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DODGING RESEARCH FOR DIGITAL AERIAL IMAGES 
M. W. Sun 3 ’*, J. Q. Zhang 3 
‘‘School of Remote Sensing and Information Engineering, Wuhan University, 129 Luoyu Road, Wuhan 430079, China 
Commission VI, WG IV/3 
KEY WORDS: Image Dodging, Seamless Mosaic, Hotspot, Homomorphic Filter, Colour Balance, Wallis Transform, Seam Lines 
Elimination 
ABSTRACT: 
Analysis two main influences of orthophoto mosaic quality, one is geometric mosaic quality and another is the colour mosaic quality. 
The uneven lightness distribution inside aerial image and the colour differences between aerial images are two factors which lead the 
colour mosaic problems. Homomorphic fdter can change the distribution of lightness inside image and Wallis transform can adjust 
the colour difference between images. Using low frequency convolution algorithm to eliminate seam lines in mosaic orthophoto. The 
finally result of DOM is showing that the dodging method mentioned in this paper is useful for solving the colour problems in 
orthophoto mosaic. 
1. INTRODUCTION 
1.1 DOM Quality’s Influences 
As an production of photogrammetry, Digital Orthophoto Map 
(DOM) has become more and more important because of its 
visual, truthful and high automatically produce. The quality of 
DOM is decided by geometric precision and colour mosaic 
quality. The details are showing in figure 1. 
Figure 1. DOM quality’s influences 
1.2 Geometric Influences 
The precision of aerial triangulation and DTM decide the 
precision and geometric quality of DOM. With the development 
of POS technology, full automatically aerial triangulation 
becomes true. Such as MATCH-AT (Inpho Company) and 
Stereo Softcopy Kit (Intergraph Company) can aerial 
triangulate automatically with POS data. Using Multi-view 
matching algorithms or Lidar data, the DTM is easy to get. 
Seam lines of orthophoto mosaic can’t cross buildings, trees etc 
which higher than ground and can’t cross areas which exist big 
gray difference between overlapping orthophotos. Researches 
about Automatic seam line detection have got achievements. 
1.3 Colour Problem 
Colour processing in orthophoto mosaic is important for DOM 
quality, include local illumination (hotspot) removing in single 
image, radiometric slope elimination between images and seam 
lines elimination in mosaic orthophoto. The following content 
of this paper discuss method of colour processing in orthophoto 
mosaic in detail. 
2. COLOUR PROCESSING IN ORTHOPHOTO 
MOSAIC 
2.1 Hotspot Removing with Homomorphic Filter 
Big frame of aerial cameras and remote sensors leads light 
illumination in cent and dark illumination in edge when 
imaging (Figure 2.A). Special ground (such as water, desert and 
metal house roof) also leads illumination problems (Figure 2.B). 
The angle between sensor director and sunshine caused top of 
the photo is light and under of the photo is dark (Figure 2.C, or 
reverse this case). Hotspot problem is very familiar in digital 
aerial photos and it lead list scene in final mosaic orthophoto, 
see Figure 3. 
The low frequency of image is related distribution of lightness 
and basic colour, high frequency of image is related texture 
characters. See Figure 4, high pass filter remove the lightness of 
image. Filtering operation can be used in space field or 
frequency field, the lightness distribution is the global 
information for images (very low frequency singles), so space 
field filtering operation need big size filter. Computer time in 
space field is related to the size of filter and big filter size lead 
more costing time, so frequency field filtering operation is 
common select. 
Homomorphic filtering is a change of high pass filtering; it split 
the irradiation information and reflection information from 
image, and filtering operation for irradiation information. The 
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