Abstract:

Objective: In order to solve the problems those the evaluation of the existing single wood permeability is greatly affected by the observer's perspective and subjectivity, and it is difficult to establish a unified judgment standard, based on the digital image of trees, this study aimed to identify and integrate the structural characteristics of the single tree. The inner and outer areas formed by the canopy were used to quantitatively evaluate the canopy permeability, with an expect to provide technical supports for the health monitoring and growth status analysis of single trees. Method: Taking cedar as the research object, the image of a single tree was obtained using Surface Pro 4. Using a pressure-sensitive stylus to manually circle the canopy area in the image, and further graying and binarizing the circled area, the binarized image of a tree was obtained. By using morphological operations, the smooth outline of the canopy was obtained, to determine the vertical symmetry axis of the canopy, establishing a mirror image of the canopy based on the symmetry axis. By using the DelaunayTri function, the triangulation of the canopy was obtained, and then the canopy Convex hull was also obtained by using the convexhull function to obtain. The shortest distance from each point on the smooth outline to the convex hull was calculated by starting from any point in the smooth outline and walking in a clockwise direction.By using K-means clustering algorithm to determine deep and light depressions, and calculating deep depressions density, the area within the smooth outline of the canopy was marked as a connected area. By using K-means clustering to divide the connected area into large holes and small holes, the density of large holes and small holes was calculated respectively. The three coefficients of density and pore density were given different weights to quantitatively evaluate the permeability of single wood. Referring to the experience of forestry experts and tree growth rules, three coefficients of deep depression density, large hole density and small hole density in the canopy profile were used to quantitatively evaluate the single tree permeability. Taking into account the effects of deep depression on the crown shape and permeability were higher than those of the large hole, a greater weight was given to the depth depression. In addition, the contribution of the small hole to the transparency was added, 6 canopy images were obtained from 6 angles of 0°, 30°, 60°, 90°, 120°, 150°. The average T_ca was used as the permeability coefficient of the single tree to reduce the fluctuation of the transparency caused by the change of viewing angle, and the true state of the single tree was reflected as accurately as possible. AutoCAD 2010 combined with Photoshop CC 2017 was used to design the single-wood verification model, and the method proposed in this study were tested and verified. Result: Ideally, the density of small hole, the density of large hole, deep depression density, and permeability was 0.125 0, 0.125 0, 0.162 9, and 0.264 6, using the method propose by this research, the density of small hole, the density of large hole, deep depression density, and permeability was 0.117 8, 0.124 1, 0.164 0, and 0.258 6. The method proposed by this research had an accuracy of up to 97.73%. Conclusion: The method proposed by this research, had a fast measurement and a small manual workload. It was expected to provide a technical support for the health monitoring and growth status analysis of single trees. At the same time, the research ideas and method could also be applied to other trees and crops. The monitoring analysis also might have a certain practical value.

Key words: symmetrical convex hull, smooth outline, deep depression, large hole, small hole, permeability