International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B5. Istanbul 2004 
  
surface patch (e E 0) with illumination directly from above 
(i = 0) ; a is only half the value of f, : 
Although the lambertian model is widely used because of it 
simplicity, it is a poor approximation to the diffuse component 
of rough surfaces. 
  
  
Figure 3. (a) Lambertian model 
(b) Lommel-seeliger model 
3. IMPLIMENTATION 
A number of techniques have been developed for modeling 
object shapes by observing real objects. However, attempts to 
model reflectance properties of real objects have been rather 
limited. In most cases, modeled reflectance properties are too 
simple or too complicated to be used for synthesizing realistic 
images of the object. In this research work, the lambertian 
model is utilized for modeling the terrain reflectivity property. 
It is very difficult to choose good test image for SFS 
algorithms. À good test image must match the assumptions of 
the algorithms, e.g. lambertian reflectance model, constant 
albedo value. But there are some images that could not have 
arisen from shading on a smooth surface with uniform 
reflecting properties and lighting. It is not difficult to satisfy 
these assumptions for synthetic image. In real image, there will 
be errors to the extent that these assumptions are not matched. 
In this research, simulate data was generated using a predefined 
bilinear surface and for real data an aerial photograph of a 
smooth hilly terrain with low information content was chosen. 
Synthetic image in scale of 1:40000 was generated by a ray 
tracing algorithm, using the synthetic DTM, together with a 
constant value for the surface albedo. The exterior orientation 
of the image and light source position were considered as 
known values. 
Also one black and white aerial image with an image scale of 
approximately 1:40000 of poorly texture area in Iran was used. 
The image was digitized using photogrammetric scanner with a 
pixel size of 14pm, resulting in a ground sample resolution of 
about 0.56m. The interior and exterior orientation were 
determined using digital stereo plotter. The illumination 
direction was calculated from known time of the image 
acquisition and geographical coordinates of surface arca. 
In order to investigate of potential of the reflectance model, we 
consider the equal profiles on the DTM and image. 
The correlation values for the slope and gray shade variation 
were obtained to assess the dependency between the gray shade 
'ariaion and the slope variation for the real data. The 
correlation for these factors were small for the real data as 
compared with the simulated data (figure 4, figure 5). This 
means that gray shade values are also significantly influenced 
by the other factors such as non-uniform terrain albedo, 
atmosphere, etc. which have not been included into Lambertian 
model. 
Also, one of the main reasons why modeling of reflectance 
properties has been unsuccessful, compared with modeling of 
object shapes, is that both diffusely reflected lights and specu- 
larly reflected lights, ie, the diffuse and specular reflection 
components are treated together, and therefore, estimation of 
reflectance properties becomes unreliable. To eliminate this 
problem, the two reflection components should be separated 
prior to estimation of reflectance properties.