Leaves of volunteer corn and soybean possess different shapes, which can be considered as a potential feature for distinction purposes
[39]. Five leaf shape features were extracted in this study—area, aspect ratio, rectangularity, circularity and eccentricity. Area is a parameter that can reflect the size of leaves. In this study, it was reasonably assumed that the dimensions of individual pixel were a constant value for all the images collected during the five days, considering the large distance between the camera and crops and the relatively small height changes of crops. The area value was converted to a pixel count individual plants, which is the number of pixels taken by the crop. The aspect ratio was defined as the length of the leaf divided by its width (
Equation (1))
[42]. In general, a corn leaf is long and thin, and a soybean leaf is short and round, which would potentially lead to different aspect ratio values. Rectangularity represents the similarity between a leaf and a rectangle and can be calculated according to
Equation (2) [41]. The more the leaf shape is similar to a rectangle, the closer the rectangularity value is to 1. The circularity, which represents the roundness of leaves, were also extracted according to
Equation (3) [41]. For a perfect circle leaf, the circularity value is 1. Furthermore, the eccentricity feature was extracted based on
Equation (4), in which
b stands for the foci of the ellipse and
a stands for the major axis length of the ellipse
[41]. It needs to be noted that the pixel square value in Equations (
1)~(
4) and the area is 1×1.