An image data processing apparatus includes a total-colorant-amount calculating unit that calculates a total amount of a metallic colorant and a process-color colorant used by an image forming apparatus to form an image having a unit area, the image forming apparatus forming an image by using a white colorant, the metallic colorant, and the process-color colorant; a white-colorant-amount specifying unit that specifies an amount of the white colorant used to form the image having a unit area on the basis of a calculation result obtained by the total-colorant-amount calculating unit; and a white-colorant-amount correcting unit that corrects the amount of the white colorant so that the amount of the white colorant used to form the image having a unit area decreases as the amount of the metallic colorant used to form the image having a unit area increases.