| The forest environment has a highly complex radiation state,and the three-dimensional structure of the canopy prevents the solar short-wave radiation from entering every corner of the forest.The traditional methods of quantifying stand radiation,such as instrumental measurement,need to organize investigators to conduct field surveys.The whole cycle is long,and at the same time,it has high requirements for operation requirements,weather and other environmental factors,and it is difficult to carry out long-term,large-scale dynamic monitoring.In addition,canopy radiation is constantly changing with time-space,season,and meteorological factors,making the measurement and modeling of stand radiative flux challenging.Lidar has important practical significance for the study of canopy radiation.Combined with computer graphics,it can further quantify and render the radiation of the forest canopy by simulating the solar radiation of the entire forest.Based on this,this paper develops a synergistic approach that uses airborne LiDAR data and ray tracing with computer graphics to simulate the forest canopy and separately calculate three forest stands(coniferous forest,broad-leaved forest and mixed forest)in a certain forest.Radiation flux at time.First,referring to the description of the shortwave solar irradiance by the Iqbal model,combined with the specific geographical parameters,atmospheric parameters and time parameters of the study area,the overall shortwave solar irradiance in the area where the stand is located is calculated.Then,on the basis of the stand point cloud data obtained by the lidar,the single tree is segmented and the specific structural parameters of the stand are obtained through parameter estimation.The canopy surfaces of conifers and hardwoods were then modeled using the surface equations of a cone and a semi-ellipsoid,respectively.After that,the ray tracing process of the forest stand is started: first,the solar altitude and azimuth angle at a certain moment are calculated according to the geographical location of the study area,and the corresponding direction vector is further calculated,and then directional discrete incident solar beams are emitted,and the distance between them is It can be adjusted to control the amount of incident light according to the needs of the study.In this experiment,a ray spacing of 0.2 meters was used,corresponding to a ray density of 25 per square meter.The forest canopy surface is then decomposed into a collection of triangular elements by triangulation.The average spacing of the vertices of the triangle can be changed by adjusting the parameter.In this study,the sampling number of the triangulation concentric circles is set to 15,and the average distance between the corresponding triangle vertices is 1.36 meters.Then use the ray tracing algorithm in computer graphics to calculate the first intersection of the incident ray and the triangle,which is the incident point.On this basis,the incident point is taken as the emission point of the subsequent reflection and transmission process,combined with the laws of reflection and transmission,after obtaining the emission vector and transmission vector corresponding to each incident point,the subsequent intersection of reflected rays and transmission rays are performed.delivery process.After obtaining all the incident points,reflection points and transmission points of the stand,combined with the angle between the ray and the normal vector of the triangle,the received incident,reflected and transmitted radiant flux of each triangle is further calculated.In addition,this paper also estimates the canopy transmittance by airborne lidar,and renders the stand radiation combined with reverse ray tracing and BRDF rendering model.Our method accurately simulates the solar incident radiation flux and shows good agreement(R2 ≥ 0.82)with the results measured by the HPEval software and the solar radiation meter based on hemispheric photographs.We selected the 15 th of each month from March to October2019,and calculated the incident,reflected and transmitted radiant fluxes of the three stands from8:00 to 17:00 at 1-hour intervals under clear sky conditions.The incident radiant flux reached a maximum value of 2047.2 k W in coniferous forests at noon on June 15 due to the largest solar elevation angle(81.21°)and the densest canopy;due to the higher reflectivity of conifers and better absorption of reflected solar beams,the taiga received the maximum reflected radiant flux(10.91-324.65 k W).However,the broad-leaved forest received more transmitted radiant flux(37.7-226.71 k W)due to the higher transmittance of the broad-leaved tree species.Our method can directly simulate the detailed distribution of canopy radiation at the stand scale,and on the basis of point cloud data,it does not require labor-intensive and material-intensive measurements such as field measurements,and is not restricted by time and space conditions.With the improvement of hardware,our method can continuously improve the running speed under the premise of ensuring accuracy,which is very valuable for studying light-dependent biophysiological processes. |
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