Precision agriculture has greatly benefited from advances in machine vision and image processing techniques. The use of feature descriptors and detectors allows to find distinctive keypoints in an image and the use of this approach for agronomical applications has become a widespread field of study. By combining near infrared (NIR) images, acquired with a modified Nikon D80 camera, and visible spectrum (VIS) images, acquired with a Nikon D300s, a proper crop identification could be obtained. Still, the use of different sensors brings an image matching challenge due to the difference between cameras and the possible distortions from each imaging technique. The aim of this paper is to compare the performance of several feature descriptors and detectors by comparing near infrared and visual spectral bands in rice crop images. Therefore, a group of 20 different scenes with different cameras and growth stages in a rice crop were evaluated. Thus, red, green, blue (RGB) and L, a, b (CIE L*a*b*) channels were extracted from VIS images in order to compare the matches obtained between each of them and the corresponding NIR image. The BRISK, SURF, SIFT, ORB, KAZE, and AKAZE methods were implemented, which act as descriptors and detectors. Additionally, a combination was made between the FAST algorithm for the detection of keypoints with the BRIEF, BRISK, and FREAK methods for features description. BF and FLANN matching methods were used. The algorithms were implemented in Python using OpenCV library. The green channel presented the highest number of correct matches in all methods. In turn, the method that presented the highest performance both in time and in the number of correct matches was the combination of the FAST feature detector and the BRISK descriptor.

A common and investigated problem in image processing is noise, especially Gaussian, because it affects image quality and the subsequent treatment. Therefore, different filtering techniques are created and improved every day. Some of these techniques require to know the approximate value of the noise level, so it is essential to develop noise estimation methods. Given that there are not comparatives studies of these techniques, in this work an evaluation of six relevant methods was developed. The techniques were implemented as java plugins for the free access software ImageJ. They were evaluated and compared using standard and synthetic images. Several of the methods assume certain parameter values. Here, the validity of these assertions was studied, observing that they were not valid in most of them. It was found, in most cases the evaluated techniques did not allow to obtain a good estimation of the noise level.