1. A method for image-based object feature description, adapted for an electronic apparatus to describe an object feature in an input image, the method comprising: detecting an object of interest in the input image, and calculating a centroid and a direction angle of the object of interest; recognizing a contour of the object of interest, and calculating, among a plurality of pixels on the contour, a distance and a relative angle of each of the pixels to the centroid; calibrating the calculated relative angle of each of the pixels by using the direction angle; dividing a 360-degree range centered on the centroid equally into a plurality of angle intervals, and separating the pixels on the contour into a plurality of groups according to a range covered by each of the angle intervals; obtaining a maximum among the distances of the pixels in each of the groups as a feature value of the group; and normalizing and collecting the feature values of the groups to form a feature vector that serves as a feature descriptor of the object of interest.