【SCIENTIFIC AMERICAN December 2019】【OPTICS】Ocean Vision

        A new algorithm takes the water out of underwater photographs

        一种新的算法可以去掉水下图片中的水。
        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.

        珊瑚丛具有自然界中最复杂多彩的生活形态之一，但是任何一个水下摄影师都知道，他们在没有人工照明的环境下拍摄的照片通常是平淡且不合预期的。甚至浅水也会选择性的吸收和散播不同程度波长的光线，难以让摄影社专注于拍摄水下美景的重点特征和混淆拍摄物颜色，特别是红色和黄色。这个影响使珊瑚科学家使用电脑视觉和机器学习算法去确定景物变得更加困难。将水下图像进行分类和计数，这不得不使用耗时的人工处理它。

        But a new algorithm called Sea-thru, developed by engineer and oceanographer Derya Akkaynak, removes the visual distortion caused by water from an image. The effects could be far-reaching for biologists 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.

        但是一种新的Sea-thru算法出现了，由工程师与海洋学家Derya Akkaynak开发，去掉了由水对图像造成的视觉变形与失真。这个影响深远的改变使得生物学家可以看到水面下真实的色彩，Akkaynak 和她在以色列海法大学的博士后导师Tali Treibitz在6月份的IEEE 计算机视觉和图像识别会议的一篇论文中详细说明了这个过程。

        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 distortion from water pixel by pixel, restoring lost colors.

        Sea-thru’s图像分析考虑了光吸收和大气散射的物理特性，和海洋相比，海洋中与光发生相互作用的的粒子更大。这个程序有效的消除了水对图片影响所引起的失真和丢失的色彩。

        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 readily work on those photographs.

        需要注意的是,这个处理过程需要拍摄的距离信息，Akkaynak 从各个不同的角度拍摄了很多同一场景的照片，通过Sea-thru算法用来估算摄像机和场景之间的距离，以此判断水中光线衰减的影响。很幸运，大多数的科学家已经对图片数据集使用摄影测量法获得了距离信息。并且Akkaynak 说，这个项目对图片的操作方式会很简便。

        “There are a lot of challenges associated 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 images, Pedersen says, adding that Sea-thru is a step in the right direction.

        在圣地亚哥加州大学的一个项目中，参与了挑战100个岛屿的研究员Nicole Pedersen说道：“在水下工作有大量的挑战，这让我们的研究进度远远落后于水面和陆地工作的研究员"。科学家们以每100平方米耗费的7000张图片去组装珊瑚礁的3D模型。由于缺乏处理这些大量图片的应用工具，导致研究进度被阻碍。Pedersen 说道，使用Sea-thru算法是提高研究进展的一个捷径。

        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

        这个应用程序的算法正如PS软件一样困难，用户可以通过人为均匀的增加红色和黄色色域来增强水下图片质量，加州科学院的珊瑚生物学家Pim Bongaerts说道：我喜欢这种方法的原因是因为这样可以获得水下更真实的色彩。获得水下的真实景象可以帮助我们从当前图像中取到更多的价值。—Erik Olsen