IEEE Robotics & Automation Magazine - September 2021 - 73

can also be used in other applications of underwater vision.
This includes the production of additional training data for
ML methods, including DL methods.
Several different image degradation methods are considered.
The first group of transformations, named pixel-based
perturbations, requires information from only a single monocular
image and transforms only pixel values; i.e., all operations
are constrained to the image domain. The second
group of geometry-contextual perturbations uses the 3D
scene geometry information obtained from stereo imagery
[39] to compute the depth relative to the camera and, in turn,
to render a more detailed simulation of underwater light
backscattering effects. Figure 6 depicts examples of each type
of distortion. The code for the degradations is available at
https://github.com/arturokkboss33/caddy-underwater
-diver-classification.
Pixel-Based Perturbations
Gaussian Blur
The image is blurred to approximate the effects caused by
moving objects, sediment clouds, material on the lens/housing,
misalignment of the camera with respect to the housing
window, or incorrect focus caused by light forward scattering.
This is done using a Gaussian kernel with standard
deviation v and size ks
" 15 93 17 ,
pairs: (. ,)(, ).
Brightness Shift
For shallow water operations
(depth 15 m1
), the ambient
light can drastically change the brightness of the
image depending on the weather conditions and time of
the day. To simulate this, a scaling factor b is applied to
each image channel with respective saturation values of
., .
b 05 2,
= "
White Balance
White balance is considered because it can lead to unexpected
image artifacts. White balancing methods are typically based
on the assumption that there is a minimum range of colors in
the scene, including neutral (white) colors. But, when there
are large regions with uniform color in the scene (water medium),
this can shift the color correction to more blueish or
reddish colors. Thus, if the white balance is not properly configured,
e.g., if a standard in-air method is used, it can
degrade the quality of the image.
To reflect this, a gray-world (GW) white balance is applied
that assumes the average of all channels should result in a gray
image. It requires a saturation threshold
t
GW 07= Not all
..
normalized pixels above this value are used during the color
correction process. Another method, denoted as simple white
balance (SWB), just stretches each input channel to generate
similar ranges for each channel. It uses a threshold
to ignore the top and bottom 5% of pixels.
t
SWB = 5 %
Underwater Alpha Blend
The image I is blended with a background image of uniform
color H that represents simple underwater haze effects. This
image operation, known as alpha blend, is defined as A=
HI ().
= "
## 1aa
+- The blending coefficients used here are
., ..
a 0250 5, Typically, a gray color for H is used based
on the color of fog on land. However, to emulate underwater
haze with higher fidelity, the Jerlov water types [33] are used
with their associated light downwelling and backscattering
attenuation factors to tune H to a more realistic color.
For our experiments, an ambient light at depth d of 10 m
is assumed. Jerlov water types wI ,I= C1 are considered, i.e.,
murky oceanic water and coastal water with low amounts of
sediment. Based on our experience, these offer challenging
visibility conditions but are within the operational range for
(a)
(b)
(c)
(d)
(e)
(f)
v 3 ;= (c) brightness shift, b . ;05=
(g)
(h)
Figure 6. Examples of underwater image perturbations: (a) the original image, with (b)-(f) pixel-based and (g)-(h) geometrycontextual
perturbations. (b) Gaussian blur,
compression, q % ;20=
(f) alpha blend, wC ,1=
a 025= .; (g) depth map from stereo; and (h) underwater haze.
SEPTEMBER 2021 * IEEE ROBOTICS & AUTOMATION MAGAZINE *
73
(d) gray-world white balance, t 07= . ; (e) JPEG
https://github.com/arturokkboss33/caddy-underwater-diver-classification https://github.com/arturokkboss33/caddy-underwater-diver-classification

IEEE Robotics & Automation Magazine - September 2021

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