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Figure 3. Change trends of brightness, contrast and average gradients when p value from 1-10 in true color aerial image shadow
removal
5.2 Shadow removal experiment with color infrared aerial
images
The test image is a color infrared aerial image of Shanghai from
1998; the resolution is 20cm/pixel. In the experiment, the
original image was reduced to 704x734pixel; the resolution is
0.4m/pixel. The Figure 4(a) is the original image. The Figure
4(b) is the shadow detection result; the black area is shadow
region. Figure 4(c) is the shadow removal image when p is 2.
(a) (b) Cute) pen.
Figure 4. Color infrared aerial image (a) the shadow detection
result (b) and the results of shadow removal (c)
Using the same method as 5.1, the brightness, contrast and
average gradients are calculated separately in the shadow and
non-shadow regions. Table 2 is the result. Figure 5 includes
three statistical trend charts of the shadow region, when p is
from | to 10 in shadow removal processing.
From Table 2 and the Figure 5, we draw the following
conclusions:
(1) Compared to true color aerial images, the brightness
decreases more sharply in shadow region, especially in red and
infrared band. The contrast and average gradients decreases
similarly; the degree of decrease is the same in all three bands,
and the result is the average gradients decreases much more
than in the true color image experiment.
(2)When p is 1,or 2, the SoG shadow removal algorithm can
recover the brightness, contrast and average gradients of
shadow region in color infrared aerial images, especially for the
brightness in red and infrared band. The values are close to non-
shadow region after shadow removal.
(3)From Figure 5, the change trend lines show that the
brightness, contrast and average gradients in red and green
bands achieve maximum value when shadows are removed
using p=2. In the infrared band the three statistical
characteristics achieve maximum when p-1; the three values
decrease a little when p-2. Infrared band measured values
sharply drop when p is from 2 to 10. When p is 2, the shadow
contrast and average gradients have a good agreement with non-
shadow. Considering the brightness, contrast, and average
gradients improvements in the shadow region across red, green,
and infrared bands, the color infrared aerial image achieves the
best overall result of shadow removal when p values 2 in this
experiment.
(4)The visual effect shows that when p is 1, 2, or 3, the shadow
region color is closer to non-shadow region after the shadow
removal; the effect is better than the resulting image when p is a
large value. There is no visible color error after the shadow
removal processing with the SoG algorithm. It follows from this
experiment that for color infrared aerial images, best results for
shadow removal are obtained when p equals 2.
Table 2. The brightness, contrast, and average gradients of the color infrared aerial image before and after shadow removal
brightness contrast average gradients
NIR R G NIR R G NIR R G
Shadow region | 12.0784 | 13.6279 | 34.3757 | 6.7721 | 7.3936 |11.2968| 0.485 | 0.5063 | 0.8062
1 76.8154 | 80.51 |100.6389| 43.0678 | 43.6788 | 33.0724 | 3.0843 | 2.9913 | 2.3602
2 75.9624 | 82.9082 |102.5798| 42.5895 | 44.9799 | 33.7102 | 3.05 3.0804 | 2.4058
3 | 64.8578] 73.6396 [101.9624] 36.3635 | 39.9515 | 33.5074 | 2.6041 | 2.736 | 2.3913
4 50.184 | 58.018 | 98.5613 | 28.1365 | 31.4763 | 32.3897 | 2.015 | 2.1556 | 2.3115
Pg 5. |39.5395 | 45.8608 | 92.5138 | 22.1684 | 24.8808 | 30.4023 | 1.5876 | 1.7039 | 2.1697
6 32.1539 | 38.1539 | 84.9458 | 18.3934 | 20.6995 | 27.9153 | 1.3172 | 1.4176 | 1.9922
7 28.3917 | 33.1515 | 77.4869 | 15.9183 | 17.9856 | 25.4641 1.14 1.2317 |]. 1.8173
8 25.3295 | 29.7025 | 71.0948 | 14.2014 1 16.1144 | 23.3635 | 1.017 | 1.1036 | 1.6674
9 23.105 | 27.1962 | 65.9188 | 12.9542 1 14.7547 | 21.6626 | 0.9277 | 1.0104 | 1.546
10 |21.4306 | 25.2988 | 61.7694 | 12.0154 | 13.7253 | 20.299 | 0.8605 0.94 1.4486
unshadow region | 76.8126 | 80.5071 |100.6347| 41.0305 | 49.1429 | 39.1242 | 3.358 | 3.4974 | 3.1397
