1. An image processing device comprising: a mixer unit that mixes a first image with a second image to generate an output image in accordance with first mask information, which defines a display region so that substantially a same number of pixels in the first and second images are displayed while the pixels are dispersed substantially uniformly, wherein the mixer unit includes: a calculation unit which reads a bit value from the first mask information on an individual pixel basis and calculates a first alpha blend value at a relevant coordinate based on the read bit value; a first multiplication unit which reads a pixel value at the relevant coordinate from the second image and calculates a first pixel value by multiplying the first alpha blend value by the read pixel value in the first image; a second multiplication unit which reads a pixel value at the relevant coordinate from the first image and calculates a second pixel value by multiplying a second alpha blend value by the read pixel value in the second image, the second alpha blend value being calculated based on the first alpha blend value; and an addition unit which sets a sum of the first pixel value and the second pixel value to be a pixel value at the relevant coordinate in the output image, and wherein the mixer unit generates the output image without generating an intermediate image.