叶面积指数（LAI）和叶倾角分布（LAD）是决定植被冠层结构的重要参数。在计算机模拟植被冠层,两个参数是植被三维真实结构生成的重要控制因子。本论文中,结合计算机图像学理论,基于实验的地面实测结构参数数据利用可改写的扩展L-system方法生成草以及白杨树的真实三维场景。RGM（A radiosity-graphics combined model）模型是基于辐射度方法的计算机模拟模型,利用此模型来计算生成的三维场景可见光及近红外波段的冠层辐射特性,如冠层波谱以及方向反射特性等。在本研究中,模拟了两种不同下垫面的白杨林地：（1）下垫面只有土壤的白杨树场景;（2）下垫面包括土壤和草的白杨树。在特定的场景组分光学特性下,模拟得到两种情况的主平面冠层BRF（bi-di-rectional reflectance factor）,并对两者的差异进行了分析。可以看出,下垫面对冠层BRF的影响不可忽视。但是,由于白杨林地结构的复杂,大尺度的场景中必须由成千上万个面元组成,因此辐射度方法不能模拟大尺度的真实结构场景。为了拓展辐射度方法应用范围,根据白杨树树冠的特点,将其抽象为椭球体,从而减少场景组成面元个数,满足了辐射度方法的要求。并结合几何光学模型的思想,在对椭球体面元赋值加入了间隙率;并考虑了整个树冠的承照面以及阴影面的差异,模拟大尺度林地冠层BRF,且与GOMS模型结果符合的很好。通过以上研究,可以看出计算机模拟为遥感研究获取多角度数据信息提供了一种很好的手段。

LAI(leaf area index) and LAD(leaf angle distribution) are two important parameters determining vegetation canopy structure.In computer simulations of vegetation canopies,these two parameters are the controlling parameters that render the 3D vegetation scene.In this paper,a computer graphics method,Modified Extended L-system(MELS),is used to render an architecturally realistic leaf canopy(grass) and tree canopy(aspen),that are based on measured structure data of typical grass and aspen trees.A radiosity-graphics combined model(RGM) is then used to calculate the radiation regime of the simulated 3D realistic structure scene,including canopy directional reflectance and spectrum in visible and NIR regions.Two aspen tree scenes are simulated:one where the underlying surface is soil and another where the underlying surface is grass and soil.Then,distributed characteristics of canopies BRF(bi-directional reflectance factor) for the two kinds of scenes are analyzed.However,for realistically structured trees,the radiosity method cannot simulate a large-scale scene because there are too many variables to be processed,so one solution is to simplify the tree crown,making it possible to simulate the canopy BRF at a larger scale.So,we conclude that computer simulations can obtain the radiation regime of forest canopies with some kinds of underlying surfaces,making computer simulation a good way of providing directional reflectance.