1. A method of generating a bit map, comprising: 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 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, wherein the detecting the first and the second edges includes scanning three lines of pixels adjacent in the first direction, the scanning being in the second direction, the scanning in the second direction including: setting a center line of the three lines adjacent in the first direction as an edge detection line; setting, in a case where an adjacent pixel in the edge detection line changes from “off” to “on”, a position after a change in combinations of three pixels adjacent in the first direction in the scanning three lines from “off-off-off” to “off-off-on” or “on-off off” as the position of the first edge, and a subsequent position after a change in combinations of the three pixels adjacent in the first direction from “off-off-on” to “off-on-on” or from “on-on-off” to “on-off-off” as the position of the second edge; setting, in a case where an adjacent pixel in the edge detection line changes from “on” to “off”, a position prior to a change in combinations of the three pixels adjacent in the first direction from “on-on-off” to “on-off-off” or from “off-on-on” to “off-off-on” as the position of the first edge, and a subsequent position prior to a change in combinations of the three pixels adjacent in the first direction from “on-off-off” or “off-off-on” to “off-off-off” as the position of the second edge; and treating as valid the positions of the first and the second edges when a pattern that is neither a pattern after a change corresponding to the first edge nor a pattern corresponding to the second edge is not detected between the positions of the first and the second edges.