The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B3b. Beijing 2008 
3. ELIMINATION OF BUILDING SHADOW 
The building shadow elimination method studied in the paper is 
based upon the Companion Area Intensity Mapping Method 
(CAIM)(Yan Li etal,2001). The first step of the CAIM method 
is to make sure the companion area of the shadow, which is 
usually taken as the frontal area of the shadow area. This area 
lies before the shadow projection direction, such as picture (a) 
in Figure 3. The writer observes that the two lines of shadow 
lateral sides are usually parallel to the sunlit direction. So in the 
paper the companion area is chosen upon this characteristic, 
such as picture (b) in Figure 3. As more regions adjacent to the 
shadow area are taken as companion area, the grey property 
will be more close to that of the area not overlaid by the 
shadow. 
(a)Common method (b)Method deciding companion 
deciding area j n t(ji s p a p er 
Figure 3. Comparison of method deciding companion area 
Where A is shadow area 
B is companion area of the shadow area 
—► is shadow projection direction 
After deciding the area, grey properties of region A and B 
should be calculated out for gamma transform, which will 
adjust grey values of shadow. The gamma transform 
parameters can be computed by putting the average grey values 
of A and B into the gamma transform equation. 
original IKONOS image. Picture (b) is the shadow extraction 
results. Picture (c) is the shadow elimination results. 
Results of the experiment show that this approach is effective 
and robust. 
(a)Original IKONOS image 
(b)Shadow extraction result 
Outpixel = 2047 * {Inpixel /2047)"' (1) 
Where Outpixel = the grey value after shadow elimination 
Inpixel = the grey value before shadow elimination 
m = parameter 
Parameter m can be calculated by putting the average grey 
value of A and B into equation (1). 
Then the whole shadow region can be restored by calculating 
the grey value after elimination from that before elimination. 
4. EXPERIMENT AND CONCLUSION 
In Experiment, the building shadow extraction is done with 
several IKONOS images and high resolution aerial images. The 
shadow extraction results of one IKONOS image are shown in 
figure 4. The shadow extraction results of the other IKONOS 
image are shown in figure 5. Picture (a) in Figure 4 and 5 is 
(c) Shadow elimination result 
Figure 4. The shadow extraction and elimination results of one 
IKONOS image