Scientific American - USA (2019-12)

16 Scientific American, December 2019

DERYA AKKAYNAK

ADVANCES

O P T I C S

Ocean Vision

A new algorithm takes the water

out of underwater photographs

Coral reefs are among nature’s most

complex and colorful living formations. But

as any underwater photographer knows,

pictures of them taken without artificial

lights often come out bland and blue. Even

shallow water selectively absorbs and

scatters light at different wavelengths,

making certain features hard to see and

washing out colors—especially reds and

yellows. This effect makes it difficult for

coral scientists to use computer vision and

machine-learning algorithms to identify,

count and classify species in underwater

images; they have to rely on time-consum-

ing human evaluation instead.

But a new algorithm called Sea-thru,

developed by engineer and oceanographer

Derya Akkaynak, removes the visual dis-

tortion caused by water from an image.

The effects could be far-reaching for biolo-

gists who need to see true colors under-

neath the surface. Akkaynak and engineer

Tali Treibitz, her postdoctoral adviser at

the University of Haifa in Israel, detailed

the process in a paper presented in June at

the IEEE Conference on Computer Vision

and Pattern Recognition.

Sea-thru’s image analysis factors in the

physics of light absorption and scattering

in the atmosphere, compared with that in

the ocean, where the particles that light

interacts with are much larger. Then the

program effectively reverses image distor-

tion from water pixel by pixel, restoring

lost colors.

One caveat is that the process requires

distance information to work. Akkaynak

takes numerous photographs of the same

scene from various angles, which Sea-thru

uses to estimate the distance between the

camera and objects in the scene—and, in

turn, the water’s light-attenuating impact.

Luckily, many scientists already capture

distance information in image data sets by

using a process called photogrammetry,

and Akkaynak says the program will readi-

ly work on those photographs.

“There are a lot of challenges associat-

ed with working underwater that put us

well behind what researchers can do

above water and on land,” says Nicole

Pedersen, a researcher on the 100 Island

Challenge, a project at the University of

California, San Diego, in which scientists

take up to 7,000 pictures per 100 square

meters to assemble 3-D models of reefs.

Progress has been hindered by a lack of

computer tools for processing these imag-

es, Pedersen says, adding that Sea-thru is

a step in the right direction.

The algorithm differs from applications

such as Photoshop, with which users can

artificially enhance underwater images

by uniformly pumping up reds or yellows.

“What I like about this approach is that it’s

really about obtaining true colors,” says Pim

Bongaerts, a coral biologist at the California

Academy of Sciences. “Getting true color

could really help us get a lot more worth

out of our current data sets.” — Erik Olsen

Large coral forma-

tion in Lembeh

Strait in Indonesia

before ( inset ) and

after processing

with the Sea-thru

algorithm.

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