International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004 
  
the corresponding wavelet coefficients among input images is 
chosen as the basic fusion rule. 
After selecting the new fused wavelet coefficients according to 
a fusion rule, an inverse wavelet transform is done to construct 
the fused image. 
4. TEST DATA 
Two types of tests are designed. First, multispectral QuickBird 
images over the Davis-Purdue Agricultural Center (DPAC) are 
fused with the QuickBird panchromatic image. The second test 
is to fuse images taken from different sensors. A QuickBird 
panchromatic image over DPAC area is fused with Ikonos 
multispectral images. The properties of the images used in this 
study are given in Table 1. 
The objective of fusing QuickBird and Ikonos images is to 
inspect the effects of different sensors on the fusion process 
such as different acquisition time, the image registration and 
alignment problems possibly caused by different platform 
attitudes, scales and projections. The principles of the fusion 
process algorithm are described in the following section. 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
Satellite QuickBird Ikonos 
Image Pan XS Pan XS 
# Bands 1 4 I 4 
XS Band # Blue: 1; Green: 2; Red: 3;Near infrared: 4 
# bits 11 B 11 [1 
CE90% (m) 23 23 25.4 25.4 
RMSE (m) 14 14 11.8 11.8 
Collection May3,2002 | April 16, 2002 
date 
Resolution (m) 0.7 2.8 1 4 
di m 2600 gam Not used goa: 
2600 650 S 650 
(row x col) 
Projection UTM WGS 84 
  
  
CE: circular error; RMSE: root mean square error; XS: multi spectral 
Table 1. Properties of test images 
5. RESULTS AND EVALUATION 
5.1 Fusion of QuickBird images 
Since both images are taken at the same time and from the 
same sensor, no registration or rectification is needed. The 
resolutions of the multispectral image and the panchromatic 
one are 2.8 m. and 0.7 m respectively. The multispectral image, 
which has four bands, is separated into four individual bands. 
As shown in Figure 3, one-level wavelet transform is applied to 
the individual bands of the multispectral image to get their 
wavelet coefficients. Since the pixel spacing of the 
panchromatic image is four times less than the multispectral 
ones, three-level wavelet decomposition is necessary for the 
panchromatic image so that its pixel spacing becomes the same 
as the multispectral images. 
The next step is to choose a fusion rule to determine the 
appropriate wavelet coefficients for the fused image. The basic 
requirement is to retain the features and realistic colors, 
respectively from the panchromatic and multispectral images. 
Since wavelet coefficients with large magnitude contain the 
information about the salient features of the images such as 
edges and lines (Li, 1994), taking the largest absolute values of 
the corresponding wavelet coefficients is chosen as the basic 
fusion rule. Therefore, the horizontal, vertical and diagonal 
detail coefficients of the one-level decomposed multispectral 
bands and three-level decomposed panchromatic image (they 
have the same pixel spacing and image dimension at this level) 
are matched pixel by pixel and the largest absolute values are 
taken to be the detail coefficients of the fused image. 
However, in order to retain color information in the 
multispectral image, the approximation coefficients are treated 
differently. In fact, the approximation coefficients of 
multispectral bands are kept unchanged in the fusion process. 
After obtaining the new approximation, horizontal, vertical and 
diagonal coefficients for the fused image, three-level inverse 
wavelet decomposition is performed. As the result, a 
multispectral image with 0.7m spatial resolution is obtained. 
* This process is repeated for each individual multispectral band. 
Finally, four fused new image is concatenated to form a new 
four-band fused image. This process is illustrated below in 
Figure 2 and the fused image is given in Figure 3. 
  
  
3-level 
Jie 
Decompo Ll N 
sition 
Panchromatic 
Image 
1-level 
ments 
Decomposition 
  
  
  
  
  
  
  
  
  
   
  
   
    
3-level Fused 
[mage 
    
wavelet 
transform 
  
  
  
One 
Multispectral 
Image Band 
Figure 2. Handling different resolutions in wavelet-based fusion 
  
  
  
  
Original Multispectral 
[Image (QB) 
Figure 3. Fusion of QuickBird pan and multispectral images 
Fused Image 
  
  
  
  
5.2. Fusion of QuickBird and Ikonos images 
This task needs registration and resampling prior fusion as 
discussed earlier. QuickBird panchromatic image is taken as 
the reference to which the Ikonos multispectral images are