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Figure 2 - High-Pass Filter used (Godoy Jünior, 1993). 
2.2 IHS transformation 
This technique, frequently used to integrate images obtained 
from different sensors, allows to substitute the Intensity 
channel of the multispectral component in IHS domain, by a 
better spatial resolution band. 
In this paper, the IHS transformation was applied to generate 
the synthetic image resulting from the fusion of the 
multispectral SPOT image with the filtered panchromatic 
SPOT image. The following procedure was used, aiming at 
preserving the spectral characteristics presented by the urban 
areas in the original multispectral image, without losing the 
spatial resolution of the panchromatic image previously 
filtered: 
* In RGB domain, the histograms averages of the three 
multispectral SPOT image bands were read (Figure 3). This 
multispectral component had been previously resampled in 
the registration operation with the panchromatic SPOT 
image. 
* These averages were equalized to 128. 
* The transformation of this equalized image to the IHS 
domain was performed. 
e The IHS image statistical parameters were read, so that the 
component I average was 127.45. 
e The statistical parameters of the filtered panchromatic SPOT 
image were read and its average was 40.61. 
This average was equalized to the multispectral SPOT image 
band I average, which would in turn substitute it. 
e The statistical parameters of the filtered panchromatic SPOT 
image were read and the equalized average was equal to 
126.58. 
* In the IHS domain, the component I of the multispectral 
SPOT image was replaced by the panchromatic SPOT image 
whose average was equalized. 
* This synthetic image was transformed from the IHS to the 
RGB domain. 
* The statistical parameters of the new RGB image were read. 
They had the following values: Band R=125.00; Band 
G=125.84; Band B=126.56. 
735 
e From each band average, the necessary values were 
subtracted in order to return to the original resampled 
multispectral SPOT image averages, before the equalization. 
e The final synthetic image statistical parameters were read, 
with the following average values for each band (Figure 4): 
Band R=39.98; Band G=31.83; Band B=41 81. 
3. RESULTS 
The high-pass filter application, previously described, allowed 
to refine the original panchromatic image visual quality. The 
filtered image enhanced the river banks, as well as the intra- 
urban features of Säo José dos Campos city and also the roads, 
bridges and railways. 
The comparison between the histograms of the resampled 
multispectral SPOT image (Figure 3) and the synthetic image 
(Figure 4) enables one to see that through the described 
processing a final image was obtained with spectral 
characteristics very similar to those of the original 
multispectral component. However, the synthetic product 
appeared with a slight alteration in its spectral characteristics, 
thus originating some shape and size changes of certain 
features, specially in areas of dense vegetation. This was due to 
the substitution of the multispectral image channel I by the 
filtered panchromatic band, as it has a low spectral response to 
the close infra-red and therefore, in the synthetic image, areas 
with vegetation as well as other targets which have a high 
degree of reflectance in the infra-red band, appeared with a 
somewhat different spectral response as opposed to that of the 
original multispectral image bands. That results in a certain 
degree of spectral information loss in those areas. On the other 
hand, the intra-urban areas were considerably improved. 
4. CONCLUSIONS 
Based on the results obtained, the following conclusions can be 
drawn: 
A. The edge enhancement obtained through the high-pass 
filtering of the original panchromatic image, favoured a 
better discrimination as well as the digitalization of several 
linear and punctual features which did not appear well 
defined in the original image, proving to be very efficient 
to obtain more enhanced synthetic bands. 
B. The merging, in the synthetic image, of the spatial 
resolution of the filtered panchromatic image with a 
spectral resolution very similar to that of the original 
multispectral component, allowed a better identification 
and extraction of targets such as industrial areas, urban 
areas limits, street sections and other intra-urban features. 
C. It is interesting to emphasize that in this methodology the 
original multispectral bands, without altering the contrast 
nor any kind of enhancement, were used. The original 
bands histograms, as well as those of the translated original 
bands to the average value used for the equalization (128) 
did not exceed the superior limit of 255 to avoid 
information loss. 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996