2.3 Pre-processing
The radiometric balancing procedure makes no attempt to correct for atmospheric effects. After
radiometric balancing, the brightness of pixels at the same location from two different scenes
will be a little different due to the atmospheric effects, especially in low-albedo vegetated areas.
The pre-processing procedure tries to make a balance between the scenes for the differences
caused mainly by atmospheric effects. After radiometric balancing, one image from the set of
images is chosen as the reference image. For each band, the pixel values of all other images in
the same set are adjusted according to
P=Eref+(S-E)*sref / s (2)
where P is the output pixel value, S is the input pixel value, E
ref is the mean pixel value of a
selected area of interest from the reference image, E is the mean pixel value of a selected area
of interest from the image to be balanced,
sref
is the standard deviation of the selected area of
interest from the reference image, s is the standard deviation of the selected area of interest
from the image to be balanced.
2.4 Pixel Ranking
The pixel ranking procedure uses the pixel intensity (weighted average of the three band pixel
values) and some suitably chosen band ratios to rank the pixels in order of "cloudiness" and
"shadowiness" according to some predefined ranking criteria. In this procedure, a shadow
intensity threshold T
s and a cloud intensity threshold T
c are determined from the intensity
histogram. The pixel ranking procedure uses these shadow and cloud thresholds to rank the
pixels in order of "cloudiness" and "shadowiness". Each of the non-cloud and non-shadow pixels
in the images is classified into one of four broad classes based on the band ratios: vegetation,
building, water and others.
For each image n from the set of N acquired images, each pixel at a location (i, j) is assigned a
rank r
n(i, j) based on the pixel intensity Y
n(i, j) and the brightness of the three display channels
R
n(i, j), G
n(i, j) and B
n(i, j) according to the following rules:
(i) For T
s£(Y
m, Y
n)
£
T
c,
if Y
m> Y
n and class="building", then r
m<r
n;
(ii) For (Y
m, Y
n)
£T
c
if Y
m < Y
n and class="vegetation", then r
m<r
n ;
(iii) For Y
m , Y
n<T
s ,
if Y
m > Y
n and class="water", then r
m < r
n;
(iv) For T
s £(Y
m , Y
n)
£T
c ,
if Y
m<Y
n and class="others", then r
m<r
n ;
(v) If (T
s £
Y
m £T
C,) and (Y
n> T
C
or Y
n<T
C) and class="others", then r
m<r
n;
(vi) For Y
M, Y
n<T
C ,
if Y
M> Y
n and class="others", then r
m <r
n;
(vii) If Y
m<T
s and Y
n > T
c then r
m<r
n;
(viii) For Y
m , Y
n > T
c ,
if Y
m < Y
n and class="others", then r
m<r
n;
In this scheme, pixels with lower rank values of r n are more superior and are more likely to be
selected. Pixels with intensities falling between the shadow and cloud thresholds are the most
superior, and are regarded as the "good pixels". Where no good pixels are available, the "shadow
pixels" are preferred over the "cloud pixels". Where all pixels at a given location are "shadow
pixels", the brightest shadow pixels will be chosen. In locations where all pixels have been
classified as "cloud pixels", the darkest cloud pixels will be selected.
After ranking the pixels, the rank-r index map n r (i, j) representing the index n of the image with
rank r at the pixel location (i, j) can be generated. In our algorithm, only the rank-1 and rank-2
index maps are generated and kept for use in generating the cloud-free mosaics.
2.5 Merging of Sub-Images
In this procedure, the rank-1 and rank-2 index maps are used to merge the multi-scenes from the
same set of images. 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 are averaged together in order
to avoid sudden spatial discontinuities in the final mosaicked image. Otherwise, the pixels from
the rank-1 image are used.
