An example of such a 3D object model is given in the following 
by Figure 2. 
A 
Species oe econ 
7 
N 
N 
N 
ES 
/ 
^ 
/ x. 
fee 
A 
/ 
N 
N 
CÁM 
y 
  
Figure 2. 3D object model 
! Each surface of the object corresponds to a single rectification 
plane. Combining all the planes of the structure the 3D model is 
| constructed. 
3.2 True Orthoimage generation 
The true orthoimage is an image that is based on an 
orthographic projection taking into account the strict structural 
condition of the object. 
The true orthoimage does not show any relief displacement as a 
result of the existence of a "perfect" structural model of the 
object. In this case, the orthographic projection directs the 
projecting rays in a perpendicular way to a horizontal plane. 
Consequently, obscured areas cannot be presented in a true 
orthoimage. 
The height information in a 3D model is used to remove the 
height of hidden effects from the perspective image by 
  
d reprojection. Frequently, for the production of an orthoimage 
more than one image can be used to acquire the full texture 
information. 
Fundamentally, there are two basic approaches (Novak, 1992; 
Mikhail et al., 2001) for the generation of an orthoimage: 
l. Forward projection 
2. Backward projection 
In the first case of forward projection, the pixels from the 
original image are projected on top of the DTM of the 3D 
model and the pixels’ object space coordinates are calculated. 
Then, the object space points are projected into the orthoimage. 
Because of the gap between the points projected into the 
orthoimage fluctuates -due to the terrain deviation and 
perspective effects- the final orthoimage pixels are generated by 
interpolation between the projected points. 
A i 
ide 
In the other case of backward projection, the object space X,Y 
coordinates related to every pixel of the final orthoimage are 
determined. The height Z at a specific X,Y point is calculated 
from the DTM or the 3D model and then the X,Y,Z object 
space coordinates are projected in the original image in order to 
acquire the gray level value for the orthoimage pixel. 
Interpolation or resampling process in the original image is 
  
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part BS. Istanbul 2004 
essential because of the fact that the projected coordinates will 
not fit accurately at the original image pixel centres. 
The main target of the true orthoimage process focuses on the 
representation of regions where abrupt changes of elevation 
appear. 
Perspective Center 
X Perspective Image 
  
/ V 
A Wu 
LF AX 
/ A 
£ / AA 
7/7 Y 
£f AN 
£P. 
/ \ 
/ 2 
/ 
Figure 3. The “cavity problem” is crucial for true orthoimage 
generation 
In Figure 3, the “cavity problem” is explained in a plan view 
outline. Due to the perspective geometry these areas must be 
eliminated during the true orthoimage generation. A 3D model 
like the one presented in Figure 2, is the appropriate material 
for the solution of the problem. 
3.3 Explaining the algorithm 
  
INPUT 3D Model 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
INPUT LO INPUT E.O 
CALCULATE DEFINE orthoimage 
orthoimage boundaries pixel size 
  
  
  
  
  
  
  
  
For every pixel of the orthoimage 
Find the position according to the 3D model 
Find the position in an original image 
Get the pixel gray value 
Assign pixel gray value in orthoimage 
End For 
  
  
  
Figure 4. The true orthoimage framework production 
  
   
  
   
    
   
    
   
  
    
    
  
   
   
   
   
  
  
  
  
   
  
  
  
  
   
  
   
  
  
   
  
   
  
   
  
   
  
   
   
  
  
  
  
  
   
Interna 
pet 7 sms 
PTR 2 
Ve AC SR racer + += 
SE ren OT As