Computer Vision for Atmospheric Turbulence: Generation, Restoration, and its Applications

Shyam Nandan Rai


Real-world images often suffer from variation in weather conditions such as rain, fog, snow, and temperature. These variations in the atmosphere adversely affect the performance of computer vision models in real-world scenarios. This problem can be bypassed by collecting and annotating images for each weather. However, collecting and annotating images in such conditions is an extremely tedious task, which is time-consuming as well as expensive. So, in this work, we address the forementioned problems. Among all the weather conditions, we focus on the distortions in the image caused by high temperature, also known as atmospheric turbulence. These distortions introduce geometrical deformation around the boundaries of an object in an image which causes a vision algorithm to perform poorly and pose a major concern. Hence, in this thesis, we address the problem of artificially generating atmospheric turbulence and restoring the images from it. In the first part of our work, we attempt to model atmospheric turbulence. Since such models are critical to extending computer vision solutions developed in the laboratory to real-world use cases. And, simulating atmospheric turbulence by using statistical models or by computer graphics is often computationally expensive. To overcome this problem, we train a generative adversarial network(GAN) which outputs an atmospheric turbulent image by utilizing less computational resources than traditional methods. We propose a novel loss function to efficiently learn the atmospheric turbulence at the finer level. Experiments show that by using the proposed loss function, our network outperforms the existing state-of-the-art image to image translation network in turbulent image generation. In the second part of the thesis, we address the ill-posed problem of restoring images degraded due to atmospheric turbulence. We propose a deep adversarial network to recover the images which are distorted due to atmospheric turbulence and show the applicability of restored images in several tasks. Unlike previous methods, our approach neither uses any prior knowledge about atmospheric turbulence conditions at inference time nor requires the fusion of multiple images to get a single restored image. To train our models, we synthesized turbulent images by following a series of efficient 2D operations. Thereafter, using our trained models we run inference on real and synthesized turbulent images. Our final restoration models DT-GAN+ and DTD-GAN+ qualitatively and quantitatively outperforms the general state-of-the-art image-to-image translation models. The improved performance of our model is due to the use of optimized residual structures along with channel attention and sub-pixel mechanism which exploits the information between the channels and removes atmospheric turbulence at the finer level. We also perform extensive experiments on restored images by utilizing them for downstream tasks such as classification, pose estimation, semantic keypoint estimation, and depth estimation. In the third part of our work, we study the problem of the semantic segmentation model in adapting to hot climate cities. This issue can be circumvented by collecting and annotating images in such weather conditions and training segmentation models on those images. But, the task of semantically annotating images for every environment is painstaking and expensive. Hence, we propose a framework that improves the performance of semantic segmentation models without explicitly creating an annotated dataset for such adverse weather variations. Our framework consists of two parts, a restoration network to remove the geometrical distortions caused by hot weather and an adaptive segmentation network that is trained on an additional loss to adapt to the statistics of the ground-truth segmentation map. We train our framework on the Cityscapes dataset, which showed a total IoU gain of 12.707 over standard segmentation models. In the last part of our work, we improve the performance of our joint restoration and segmentation network via a feedback mechanism. In, the previous approach the restoration network does not learn directly from the errors of the segmentation network. In other words, the restoration network is not task aware. Hence, we propose a semantic feedback learning approach, which improves the task of semantic segmentation giving a feedback response into the restoration network. This response works as an attend and fix mechanism by focusing on those areas of an image where restoration needs improvement. Also, we proposed loss functions: Iterative Focal Loss (iFL) and Class-Balanced Iterative Focal Loss (CBiFL), which are specifically designed to improve the performance of the feedback network. These losses focus more on those samples that are continuously miss-classified over successive iterations. Our approach gives a gain of 17.41 mIoU over the standard segmentation model, including the additional gain of 1.9 mIoU with CB-iFL on the Cityscapes dataset

Year of completion:  December 2020
 Advisor : C V Jawahar,Vineeth Balasubramanian,Anbumani Subramanian

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