Image processing device, display device, image processing method, display method, and image processing program

The purpose of the present invention is to display a main image and a sub-image simultaneously or by switching therebetween, according to a display mode, with an appropriate image quality. An image processing unit comprises: a first feature amount calculation unit for calculating the feature amount of main image data; a second feature amount calculation unit (25) for calculating the feature amount of sub-image data; an image quality adjustment unit (26) for performing an image quality adjustment on the sub-image data after resizing; and a display image data generation unit for generating, according to display mode information and from the main image data and the sub-image data after the adjustment, display image data used for displaying the main image and the sub-image simultaneously or by switching therebetween. The image quality adjustment unit (26) performs the image quality adjustment so that the difference between the feature amount of the sub-image data after the adjustment and the feature amount of the main image data becomes small as compared to the difference between the calculated feature amount of the sub-image data and the calculated feature amount of the main image. When performing the image quality adjustment on a per-content basis, the image quality adjustment unit (26) updates, at the timing of switching between contents, the feature amount of the main image data used for the image quality adjustment.