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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B3. Istanbul 2004 
  
Er tte 
Input 
Images 
  
Balancing of 
Grey Level 
— 
  
  
  
  
c | 
  
D 
  
  
  
  
Classification 
  
  
  
M orphological 
Pixel/Patch 
  
  
  
  
  
  
  
  
  
  
  
  
   
   
and Mask Filter Ranking 
C loud -free M osaic M erging of 
M osaics 
  
Production 
  
  
Sub-images 
  
  
  
  
  
Figure l. A schematic diagram of the cloud-free and cloud shadow-free mosaic generating system 
The initial cloud mask produced using a fixed intensity 
threshold usually excludes the thin clouds at cloud edges. To 
solve this problem, a morphological filter is applied to dilate the 
initial cloud mask patch so that the thin clouds at cloud edges 
are included in the cloud mask. 
2.4 Pixel/Patch Ranking 
In the masking process, some bright patches on bare soil can 
still be mistaken as cloud patches due to the similar intensity 
level and colour. These patches can be unfilled because no good 
data patches in the set of co-registered cloudy images can be 
used to mosaic the composite image. We therefore introduce a 
pixel/patch ranking procedure that employs the pixel/patch 
intensity and some suitably chosen band ratios to rank the 
pixels/patches in the order of "cloudness" and “shadowiness” 
according to some predefined ranking criteria described below. 
In this procedure, a shadow intensity threshold 7, a vegetation 
intensity threshold 7,, à cloud threshold 7, are determined from 
the intensity histogram. The pixel/patch ranking procedure uses 
these shadow and cloud thresholds to rank the pixels/patches in 
order of *cloudiness" and “shadowness”. Each of the non-cloud 
and non-shadow pixels/patches in the images is classified into 
one of three broad classes based on the band ratios: vegetation, 
open land and others. 
For each image n from the set of N acquired images, each 
pixel/patch at a location (i, /) is assigned a rank r,(i, /) based on 
the pixel/patch intensity Y,(/, /) according to the following 
rules: 
^ar , 7 7 < (4 V SY + since = 
D For 7,.$ (Yo "95$ TOW fr, Y, and class 
" 
» stafi " » . Cp + 
vegetation", then ry, € ry; 
2) Por 7,50 
€ " a ^ M y + 
land", then r,, € rj; 
3 Cy « € / E ^ ey 
3) Hy 7, and Yı, > Te; then r, rye 
S | V y a i£ V ~ J » ‘ «p 
4) ForY,» J^ Tof p" then rj; <r; 
5) For! 
S ForY,, 
ups T if Y, Y, and class open 
Y, > le ity, < Y then ry, <r; 
In this scheme, pixels/patches with lower rank values of r, are 
more superior and are more likely to be selected. Pixels/patches 
with intensities falling between the shadow and cloud 
thresholds are the most superior, and are regarded as the "good 
pixels/patches". The “good pixels/patches” are further classified 
into “vegetation pixels/patches” or “open land pixels/patches” 
depending on whether the pixel/patches intensity is below or 
above the vegetation threshold. As rule of thumb, the darker 
*good pixels/patches" are preferred over the brighter "good 
pixels/patches" because the brighter "good pixels/patches" may 
be contaminated by thin clouds. Where no good pixels/patches 
are available, the "shadow pixels/patches" are preferred over the 
"cloud pixels/patches". Where all pixels/patches at a given 
location are "shadow pixels/patches", the brightest shadow 
pixels/patches will be chosen. In locations where all 
pixels/patches have been classified as "cloud pixels/patches", 
the darkest cloud pixels/patches will be selected. After ranking 
the pixels/patches, the rank-r index map ni, /) representing the 
index n of the image with rank r at the pixel/patch location (7, /) 
can be generated. It is preferred that only the rank-1 and rank-2 
index maps are generated and kept for use in generating the 
cloud-free and cloud-shadow free mosaics. 
2.5 Merging of Sub-images 
The rank-1 and rank-2 index maps generated from the 
pixel/patch ranking procedure are used to merge the input 
multi-scenes that have been processed by the grey-level 
balance. If the pixel at a given location has been classified as 
“vegetation pixel”, the pixels from the rank-1 image and the 
rank-2 image at that location may be averaged together in order 
to avoid sudden spatial discontinuities in the final mosaic 
image. Otherwise, the pixels from the rank-1 image are used. 
2.6 Mosaic Production 
The basic idea of the cloud-free and cloud shadow-free mosaic 
algorithm is to mask the clouds, cloud-shadows and select good 
image data from the different scenes in a selected set of co- 
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