# Prediction of Oleic Acid Content of Rapeseed Using Hyperspectral Technique
## 3 Result and Discussion  
### Feature Selection  
The red-edge parameter is one of the most significant characteristics of the green plant spectrum. It refers to the spectral position (wavelength) corresponding to the maximum value of the first derivative spectrum in the red-light range (680-760 nm). The red-edge amplitude refers to the maximum value of the first derivative spectrum in the red-light range. The red-edge area is the integral of the first derivative in the red-light range. Similarly, the blue-edge (490-530 nm) parameter and the yellow-edge parameter (560-640 nm) are also regarded as important features of the green plant. They are collectively called trilateral parameter (TriP) [22].  
Spectral index is a linear or nonlinear combination of spectral reflectance at some specific bands, which has a certain meaning for the object. Generally, the Ratio spectral index (RSI), Normalized difference spectral index (NDSI), and Difference spectral index (DSI) are the three most significant spectral indexes, which are selected to our consideration and shown in Equations (14)-(16) (\(R_{\lambda_{1}}\) and \(R_{\lambda_{2}}\) denote the reflectance of the wavelength \(\lambda_{1}\) and \(\lambda_{2}\), respectively). The hyperspectral parameters mentioned in this paper are summarized in Table 2.  
\[\text{RSI }(\lambda_{1},\,\lambda_{2})=\ \frac{R_{\lambda_{1}}}{R_{\lambda_{2}}} \tag{14}\]  
\[\text{NDSI }(\lambda_{1},\,\lambda_{2})=\ \frac{R_{\lambda_{1}}-R_{\lambda_{2}}}{R _{\lambda_{1}}+R_{\lambda_{2}}} \tag{15}\]  
\[\text{RSI }(\lambda_{1},\,\lambda_{2})=\ \ R_{\lambda_{1}}-R_{\lambda_{2}} \tag{16}\]  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameters** & **Symbol** & **Description** & **Ref** \\ \hline \multirow{8}{*}{Multifractal feature} & MF-\(h(0)\) & Generalized Hurst exponent & [8; 9; 10] \\  & MF-\(h(\pm 1)\) & Generalized Hurst exponent & [8; 9; 10] \\  & MF-\(h(\pm 2)\) & Generalized Hurst exponent & [8; 9; 10] \\  & MF-\(h(\pm 3)\) & Generalized Hurst exponent & [8; 9; 10] \\  & MF-\(\Delta h\) & Span of h(q) & [8; 9; 10] \\  & MF-\(\alpha_{max}\) & Maximum of \(\mathit{H\ddot{o}}\)_lder_ index & [8; 9; 10] \\  & MF-\(\alpha_{min}\) & Minimum of \(\mathit{H\ddot{o}}\)_lder_ index & [8; 9; 10] \\  & MF-\(\Delta\alpha\) & Span of \(\alpha(q)\) & [8; 9; 10] \\ \hline \multirow{2}{*}{Sensitive band} & SB-R & Sensitive band of reflectance & [23] \\  & SB-DR & Sensitive band of the first derivative & [23] \\  & & reflectance & \\ \hline \multirow{8}{*}{Trilateral Parameter} & TriP-rep & Red edge position & [24] \\  & TriP-Dr & Red edge amplitude & [24] \\  & TriP-SDr & Red edge area & [24] \\  & TriP-yep & Yellow edge position & [25] \\  & TriP-Dy & Yellow edge amplitude & [25] \\  & TriP-SDy & Yellow edge area & [22] \\  & TriP-bep & Blue edge position & [25] \\  & TriP-Db & Blue edge amplitude & [25] \\  & TriP-SDb & Blue edge area & [22] \\ \hline \multirow{2}{*}{Spectral index} & SPI-RSI & Ratio spectral index & [26] \\  & SPI-DSI & Difference spectral index & [27] \\ \cline{1-1}  & SPI-NDSI & Normalized difference spectral index & [28] \\ \hline \hline \end{tabular}
\end{table}
Table 2: Hyperspectral characteristic parameters.  
The multifractal feature captures the global singularity and correlation of the hyperspectral reflectance, which may reflect the essential characteristics of the spectral reflectance of rapeseed samples with different oleic acid content. The spectral index expresses the combined characteristics of the reflectance at different bands. The trilateral parameter focuses on the hyperspectral characteristics change at special locations. The sensitive band locates the band where the hyperspectral reflectance has the most significant correlation. In the following, we use these four types of parameters as features to predict the oleic acid content of rapeseed.