1. An image processing device for generating a composite image using multi-viewpoint image data before color interpolation captured by a camera array image capturing device, the image processing device comprising: a unit configured to acquire information of a pixel value and a pixel position in the multi-viewpoint image data; a pixel position determination unit configured to determine the pixel position in a composite image in the pixel position of each pixel of the multi-viewpoint image data in accordance with an arbitrary focus position based on optical parameters at the time of the image capturing; a color derivation unit configured to derive the color of each pixel of the multi-viewpoint image data; and a pixel value calculation unit configured to calculate the pixel value of each pixel of the composite image using the determined pixel position in the composite image and the pixel value of the multi-viewpoint image data corresponding to the derived pixel color.