If it is determined in step S20 that the count of the counter i is not equal to the number of special toner colors, the process proceeds to step S22. In step S22, the server 12 determines a coefficient k from the arrangement or color of the special toner, which is obtained in step S14. The coefficient k is set in advance in the way illustrated in FIG. 8, for example, as described above.

Then, in step S24, the server 12 combines a basic color image or the previous composite image with a special-toner image for each RGB color on a coordinate-by-coordinate basis. Here, the images are combined sequentially, starting from the special-toner plate used in the background.

After the processing of step S24, in step S26, the server 12 increments the counter i. Then, the process returns to step S20. If it is determined in step S20 that the count of the counter i is equal to the number of special toner colors, the process proceeds to step S28. In step S28, the server 12 converts the current composite image into a bitmap image and displays a preview screen on the display device 17. Then, the process ends.

While the exemplary embodiments have been described in the context of xerographic printing, the exemplary embodiments may also be applied to any other type of printing, for example, inkjet printing.