1. An image processing device comprising: a phase calculation unit configured to calculate a phase of pixels of a 3D image projected onto a light shielding unit that limits a region seen by a left eye and a region seen by a right eye of the 3D image displayed on a display unit; and a blend processing unit configured to combine an image for the left eye with an image for the right eye for each pixel based on the phase calculated by the phase calculation unit, the image for the left eye and the image for the right eye being included in the 3D image, wherein the phase calculation unit calculates the phase based on a viewing position using an expression that considers that a position of the light shielding unit in a vertical direction to a screen of the display unit differs according to a position thereof in a horizontal direction to the screen of the display unit, wherein the display unit displays the combined images of the left and right eyes, and wherein the phase calculation unit and the blend processing unit are each implemented via at least one processor.