A first sum is calculated which is a sum of light intensities of dispersed lights generated when a pixel light by a target pixel passes through an optical system, and a first light intensity is calculated by adding the first sum to a light intensity of the pixel light of the target pixel. A second sum is calculated which is a sum, at a position of the target pixel, of each of light intensities of dispersed lights generated when each of pixel lights by pixels around the target pixel pass through the optical system. A second light intensity of the pixel light by the target pixel is calculated by subtracting the second sum from the first light intensity.