Modeling the radiance fieldwithin 3D crop canopies

Michaël Chelle, Bruno AndrieuUMR Environnement et Grandes Cultures

INRA Thiverval-Grignon - France

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Modeling 3D light transferLight-leaf interaction

incident

reflection

transmissionabsorption

Sanz et al, 1997

Maize leaf BRDF

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Modeling 3D light transfer

interception

scattering

Light-leaves interactions

=> Not working on a whole canopy, but on a significant pattern ∞ duplicated

The radiance equation

L(y,yx)

Complexity of solving this equation depends on the number of surfaces Sy

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First order of scatteringProjection (Z-buffer)

Efficient treatment of periodic infinite canopy

Canopy BRDF => double Z-buffer : Bvis (B. Andrieu, 1999)

Canopy gap fraction => single Z-buffer : Monogap

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First order of scatteringExample of application

Estimation of the clumping parameter

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Multiple scatteringMonte 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

* Classic CG algorithms

•Polygons set, various leaf BRDF

• Multispectral: work in progress

* Estimation of the variance of each output

Few assumptions, but Computing-time consuming

* 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

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Multiple scatteringIllustrations of parcinopy uses

Generation of reference dataset: nested radiosity, Kuusk (97), Shabanov (2000) Analysis of sensitivity : leaf BRDF, Plant geometry (Espana et al)

an erectophile canopy lit with a zenith source

NIR

?

Study of radiative transfer: what about fluxes isotropy? scattering order?

LAI 0.5,

LAI 2

LAI 3.7

TM, LAI 4, 60°, NIR

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Multiple scatteringA more efficient method : radiosity

Borel (1991); Goel (1991), Garcia-Haro (2002),

fr(x) i

L(y,r)

B i (radiosity)H Lambertian

Thus, the radiance equation is simplified:

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

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Multiple scatteringA dedicated radiosity method for canopy

the nested radiosity (Chelle et Andrieu, 1998)

Designed to estimate leaf irradiances, a Z-buffer projection may be used to estimate canopy BRDF from these…

For each triangle, a sphere defines the close objects

The far radiations are estimated by a TM model: SAIL

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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?

Modeling 3D light transfer

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Conclusion

Provide tools to investigate light-canopy interactions and the properties of resulting fluxes

Provide reference dataset

Combining accurate 3D canopies and 3D RT tools

Basis to develop efficient, but correct RT models to analyze remote sensing data

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0 ~ 1 0 <1

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Sensivity to the sphere diameter : the case of maize