4.2. Estimation of structural parameters 
The results of the foliage zenith angle estimates is not surprising knowing that after tasselling maize leaves 
spread horizontally under their own weight. One can find different results in the littérature, according to the 
stage of development and the maize variety, as for example Prevot and Brunet (1993) who observed similar 
zenith distribution just after tasselling or a maximum between 50° and 60° before silking (Prevot et al 1991; 
Perrier 1976). As far as the leaf azimuth orientation is concerned, Lemeur (1973) observed that the maximum 
of the azimuth density function corresponded to the direction perpendicular to the rows and the minimum to the 
parallel to them. On the contrary, Ross (1981) observed a weak maximum at 45 degrees from the rows 
direction. The differences between these results and our estimations could be due to the leaf lamina, the wavy 
feature of which was not considered in our model, and therefore the orientation angle calculated from the 3-D 
stereo model should be not the same as the real angles. 
The computed value of the leaf area density was slightly underestimated compared to the Plant Profiles 
measurements. We believe that this difference is mostly due to the leaf parts near the insertion point and the 
tip, which were in general hidden and therefore not reproduced in the 3-D image processing. Nevertheless one 
should question the comparison between stereovision estimates and plant profiles measurements. As a matter of 
fact, we must point out that the first method was performed on 100 maize plants which were present in the 
stereo photographs, while the second one on only 20 arbitrarily chosen in the field. 
The method presented in this paper gives a real description of the plant 3D geometry. It will be very 
interesting to combine this approach with the Botanical Plant Modelling System (Lewis and Muller 1990, 
Lewis et al. 1991) which enables to generate plant canopies starting from parameters such as plant density and 
number of leaves per stem, leaf length, width, and twist, leaf zenith and azimuth angles, etc.... All these needed 
parameters should be easily obtained from our 3-D geometric model. It should be also possible to study light 
penetration inside the canopy with a suitable ray tracing algorithm, such as for example the ARARAT 
procedure (Advanced RAdiometric RAy Tracer) (Lewis and Muller 1992), directly applied to our 3-D 
vegetation model. This kind of application should provide a realistic simulation of the directional canopy 
reflectance provided that optical properties of leaves are known. In the future this kind of approach should 
enable us to compare the radiative transfer involved by various geometrical structures (species, crops) in order 
to build directional reflectance models suited to various types of vegetation. 
Acknowledgements 
The authors thank B. Andrieu for useful discussions and for providing the software to analyse the leaf 
orientation distributions. We also thank A. Allix, Y. Tymen, P. Belluomo and E. Delbaere for their 
contributions to this work. 
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