Leveraging Structural Cues for Better Training and Deployment in Computer Vision

Shyamgopal Karthik


With the growing use of computer vision tools in wide-ranging applications, it becomes imperative to understand and resolve issues in computer vision models when they are used in production settings for various applications. In particular, it is essential to understand that the model can be wrong quite frequently during deployment. Developing a better understanding of the mistakes made by a model can help mitigate and handle them without catastrophic consequences. To investigate the severity of mistakes, we first explore this in a simple classification setting. Even in this setting, understanding the severity of mistakes of difficult to quantify, especially since manually defining pairwise costs does not scale well for large-scale classification datasets. Therefore most works have used class taxonomies/hierarchies, which allow pairwise costs to be defined using graph distances. There has been increasing interest in building deep hierarchy-aware classifiers, aiming to quantify and reduce the severity of mistakes and not just count the number of errors. However, most of these works require the hierarchy to be available during training and cannot adapt to new hierarchies or even small modifications to the existing hierarchy without having to re-train the model. We explore a different direction for hierarchy-aware classification – amending mistakes by making post-hoc corrections by resorting to the classical Conditional Risk Minimization(CRM). Surprisingly, we find that this method is a far more suitable alternative than the works on deep hierarchy-aware classification; CRM preserves the base model’s top-1 accuracy and brings the most likely predictions of the model closer to the ground truth and is able to provide reliable probability estimates, unlike hierarchy-aware classifiers. We firmly believe that this serves as a very strong and useful baseline for future exploration in this direction. We turn our attention to a crucial problem in many video processing pipelines: visual(single) object tracking. In particular, we explore the long-term tracking scenario where given a target in the first frame of the video; the goal is to track the object throughout a (long) video during which the object may undergo occlusion, vary in appearance, or go out-of-view. The temporal aspect of videos also makes it an ideal scenario to understand the accumulation of errors that would not be otherwise seen if every image is independent. We hypothesize that there are three crucial abilities that a tracker must possess to be effective in the long-term tracking scenario, namely Re-Detection, Recovery and, Reliability. The tracker must be able to re-detect the target when the target goes out of the scene, and returns must recover from failure and track an object contiguously to be of practical utility. We propose a set of novel and comprehensive experiments to understand each of these aspects which give a thorough understanding of the strengths and limitations of various state-of-the-art tracking algorithms. We finally visit the problem of multi-object tracking. Unlike the problem of single-object tracking where the target is initialized in the first frame, the goal here is to track all objects of a particular category(such as pedestrians, vehicles, animals etc.). Since this problem does not require user-initialization, it has found use in wide-ranging real-time applications such as autonomous driving. The typical multiobject tracking pipeline follows the tracking-by-detection paradigm, i.e. an object detector is first used to detect all the objects in the scene. These detections are linked together to form the final trajectories using a combination of Spatio-temporal features and appearance/Re-Identification(ReID) features. The appearance features are extracted using a Convolutional Neural Network(CNN) trained on a corpus of labelled videos. Our central insight is that only the appearance model requires labelled videos in the entire pipeline, while the rest of the pipeline can be trained with just image-level supervision. Inspired by the recent successes in unsupervised contrastive learning which enforces the similarity in feature space between an image and its augmented version, we resort to a simple method that leverages the spatio-temporal consistency in videos to generate “natural” augmentations which are then used as pseudo-labels to train the appearance model. When integrated into the overall tracking pipeline, we find that this unsupervised appearance model can match the performance of its supervised counterparts in reducing the identity switches present in the trajectories, thereby saving costly video annotations that are impractical to scale up without sacrificing performance.

Year of completion:  April 2021
 Advisor : Vineet Gandhi

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