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Modeling the radiance field within 3D crop canopies Michaël Chelle, Bruno Andrieu UMR Environnement et Grandes Cultures INRA Thiverval-Grignon - France. Maize leaf BRDF. Sanz et al, 1997. Modeling 3D light transfer. Light-leaf interaction. incident. reflection. absorption.
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Modeling the radiance field within 3D crop canopies Michaël Chelle, Bruno Andrieu UMR Environnement et Grandes Cultures INRA Thiverval-Grignon - France
Maize leaf BRDF Sanz et al, 1997 Modeling 3D light transfer Light-leaf interaction incident reflection absorption transmission
Modeling 3D light transfer scattering interception The radiance equation L(y,yx) Light-leaves interactions Complexity of solving this equation depends on the number of surfacesSy => Not working on a whole canopy, but on a significant pattern ∞ duplicated
First order of scattering Projection (Z-buffer) Efficient treatment of periodic infinite canopy Canopy gap fraction => single Z-buffer : Monogap Canopy BRDF => double Z-buffer : Bvis(B. Andrieu, 1999)
First order of scattering Example of application Estimation of the clumping parameter
Multiple scattering Monte Carlo ray tracing Ross & Marshak (1988); ART (Dauzat, 1991) Raytran (Govaerts, 1994), North(1996), BPMS (Lewis, 1999),… Following stochastically the propagation of light rays within a 3D canopy Our Monte Carlo ray tracing : PARCINOPY • Polygons set, various leaf BRDF • Multispectral: work in progress * Classic CG algorithms * Numerous output variables (not only canopy reflectance) + Canopy BRDF, gap fraction,… + Profile of mean fluxes, radiance distrib° + virtual sensors + polygons irradiance each variable may be given by scattering order * Estimation of the variance of each output Few assumptions, but Computing-time consuming
? an erectophile canopy lit with a zenith source NIR TM, LAI 4, 60°, NIR LAI 0.5, LAI 2 LAI 3.7 Multiple scattering Illustrations of parcinopy uses • Generation of reference dataset: nested radiosity, Kuusk (97),Shabanov (2000) • Analysis of sensitivity : leaf BRDF, Plant geometry (Espana et al) • Study of radiative transfer: what about fluxes isotropy? scattering order?
Thus, the radiance equation is simplified: i fr(x) H Lambertian L(y,r) Bi (radiosity) Multiple scattering A more efficient method : radiosity Borel (1991); Goel (1991), Garcia-Haro (2002), A radiosity model consists in: • computing the N2 form factors between each leaf • solving the resulting system of linear equations • Two limitations of the radiosity method: • the N2 complexity • the Lambertian approximation
Multiple scattering A dedicated radiosity method for canopy the nested radiosity(Chelle et Andrieu, 1998) For each triangle, a sphere defines the close objects The far radiations are estimated by a TM model: SAIL Designed to estimate leaf irradiances, a Z-buffer projection may be used to estimate canopy BRDF from these…
Modeling 3D light transfer Several questions remains: • What about the 3D structure accuracy? • Quid about moving plants ? • How detailed should be the optical properties ? • Are these approaches also suitable for forest canopy? • What about needles? • Experimental dataset ? • Should the 3D approaches be restricted to the theoretical studies • to improve efficient TM models (hot spot, clumping,…) • or be used to design operational methods?
Conclusion Combining accurate 3D canopies and 3D RT tools • Provide tools to investigate light-canopy interactions and the properties of resulting fluxes • Provide reference dataset Basis to develop efficient, but correct RT models to analyze remote sensing data
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