1. A print data processing method for processing print data, the print data being raster data including white-plate data representing a white-plate graphic corresponding to a region to be applied with a white ink; and color-plate data representing a color-plate graphic corresponding to a region to be applied with an ink of a color other than white, the method comprising: an erosion candidate region calculating step of determining an erosion candidate region, the erosion candidate region being a candidate for a region in which the white-plate graphic represented by the white-plate data is eroded; a colored-region calculating step of determining a colored region based on the color-plate data, the colored region being the region to be applied with an ink of a color other than white; an application target region calculating step of determining, as an application target region, a region of a set of pixels included in at least one of the colored region and the region to be applied with a white ink based on the white-plate data; an erosion allowed region calculating step of determining an erosion allowed region by performing an erosion process on the application target region, the erosion allowed region being a region allowing for erosion of the white-plate graphic represented by the white-plate data; an erosion target region identifying step of defining a region of a set of pixels included in all of the erosion candidate region, the colored region, and the erosion allowed region as an erosion target region; and a white-plate data updating step of rewriting values of data of the pixels included in the erosion target region among the white-plate data such that an amount of white ink applied to the pixels is reduced.