An image data processing apparatus receives trichromatic image data with values expressing red, green, and blue and generates information indicating the gamut of colors of the image data. The gamut information is obtained from maximum and minimum values, or values equivalent to the maximum and minimum values, pertaining to different color components during a prescribed time interval, and indicates the actual gamut of colors that appear during the prescribed time interval. The gamut information is appended to the image data, or output separately from the image data, for use in subsequent image processing.