In RE3, the CPU 40 detects the most downstream lateral-end position DSP. It may be noted that, while in the most upstream lateral-end position detecting process RB3 the CPU 40 searched through the pixel positions upstream from the objective position TMDP appointed in RB1 along the main-scanning direction MD to detect the edges, in RE3, the CPU 40 searches through the pixel positions downstream from the objective position TMDP appointed in RE1 along the main-scanning direction MD for edges. Therefore, description of processes which are substantially similar to those in RB3 will be herein omitted. Specifically, processes in RH2, RH3, RH4, RH6, RH7, RH9, and RH11 are the same as those in RD2, RD3, RD4, RD6, RD7, RD9, and RD11, respectively; therefore, description of those will be omitted.

The most downstream lateral-end position detecting process RE3 will be described in the following paragraphs with reference to FIG. 14. As the most downstream lateral-end position detecting process starts, in RH1, the CPU 40 appoints a new objective position TMDP. In particular, the CPU 40 may appoint a pixel position, which is apart downstream from the currently appointed objective position TMDP for the predetermined quantity of, e.g., 10, pixels along the main-scanning direction MD, as the new objective position TMDP. The currently appointed objective position TMDP may be one of the objective position TMDP appointed in RE1 and the objective position TMDP appointed in RE3.