US Patent 7724393 Judging whether trapping process is required or not

In trapping process of a multicolor image, it is judged whether trapping process is required or not, and trapping process is performed only when required. Specifically, trapping process is executed when in a portion where a plurality of figures constituting an image overlap, the plate color value of a relatively lower figure is erased or overwritten. This enables to execute trapping process only when there is the danger that a gap occurs in a boundary portion where different two colors are adjacent each other.