1. An image generating apparatus comprising: an image acquirer which acquires a photo shooting image formed by a plurality of sub images shot from each of a plurality of viewpoints; an extractor which extracts, from sub-pixels forming the sub images, sub-pixels corresponding to an object whose composition is to be modified as pixels to be modified; a modifier which modifies a correspondence relationship with sub-pixels extracted as the pixels to be modified among correspondence relationships between the sub-pixels and pixels to be reconstructed forming a reconstructed image which is defined on a predetermined reconstruction plane; and a generator which generates a reconstructed image in which the composition of the object has been modified by calculating pixel values of the pixels to be reconstructed from pixel values of the sub-pixels using the modified correspondence relationship.