This paper describes a method for measuring the diameter of fruits with a near spherical shape using an RGB-D camera in order to investigate fruit enlargement at different times of the year. In general depth-based measurement methods, when considering the diameter of an object with a near-spherical shape, the diameter can be calculated by obtaining the Euclidean distance from the coordinates of the object's sides or by using the depth at the center of the object and the size of the object on the RGB image. However, it is difficult to accurately determine the diameter of an object because of errors in the calculated results due to the perspective projection of a general camera. Therefore, this study proposes a method to measure the diameter of fruits that have a shape similar to a sphere. Although this study focuses on young apple fruits, the proposed method can be applied to other agricultural crops, as well as to objects that are similar to spheres. In addition, we have also studied a correction that takes into account the rotation of the object so that the method can be applied to objects with circular cross-sections.
Fukuda et al. (Sat,) studied this question.