The nested radiosity model for the distribution of light within plant canopies

We present a new approach to simulate the distribution of natural light within plant canopies. The canopy is described in 3D, each organ being represented by a set of polygons. Our model calculates the light incident on each polygon. The principle is to distinguish for each polygon the contribution of the light coming directly from light sources, the light scattered from close polygons and that scattered from far polygons. Close polygons are defined as located inside a sphere surrounding the studied polygon and having a diameter Ds. The direct light is computed by projection. The exchanges between close polygons are computed by the radiosity method, whereas the contribution from far polygons is estimated by a multi-layer model. The main part of computing time corresponds to the calculations of the geometric coefficients of the radiosity system. Then radiative exchanges can be quickly simulated for various conditions of the angular distribution of incoming light and various optical properties of soil and phytolelements. Simulations compare satisfactorily with those produced by a Monte Carlo ray tracing. They show that considering explicitly the close neighboring of each polygon improves the estimation of organs irradiance, by taking into account the local variability of fluxes. For a virtual maize canopy, these estimations are satisfying with Ds=0.5 m; in these conditions, the simulation time on a workstation was 25 min for a canopy of 100 plants.