| An image with two or more repetitive patterns spatially repeated at a constant pitch is acquired. A phase computing unit thins out the repetitive patterns at a predetermined thinned-out interval to generate a thinned-out image, and computes a phase of a moire image obtained by interpolating the thinned-out image. A displacement computing unit calculates a displacement of the repetitive patterns based on a difference between the phase at a time and the phase at another time. A corrective operation unit subtracts an offset from the displacement of one repetitive pattern of the repetitive patterns, the offset being a product of the displacement of another repetitive pattern of the repetitive patterns by a pitch ratio of a pitch of the another repetitive pattern to a pitch of one repetitive pattern. |
