表面矢量编码的查找表算法

针对体绘制计算中严重影响绘制速度的光照效果处理运算,提出了利用表面矢量编码为索引,构造光照效果计算查找表的规划方法.算法编码简单,数据运算中内存的开销小,运算速度快,实际应用效果良好.     

基于三维激光扫描点云的树冠体积计算方法

针对树冠形状不规则,树冠体积难以测量和计算的问题,提出一种基于三维激光扫描点云的树冠体积计算方法——体元模拟法,即以固定大小的体元模拟不规则的树冠形状.首先将树冠沿树高方向以k为步长进行等距离分段,把每一段树冠的点投影到垂直于树高方向的平面上,再对该平面划分成大小为k×k的像元,根据投影到各个像元内点的数量,判断该像元的有效性,统计有效像元的数量T,则树冠体积为T个k×k×k的体元之和.经试验,当体元边长等于冠径的1/10时,计算的树冠体积达到稳定.该算法对于任何树种不用考虑树冠的形状,减少了人为判别导致的差异,适用于三维激光扫描树木树冠体积的计算.     

Estimating leaf area distribution in savanna trees from terrestrial LiDAR measurements

Vegetation structure parameters are key elements in the study of ecosystem functioning and global scale ecosystemic interactions. The detailed retrieval of many of these parameters by direct measurements is impractical due to the quantity of plant material in trees. Terrestrial LiDAR Scanners (TLSs) have been shown to hold great potential as an indirect means of estimating plant structure parameters with a high level of detail, while some studies identified a number of challenges inherent to this approach. In this study we investigate the use of a voxel-based approach to retrieve leaf area distribution of individual trees. The approach is based on the contact frequency method applied to co-registered TLS returns from two or more scanning positions. The contact frequency was computed for voxels being 10, 30, and 50 cm in size and subsequently corrected for the influence of occlusion effects, leaf inclination, the presence of non-photosynthetic material, and the laser beam size. The leaf area of voxels for which occlusion effects were too pronounced was estimated using modeled values based on the availability of light. We compared the TLS derived leaf area estimates against direct measurements, obtained by the harvesting of leaves, in a broad-leaved savanna of central Mali. The measured leaf area values of the sampled trees ranged from 30 to 530 m², and crown LAI values between 0.8 and 7.2. The leaf area estimates lay on average 14% from the reference measurements (general bias). Our method provides vertical as well as radial distributions of leaf area in individual trees, and lends itself to the estimation of savanna vegetation structural parameters with a high level of detail.