1. An image data processing device for processing structured image data including background image data representing a background image, foreground image data representing a foreground image indicating a color and a gross shape, and mask data representing a mask image specifying a prescribed area of the foreground image to be overlaid on the background image, the image data processing device lightening a shade of the background image by applying a background color lightening process to the background data, and lightening a shade of the foreground image less than the shade of the background image by applying a foreground color lightening process to the foreground data.