Due to the low recognition rate of weeds in wheat fields and the inability to accurately locate weeds, we propose a recognition method for weeds in natural wheat fields based on the fusion of RGB image features and depth features. The method breaks through the limitations of the two-dimensional spatial features extracted from RGB images when recognizing grass weeds similar to wheat. According to the species, distribution of weeds in wheat fields, we extracted the color, position, texture, and depth features of weeds in wheat fields from RGB and depth images during the tillering and jointing stages. And then used the AdaBoost algorithm for the integrated learning of multiple classifiers, thereby achieving the recognition of weeds in wheat fields. The experimental results revealed that the recognition speed of weeds during the tillering stage was 0.2 s and the accuracy rate was 88%. The recognition speed of weeds during the jointing stage was 0.69 s, and the accuracy rate of weed recognition was 81.08%. These results are significantly higher than the weed recognition rate based on features extracted from RGB images.
Canopy spectral reflectance can indicate both crop nutrient and canopy structural information. Differences in canopy structure can affect spectral reflectance. However, a non-imaging spectrometer cannot distinguish such differences while monitoring crop nutrients, because the results are likely to be influenced by the canopy structure. In addition, nitrogen application rate is one of the main factors influencing the canopy structure of crops. Strong correlations exist between indices of canopy structure and leaf nitrogen, and thus, these can be used to compensate for the spectral monitoring of nitrogen content in wheat leaves. In this study, canopy structural indices (CSI) such as wheat coverage, height, and textural features were obtained based on the RGB and height images obtained by the RGB-D camera. Moreover, canopy spectral reflectance was obtained by an ASD hyperspectral spectrometer, based on which two vegetation indices—ratio vegetation index (RVI) and angular insensitivity vegetation index (AIVI)—were constructed. With the vegetation indices and CSIs as input parameters, a model was established to predict the leaf nitrogen content (LNC) and leaf nitrogen accumulation (LNA) of wheat based on partial least squares (PLS) and random forest (RF) regression algorithms. The results showed that the RF model with RVI and CSI as inputs had the highest prediction accuracy for LNA, the coefficient of determination (R2) reached 0.79, and the root mean square error (RMSE) was 1.54 g/m2. The vegetation indices and coverage were relatively important features in the model. In addition, the PLS model with AIVI and CSI as input parameters had the highest prediction accuracy for LNC, with an R2 of 0.78 and an RMSE of 0.35%, among the vegetation indices. In addition, parts of both the textural and height features were important. The results suggested that PLS and RF regression algorithms can effectively integrate spectral and canopy structural information, and canopy structural information effectively supplement spectral information by improving the prediction accuracy of vegetation indices for LNA and LNC.
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