1. A method of generating a bit map, comprising: a first detection step of detecting, when a second bit map, improved in first-direction resolution, is generated from a first bit map in which a plurality of image-forming pixels are arranged two-dimensionally, a first edge and a second edge appearing in a second direction orthogonal to a first direction in the first bit map; and a first addition step of adding, between a third edge and a fourth edge in the second bit map that respectively correspond to the first edge and the second edge, at least an edge extending in the first direction so as to effect interpolation between the third edge and the fourth edge.