1. A non-transitory computer-readable recording medium having stored therein an image processing program for causing a computer to execute a process comprising: identifying, on a taken image of a jig having a plurality of first color regions and a plurality of second color regions different in color from the first color regions, the first color regions; comparing each of the identified first color regions with a predetermined convex shape and retracting each of the first color regions until it is determined that each of the first color regions has a similarity in shape to the predetermined convex shape; and determining a feature point of the jig on the image on the basis of a positional relationship of a plurality of first retracted regions on the image, the first retracted regions being obtained from the first color regions as a result of retraction at the retracting, wherein the identifying includes comparing each of pixel values of pixels composing a grayscale image with a predetermined threshold, binarizing a pixel of which the pixel value is larger than the predetermined threshold to “1”, binarizing a pixel of which the pixel value is equal to or smaller than the predetermined threshold to “0”, generating a binary image from the grayscale image, and identifying white regions as the first color regions and black regions as the second color regions on the grayscale image, and the retracting includes performing a retracting process to repeatedly retract each of color regions in the binary image generated at the identifying until the shape of a binarized region in the binary image is judged as having a similarity to a predetermined elliptical shape, separating the white regions in a circular portion of the jig from the white regions outside of the circular portion of the jig, and separating the black regions in the circular portion of the jig from the black regions outside of the circular portion of the jig.