one of the second pixels; storing a plurality of first values, each associated with one of the first pixels existing on each of the rays; storing a plurality of first coordinates, each corresponding to one of the first values; and determining a noise position in the first pixels based on the first coordinates.