Image processing apparatus, image processing method, and computer-readable storage medium

An image processing apparatus includes a detection unit, a size changing unit, and a determining unit. The detection unit detects, from a first captured image obtained by capturing a projection image projected by a projection unit, a vertex of the projection image. The size changing unit reduces a size of the projection image in response to the detection unit not detecting a predetermined number of vertexes of the projection image from the first captured image. The determining unit determines a distortion correction parameter to be used for correcting a distortion of a projection image projected by the projection unit, in accordance with a detection result of vertexes by the detection unit for a second captured image obtained by capturing the size-reduced projection image in a case where the detection unit detects the predetermined number of vertexes of the size-reduced projection image from the second captured image.