The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B4. Beijing 2008 
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Figure 6. The section diagram of homomorphic filtering 
Figure 7. Mosaic result after homomorphic filtering 
2.2 Colour balance with Wallis Transform 
Figure 9. Colour is different between aerial photos, some of 
them are green, and others are yellow, the result of mosaic 
orthophoto has blocks 
Wallis filter can adjustment the colour different between images, 
it adjust the gray mean and variance to a given mean and 
variance. The gray mean of image define hue and light 
information, and variance of image define the range of gray 
values (contrast information). Select a stand image from aerial 
images and get mean and variance information about red band, 
green band and blue band of stand image as the stand values of 
mean and variance. All images operated with wallis filtering 
and adjust the means and variances to stand values. 
The formula of wallis filtering as follows. ^ 
Digital aerial camera is easily to deviate colours than simulation 
camera because the CCD’s charge deviation (Figure 8), it leads 
blocks in final mosaic orthophoto (Figure 9). 
g(x, y) = m s + v s (g c (x, y)-m c )!v c (2) 
Figure 8. The deviate colour problem in digital aerial image 
m s is stand image’s mean, v s is stand image’s variance, m c is 
current image’s mean, v c is current image’s variance, gc(x,y) is 
the gray value of current image, g(x,y) is the gray value after 
wallis filtering operation. The mean value m and variance value 
v of image is computer with formula 3. 
fi( x >y) ~ 
0 g(x,yV=i 
1 = i 
w h 
h >=ZZ/( x >^) 
x=0 y=0 
m = 
/=0 
w*h 
v = 
/=0 
w*h 
(3) 
w, h is the image width and height, k is the colour resolution per 
pixel, for example ¿=255 if the resolution is 8bits per colour 
channel.