ISPRS Commission III, Vol.34, Part 3A „Photogrammetric Computer Vision“, Graz, 2002 
  
entity is either sunny or shadowed. Problems rise with respect 
to the number of voxels multiplied by the computation time 
used for energy flux balance and temperature determination for 
a voxel. In a similar way, geometrical facets are heterogeneous 
with respect to boundary conditions existing on these entities; if 
computed on facet, surface temperature would not be physically 
relevant. 
Taking into account variations in time of all the interactions 
differentiates the image synthesis in the infrared range from that 
in the visible domain. The synthesis methods used for the 
simulation of landscapes for short wavelengths do not need to 
reproduce the recent past of the landscape as it is observed in 
real infrared images. The radiosity method can be used in such a 
case. In the thermal infrared range, it cannot, except for 
considering the radiosity method for each voxel and each time- 
step. An entity permitting to compute both temperatures at each 
instant during the process and the radiance leaving the scene 
has to be used. 
In addition, a 3-D description of the landscape is necessary to 
perform a simulation with a very high spatial resolution. In this 
respect, objects forming the landscape interact each other. 
Realistic image simulation should reproduce these physical 
interactions between objects (Poglio ef al., 2001a). This adds to 
the complexity of the simulation process. 
3.3 The landscape representation 
We defined an element as an entity that exhibits homogeneous 
properties with respect to geometrical considerations, material 
constitution, and the occurring physical phenomena at each 
instant (Poglio et al., 2001b). To reduce as many as possible the 
number of elements, and afterward the computation time, 
element is defined as the largest entity included in a facet. 
The element is a part of an object; it is a 3-D entity. Its external 
surface is flat and oriented. The in-depth constitution of the 
element is made of one or several layers of primary materials: 
for instance, a wall can be made of two layers; construction 
concrete and insulation. The element is homogeneous with 
boundary conditions, both internal and external. Figure 1 
illustrates the element. 
To better understand the element, one may consider a facet; this 
facet is partly shadowed at instant /. These portions of the facet 
are a preliminary set of elements, which will be subsequently 
subdivided by taking into account other phenomena and other 
instants. 
The mesh supporting objects constituting the landscape is made 
of the union of elements for the period of simulation of 
temperature. Figure 2 illustrates the representation of the 
landscape. The landscape is divided into objects, objects into 
facets, facets into elements. In each subdivision, properties and 
characteristics are inherited from the previous hierarchical level 
and are added to perform an element-based representation of the 
landscape. 
A - 242 
  
Homogeneous flux 
balance at each point 
on the external surface 
   
   
Multi layered | 
constitution in 
the in-depth 
dimension 
Same boundary conditions 
on internal surface 
  
  
  
Figure 1: illustration of the multi-layered in-depth constitution 
of an element and the homogeneity with respect to 
boundary conditions on both the internal and 
external surface. 
  
  
| Landscape | 
T 
v 
Facet k-1 and 
3-D description 
  
  
Objectk | 
Facet k-p and 
3-D description 
Element k-p-1 
  
Y 
Facet Æ-2 and 
3-D description 
  
  
    
  
   
Element &-2-5 
   
     
| 
Element &-/-/ 
  
Element &-2-/ 
  
  
Figure 2: illustration of the landscape element representation. 
4. ARCHITECTURE OF THE SIMULATOR OSIRIS 
The simulator OSIRIS is divided into four primary simulators, 
each of them dealing with a particular sub-part of the simulation 
(Poglio et al., 2001c). These four primary simulators, called 
(S0) to (S3), operate successively. 
The main technical originality of the simulator OSIRIS is the 
pre-processing simulator, (SO), generating the elements. The 
simulator (S0) operates with the following scheme (figure 3). 
The shadow maps are made and combined with the facet 
description of the scene to obtain the element-based 
representation. In addition, the radiative and conductive 
surroundings, the wind velocity for each element and the form- 
factor matrix are computed.