1. An image processing apparatus, comprising: a feature point calculator configured to calculate feature points included in input image data; and a features calculator configured to calculate features of the input image data in accordance with relative positions of the feature points calculated by the feature point calculator, the feature point calculator including: a line detector configured to detect, from the input image data, a line image extending in a first direction and a line image extending in a second direction that is different from the first direction, wherein the first direction is a first oblique direction that is oblique to lower right with respect to a horizontal direction of the input image data, and the second direction is a second oblique direction that is oblique to lower left with respect to the horizontal direction of the input image data, and wherein further the line detector is configured to detect a line image extending in a horizontal direction and a line image extending in a vertical direction; and an intersection calculator configured to calculate, as a feature point, an intersection of the line image extending in the first direction and the line image extending in the second direction, and also configured to detect an intersection of the line image extending in the horizontal direction and the line image extending in the vertical direction.