is the same 
y red frames, the 
, = 4. However, 
nd the houses in 
h influences the 
nproved method 
nage into blocks 
re we divide the 
ie value range of 
ing image is as 
Figure 5. The resulting image when Dy, is different 
Through computation, the definition of the lower right block 
is the maximum, so the cut-off frequency should be set to be 
minimum. In this way the region around the ground objects 
will not be blurred. The definition of the lower left block is 
the minimum, so the cut-off frequency should be set 
maximum. Through this we can reach a more satisfactory 
dodging effect. 
In order to reflect the experiment result more accurately, we 
use some objective indicators to assess the original and 
resulting images. At present, a common assessment method 
is to first select five regions from the image, the upper left, 
the lower left, the upper right, the lower right and the center, 
then compare their mean and variance. However, there may 
be differences between different regions in an image itself. 
For example, if the upper left area of an image is forest and 
the lower left area is houses, there should be different 
lightness and contrast in an image. The approximately same 
mean and variance of the two regions doesn't indicate a good 
dodging effect. 
Therefore, this paper adopts a new assessment method: select 
several same ground objects from the different image regions 
manually as shown in Figure 6, and calculate their mean and 
variance(Table 1). If these values are approximately the same, 
that means a satisfactory dodging effect was achieved. 
Figure 6. The original image and the image after dodging 
  
Mean standard deviation 
  
Before After Before After 
dodging | dodging | dodging | dodging 
  
Vegetation] | 171.39 106.93 28.37 24.35 
  
Vegetation? | 61.22 95.30 15.64 20.13 
  
Vegetation3 | 33.45 88.35 6.8 19.08 
  
Buildingsl 151.58 105.37 46.42 40.58 
  
Buildings2 180.69 103.23 46.90 39.41 
  
  
  
  
  
  
Buildings3 160.45 104.19 49.69 41.29 
  
Table 1. The image information before and after dodging 
It can be seen that the means of the same kind of ground 
objects are approximately the same after dodging, in addition, 
the uneven contrast of the whole image is greatly improved. 
5. CONCLUSION 
This paper puts forward an improved image dodging