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.