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 comprising: a processing unit to lighten a shade of the background image by applying a background color lightening process to the background data, and to lighten 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, wherein a plurality of color coefficient series is generated, one series per frequency band, by applying discrete wavelet transformation to each of the background data and foreground data, wherein a brightness coefficient increasing process is applied only to a color coefficient series corresponding to a relatively lowest frequency band, and wherein, as an original brightness coefficient positively relatively increases a growth rate of the increased brightness coefficient relatively decreases.